arXiv daily: Quantum Physics (quant-ph)

Authors:Kohtaro Kato

Abstract: We study an exact local compression of a quantum bipartite state; that is, applying local quantum operations to the state to reduce the dimensions of Hilbert spaces while perfectly maintaining the correlation. We provide a closed formula for calculating the minimal achievable dimensions, provided as a minimization of the Schmidt rank of a particular pure state constructed from that state. Numerically more tractable upper and lower bounds of the rank were also obtained. Subsequently, we consider the exact compression of quantum channels as an application. Using this method, a post-processing step that can reduce the output dimensions while retaining information on the output of the original channel can be analyzed.

Authors:Esteban Marulanda, Andrés Gómez

Abstract: We demonstrate that the temperature affects the validity of the CHSH inequality in an open bipartite two-qubit system. Specifically, for initial entangled states within the decoherence-free subspace (DFS), the CHSH inequality remains temperature-independent. In contrast, other entangled states exhibit a temperature threshold beyond which the inequality holds.

Authors:Priya Malpani, Satish Kumar, Anirban Pathak

Abstract: In the present work, we report experimental realization of an optical fiber based COW protocol for QKD in the telecom wavelength (1550 nm) where the attenuation in the optical fiber is minimum. A laser of 1550 nm wavelength, attenuator and intensity modulator is used for the generation of pulses having average photon number 0.5 and repetition rate of 500 MHz. The experiment is performed over 40 km, 80 km and 120 km of optical fiber and several experimental parameters like disclose rate, compression ratio, dead time and excess bias voltage of the detector are varied for all the cases (i.e., for 40 km, 80 km and 120 km distances) to observe their impact on the final key rate. Specifically, It is observed that there is a linear increase in the key rate as we decrease compression ratio or disclose rate. The key rate obtains its maximum value for least permitted values of disclose rate, compression ratio and dead time. It seems to remain stable for various values of excess bias voltage. While changing various parameters, we have maintained the quantum bit error rate (QBER) below 6%. The key rate obtained is also found to remain stable over time. Experimental results obtained here are also compared with the earlier realizations of the COW QKD protocol. Further, to emulate key rate at intermediate distances and at a distance larger than 120 km, an attenuator of 5 dB loss is used which can be treated as equivalent to 25 km of the optical fiber used in the present implementation. This has made the present implementation equivalent to the realization of COW QKD upto 145 km.

Authors:Maximilian Rieger, Moritz Reh, Martin Gärttner

Abstract: We explore a supervised machine learning approach to estimate the entanglement entropy of multi-qubit systems from few experimental samples. We put a particular focus on estimating both aleatoric and epistemic uncertainty of the network's estimate and benchmark against the best known conventional estimation algorithms. For states that are contained in the training distribution, we observe convergence in a regime of sample sizes in which the baseline method fails to give correct estimates, while extrapolation only seems possible for regions close to the training regime. As a further application of our method, highly relevant for quantum simulation experiments, we estimate the quantum mutual information for non-unitary evolution by training our model on different noise strengths.

Authors:Xiang Wang, Feng-Yu Lu, Ze-Hao Wang, Zhen-Qiang Yin, Shuang Wang, Wei Chen, De-Yong He, Guang-Can Guo, Zheng-Fu Han

Abstract: Measurement-device-independent quantum key distribution (MDI-QKD) can resist all attacks on the detection devices, but there are still some security issues related to the source side. One possible solution is to use the passive protocol to eliminate the side channels introduced by active modulators at the source. Recently, a fully passive QKD protocol has been proposed that can simultaneously achieve passive encoding and passive decoy-state modulation using linear optics. In this work, we propose a fully passive MDI-QKD scheme that can protect the system from both side channels of source modulators and attacks on the measurement devices, which can significantly improve the implementation security of the QKD systems. We provide a specific passive encoding strategy and a method for decoy-state analysis, followed by simulation results for the secure key rate in the asymptotic scenario. Our work offers a feasible way to improve the implementation security of QKD systems, and serves as a reference for achieving passive QKD schemes using realistic devices.

Authors:Kfir Blum, Omri Rosner

Abstract: We study tunneling in one-dimensional quantum mechanics using the path integral in real time, where solutions of the classical equation of motion live in the complex plane. Analyzing solutions with small (complex) energy, relevant for constructing the wave function after a long time, we unravel the analytic structure of the action, and show explicitly how the imaginary time bounce arises as a parameterization of the lowest order term in the energy expansion. The real time calculation naturally extends to describe the wave function in the free region of the potential, reproducing the usual WKB approximation. The extension of our analysis to the semiclassical correction due to fluctuations on the saddle is left for future work.

Authors:Jonte R Hance, John Rarity, James Ladyman

Abstract: We here reply to a recent comment by Vaidman on our paper, ``Weak values and the past of a quantum particle'', which we published in Physical Review Research. In his Comment, he first admits that he is just defining (assuming) the weak trace gives the presence of a particle -- however, in this case, he should use a term other than presence, as this already has a separate, intuitive meaning other than ``where a weak trace is''. Despite this admission, Vaidman then goes on to argue for this definition by appeal to ideas around an objectively-existing idea of presence. We show these appeals are flawed, and rely on their own conclusion -- that there is always a matter of fact about the location of a quantum particle.

Authors:L. A. Markovich, S. Malikis, S. Polla, J. T. Brugués

Abstract: The phase shift rules enable the estimation of the derivative of a quantum state with respect to phase parameters, providing valuable insights into the behavior and dynamics of quantum systems. This capability is essential in quantum simulation tasks where understanding the behavior of complex quantum systems is of interest, such as simulating chemical reactions or condensed matter systems. However, parameter shift rules are typically designed for Hamiltonian systems with equidistant eigenvalues. For systems with closely spaced eigenvalues, effective rules have not been established. We provide insights about the optimal design of a parameter shift rule, tailored to various sorts of spectral information that may be available. The proposed method lets derivatives be calculated for any system, regardless of how close the eigenvalues are to each other. It also optimizes the number of phase shifts, which reduces the amount of gate resources needed.

Authors:Aviv Aroch, Ronnie Kosloff, Shimshon Kallush

Abstract: All quantum systems are subject to noise from the environment or external controls. This noise is a major obstacle to the realization of quantum technology. For example, noise limits the fidelity of quantum gates. Employing optimal control theory, we study the generation of quantum single and two-qubit gates. Specifically, we explore a Markovian model of phase and amplitude noise, leading to the degradation of the gate fidelity. We show that optimal control with such noise models generates control solutions to mitigate the loss of gate fidelity. The problem is formulated in Liouville space employing an extremely accurate numerical solver and the Krotov algorithm for solving the optimal control equations.

Authors:Jinjie Li, Wenyuan Wang, Hoi-Kwong Lo

Abstract: A recently proposed fully passive QKD removes all source modulator side channels. In this work, we combine the fully passive sources with MDI-QKD to remove simultaneously side channels from source modulators and detectors. We show a numerical simulation of the passive MDI-QKD, and we obtain an acceptable key rate while getting much better implementation security, as well as ease of implementation, compared with a recently proposed fully passive TF-QKD, paving the way towards more secure and practical QKD systems. We have proved that a fully passive protocol is compatible with MDI-QKD and we also proposed a novel idea that could potentially improve the sifting efficiency.

Authors:Edoardo Pedicillo, Andrea Pasquale, Stefano Carrazza

Abstract: Quantum computing is a growing field where the information is processed by two-levels quantum states known as qubits. Current physical realizations of qubits require a careful calibration, composed by different experiments, due to noise and decoherence phenomena. Among the different characterization experiments, a crucial step is to develop a model to classify the measured state by discriminating the ground state from the excited state. In this proceedings we benchmark multiple classification techniques applied to real quantum devices.

Authors:Giovanni Francesco Diotallevi, Björn Annby-Andersson, Peter Samuelsson, Armin Tavakoli, Pharnam Bakhshinezhad

Abstract: Quantum thermal machines can generate steady-state entanglement by harvesting spontaneous interactions with local environments. However, using minimal resources and control, the entanglement is typically very noisy. Here, we study entanglement generation in a two-qubit quantum thermal machine in the presence of a continuous feedback protocol. Each qubit is measured continuously and the outcomes are used for real-time feedback to control the local system-environment interactions. We show that there exists an ideal operation regime where the quality of entanglement is significantly improved, to the extent that it can violate standard Bell inequalities and uphold quantum teleportation. In particular, we find, for ideal operation, that the heat current across the system is proportional to the entanglement concurrence. Finally, we investigate the robustness of entanglement production when the machine operates away from the ideal conditions.

Authors:Antoine Soulas

Abstract: A mathematical consistency condition constraining any relativistic quantum theory is formulated. It turns out to be equivalent to the locality of physics as well as, in the context of quantum field theory, microcausality, thereby revealing that these are actually two redundant hypotheses. It also promotes an epistemic interpretation of the wavefunction collapse, helps address unsolved problems related to nonlocal measurements and provides a new proof of the non-measurability of fermionic fields.

Authors:Aleksey N. Bolgar, Daniil D. Kirichenko, Rais. S. Shaikhaidarov, Shtefan V. Sanduleanu, Alexander V. Semenov, Aleksey Yu. Dmitriev, Oleg V. Astafiev

Abstract: We study a phononic crystal interacting with an artificial atom { a superconducting quantum system { in the quantum regime. The phononic crystal is made of a long lattice of narrow metallic stripes on a quatz surface. The artificial atom in turn interacts with a transmission line therefore two degrees of freedom of different nature, acoustic and electromagnetic, are coupled with a single quantum object. A scattering spectrum of propagating electromagnetic waves on the artificial atom visualizes acoustic modes of the phononic crystal. We simulate the system and found quasinormal modes of our phononic crystal and their properties. The calculations are consistent with the experimentally found modes, which are fitted to the dispersion branches of the phononic crystal near the first Brillouin zone edge. Our geometry allows to realize effects of quantum acoustics on a simple and compact phononic crystal.

Authors:Haowei Xu, Uroš Delić, Guoqing Wang, Changhao Li, Paola Cappellaro, Ju Li

Abstract: Certain non-Hermitian systems exhibit the skin effect, whereby the wavefunctions become exponentially localized at one edge of the system. Such exponential amplification of wavefunction has received significant attention due to its potential applications in e.g., classical and quantum sensing. However, the opposite edge of the system, featured by the exponentially suppressed wavefunctions, remains largely unexplored. Leveraging this phenomenon, we introduce a non-Hermitian cooling mechanism, which is fundamentally distinct from traditional refrigeration or laser cooling techniques. Notably, non-Hermiticity will not amplify thermal excitations, but rather redistribute them. Hence, thermal excitations can be cooled down at one edge of the system, and the cooling effect can be exponentially enhanced by the number of auxiliary modes, albeit with a lower bound that depends on the dissipative interaction with the environment. Non-Hermitian cooling does not rely on intricate properties such as exceptional points or non-trivial topology, and it can apply to a wide range of Bosonic modes such as photons, phonons, magnons, etc.

Authors:Jianming Yi, Kalyani Suresh, Ali Moghiseh, Norbert Wehn

Abstract: Quantum Support Vector Machines (QSVM) play a vital role in using quantum resources for supervised machine learning tasks, such as classification. However, current methods are strongly limited in terms of scalability on Noisy Intermediate Scale Quantum (NISQ) devices. In this work, we propose a novel approach called the Variational Quantum Linear Solver (VQLS) enhanced QSVM. This is built upon our idea of utilizing the variational quantum linear solver to solve system of linear equations of a least squares-SVM on a NISQ device. The implementation of our approach is evaluated by an extensive series of numerical experiments with the Iris dataset, which consists of three distinct iris plant species. Based on this, we explore the practicality and effectiveness of our algorithm by constructing a classifier capable of classification in a feature space ranging from one to seven dimensions. Furthermore, by strategically exploiting both classical and quantum computing for various subroutines of our algorithm, we effectively mitigate practical challenges associated with the implementation. These include significant improvement in the trainability of the variational ansatz and notable reductions in run-time for cost calculations. Based on the numerical experiments, our approach exhibits the capability of identifying a separating hyperplane in an 8-dimensional feature space. Moreover, it consistently demonstrated strong performance across various instances with the same dataset.

Authors:Paolo Perinotti, Alessandro Tosini, Leonardo Vaglini

Abstract: A causal relation between quantum agents, say Alice and Bob, is necessarily mediated by an interaction. Modelling the last one as a reversible quantum channel, an intervention of Alice can have causal influence on Bob's system, modifying correlations between Alice and Bob's systems. Causal influence between quantum systems necessarily allows for signalling. Here we prove a continuity relation between the strength of causal influence and that of signalling. The continuity with respect to the intensity of the interaction is also shown for bipartite channels having equal input and output subsystems.

Authors:A. Mariani, U. -J. Wiese

Abstract: Based on the recent construction of a self-adjoint momentum operator for a particle confined in a one-dimensional interval, we extend the construction to arbitrarily shaped regions in any number of dimensions. Different components of the momentum vector do not commute with each other unless very special conditions are met. As such, momentum measurements should be considered one direction at a time. We also extend other results, such as the Ehrenfest theorem and the interpretation of the Heisenberg uncertainty relation to higher dimensions.

Authors:Cameron Calcluth, Nicolas Reichel, Alessandro Ferraro, Giulia Ferrini

Abstract: We present a new method for quantifying the resourcefulness of continuous-variable states in the context of promoting otherwise simulatable circuits to universality. The simulatable, albeit non-Gaussian, circuits that we consider are composed of Gottesman-Kitaev-Preskill states, Gaussian operations, and homodyne measurements. We first introduce a general framework for mapping a continuous-variable state into a qubit state. We then express existing maps in this framework, including the modular subsystem decomposition and stabilizer subsystem decomposition. Combining these results with existing results in discrete-variable quantum computation provides a sufficient condition for achieving universal quantum computation. These results also allow us to demonstrate that for states symmetric in the position representation, the modular subsystem decomposition can be interpreted in terms of resourceless (simulatable) operations - i.e., included in the class of Gaussian circuits with input stabilizer Gottesman-Kitaev-Preskill states. Therefore, the modular subsystem decomposition is an operationally relevant mapping to analyze the logical content of realistic Gottesman-Kitaev-Preskill states, among other states.

Authors:Gustavo de Oliveira, Lucas C. Céleri

Abstract: We address the question of entropy production in the context of the dynamical Casimir effect. Specifically, we consider a one-dimensional ideal cavity with one of its mirrors describing a prescribed trajectory. Inside the cavity we have a scalar quantum field and we ask about the changes in the thermodynamic entropy of the field induced by the non-trivial boundary conditions imposed by the moving mirror. By employing an effective Hamiltonian approach, we compute the entropy production and show that it scales with the number of particles created in the short-time limit. Moreover, such approach allows us to demonstrate that this entropy is directly related to the developments of quantum coherences in the mode basis of the field. A distinct approach, based on the time evolution of Gaussian states, allows us to study the long-time limit of the entropy production in single mode of the field. This results in a relation between the thermodynamic entropy production in the field mode with the entanglement between the considered mode and all the other modes. In this way, we link the entropy production in the field due to the dynamical Casimir effect with two fundamental features of quantum mechanics, coherence and entanglement.

Authors:Gian Gentinetta, Friederike Metz, Giuseppe Carleo

Abstract: Simulating the dynamics of large quantum systems is a formidable yet vital pursuit for obtaining a deeper understanding of quantum mechanical phenomena. While quantum computers hold great promise for speeding up such simulations, their practical application remains hindered by limited scale and pervasive noise. In this work, we propose an approach that addresses these challenges by employing circuit knitting to partition a large quantum system into smaller subsystems that can each be simulated on a separate device. The evolution of the system is governed by the projected variational quantum dynamics (PVQD) algorithm, supplemented with constraints on the parameters of the variational quantum circuit, ensuring that the sampling overhead imposed by the circuit knitting scheme remains controllable. We test our method on quantum spin systems with multiple weakly entangled blocks each consisting of strongly correlated spins, where we are able to accurately simulate the dynamics while keeping the sampling overhead manageable. Further, we show that the same method can be used to reduce the circuit depth by cutting long-ranged gates.

Authors:David Jennings, Matteo Lostaglio, Robert B. Lowrie, Sam Pallister, Andrew T. Sornborger

Abstract: How well can quantum computers simulate classical dynamical systems? There is increasing effort in developing quantum algorithms to efficiently simulate dynamics beyond Hamiltonian simulation, but so far exact running costs are not known. In this work, we provide two significant contributions. First, we provide the first non-asymptotic computation of the cost of encoding the solution to linear ordinary differential equations into quantum states -- either the solution at a final time, or an encoding of the whole history within a time interval. Second, we show that the stability properties of a large class of classical dynamics can allow their fast-forwarding, making their quantum simulation much more time-efficient. We give a broad framework to include stability information in the complexity analysis and present examples where this brings several orders of magnitude improvements in the query counts compared to state-of-the-art analysis. From this point of view, quantum Hamiltonian dynamics is a boundary case that does not allow this form of stability-induced fast-forwarding. To illustrate our results, we find that for homogeneous systems with negative log-norm, the query counts lie within the curves $11900 \sqrt{T} \log(T)$ and $10300 T \log(T)$ for $T \in [10^6, 10^{15}]$ and error $\epsilon = 10^{-10}$, when outputting a history state.

Authors:Enrico Fontana, Dylan Herman, Shouvanik Chakrabarti, Niraj Kumar, Romina Yalovetzky, Jamie Heredge, Shree Hari Sureshbabu, Marco Pistoia

Abstract: Using tools from the representation theory of compact Lie groups we formulate a theory of Barren Plateaus (BPs) for parameterized quantum circuits where the observable lies in the dynamical Lie algebra (DLA), a setting that we term Lie-algebra Supported Ansatz (LASA). A large variety of commonly used ans\"atze such as the Hamiltonian Variational Ansatz, Quantum Alternating Operator Ansatz, and many equivariant quantum neural networks are LASAs. In particular, our theory provides for the first time the ability to compute the gradient variance for a non-trivial, subspace uncontrollable family of quantum circuits, the quantum compound ans\"atze. We rigorously prove that the variance of the circuit gradient, under Haar initialization, scales inversely with the dimension of the DLA, which agrees with existing numerical observations.

Authors:Rubayet Al Maruf, Sreesh Venuturumilli, Divya Bharadwaj, Paul Anderson, Jiawei Qiu, Yujia Yuan, Mohd Zeeshan, Behrooz Semnani, Philip J. Poole, Dan Dalacu, Kevin Resch, Michael E. Reimer, Michal Bajcsy

Abstract: Hybrid quantum technologies aim to harness the best characteristics of multiple quantum systems, in a similar fashion that classical computers combine electronic, photonic, magnetic, and mechanical components. For example, quantum dots embedded in semiconductor nanowires can produce highly pure, deterministic, and indistinguishable single-photons with high repetition, while atomic ensembles offer robust photon storage capabilities and strong optical nonlinearities that can be controlled with single-photons. However, to successfully integrate quantum dots with atomic ensembles, one needs to carefully match the optical frequencies of these two platforms. Here, we propose and experimentally demonstrate simple, precise, reversible, broad-range, and local method for controlling the emission frequency of individual quantum dots embedded in tapered semiconductor nanowires and use it to interface with an atomic ensemble via single-photons matched to hyperfine transitions and slow-light regions of the cesium D1-line. Our approach allows linking together atomic and solid-state quantum systems and can potentially also be applied to other types of nanowire-embedded solid-state emitters, as well as to creating devices based on multiple solid-state emitters tuned to produce indistinguishable photons.

Authors:Zi-Yong Ge, Heng Fan, Franco Nori

Abstract: The existence of zero-dimensional superradiant quantum phase transitions seems inconsistent with conventional statistical physics, which has not been explained so far. Here we demonstrate the corresponding effective field theories and finite-temperature properties of light-matter interacting systems, and show how this zero-dimensional quantum phase transition occurs. We first focus on the Rabi model, which is a minimum model that hosts a superradiant quantum phase transition. With the path integral method, we derive the imaginary-time action of the photon degrees of freedom. We also define a dynamical exponent as the rescaling between the temperature and the photon frequency, and perform dimensional analysis to the effective action. Our results show that the effective theory becomes a free scalar field or $\phi^4$-theory for a proper dynamical exponent, where a true second-order quantum phase transition emerges. These results are also verified by numerical simulations of imaginary-time correlation functions of the order parameter. Furthermore, we also generalize this method to the Dicke model. Our results make the zero-dimensional superradiant quantum phase transition compatible with conventional statistical physics, and pave the way to understand it in the perspective of effective field theories.

Authors:Shun Okumura, Masayuki Ohzeki

Abstract: We show the relationship between the Fourier coefficients and the barren plateau problem emerging in parameterized quantum circuits. In particular, the sum of squares of the Fourier coefficients is exponentially restricted concerning the qubits under the barren plateau condition. Throughout theory and numerical experiments, we introduce that this property leads to the vanishing of a probability and an expectation formed by parameterized quantum circuits. The traditional barren plateau problem requires the variance of gradient, whereas our idea does not explicitly need a statistic. Therefore, it is not required to specify the kind of initial probability distribution.

Authors:K. Matsumoto, S. Kagami, T. Fujisaku, A. Kirihara, S. Yanagimachi, T. Ikegami, A. Morinaga

Abstract: Coherent-population-trapping resonance is a quantum interference effect that appears in the two-photon transitions between the ground-state hyperfine levels of alkali atoms and is often utilized in miniature clock devices. To quantitatively understand and predict the performance of this phenomenon, it is necessary to consider the transitions and relaxations between all hyperfine Zeeman sublevels involved in the different excitation processes of the atom. In this study, we constructed a computational multi-level atomic model of the Liouville density-matrix equation for 32 Zeeman sublevels involved in the $D_1$ line of $^{133}$Cs irradiated by two frequencies with circularly polarized components and then simulated the amplitude and shape of the transmitted light through a Cs vapor cell. We show that the numerical solutions of the equation and analytical investigations adequately explain a variety of the characteristics observed in the experiment.

Authors:Mátyás Koniorczyk, Krzysztof Krawiec, Ludmila Botelho, Nikola Bešinović, Krzysztof Domino

Abstract: We address the applicability of quantum-classical hybrid solvers for practical railway dispatching/conflict management problems, with a demonstration on real-life metropolitan-scale network traffic. The railway network includes both single-and double segments and covers all the requirements posed by the operator of the network. We build a linear integer model for the problem and solve it with D-Wave's quantum-classical hybrid solver as well as with CPLEX for comparison. The computational results demonstrate the readiness for application and benefits of quantum-classical hybrid solvers in the a realistic railway scenario: they yield acceptable solutions on time; a critical requirement in a dispatching situation. Though they are heuristic they offer a valid alternative and outperform classical solvers in some cases.

Authors:Tanmoy bera, Vibhor Singh

Abstract: Cavity electromechanical systems, consisting of a mechanical resonator coupled to an electromagnetic mode, are extensively used for sensing of various forces and controlling the vibrations of a mechanical mode down to their quantum limit. In the microwave domain, such devices based on magnetic-flux coupling have emerged as a promising platform with the potential to reach a single-photon strong coupling regime. Here, we demonstrate a flux-coupled electromechanical device using a frequency tunable superconducting transmon qubit, and a microwave cavity. By tuning the qubit in resonance with the cavity, the hybridized state (dressed mode) of the qubit and the cavity mode is used to achieve a magnetic field-dependent electromechanical coupling. It is established by performing an electromagnetically-induced transparency (EIT)-like experiment. At the largest applied field, we estimate the single-photon coupling rate of 60 kHz. Further, in the presence of the pump signal, we observe backaction, showing both cooling and heating of the mechanical mode. With a stronger pump, the dressed mode shows the signature of "super-splitting", and a strong backaction on the mechanical resonator, reflected in the broadening of the mechanical linewidth by a factor of 42 while using less than 1 photon in the dressed mode.

Authors:Eun Mi Kim, Sun Kyung Lee, Sang Min Lee, Myeong Soo Kang, Hee Su Park

Abstract: Quantum interferometry based on induced-coherence phenomena has demonstrated the possibility of undetected-photon measurements. Perturbation in the optical path of probe photons can be detected by interference signals generated by quantum mechanically correlated twin photons propagating through a different path, possibly at a different wavelength. To the best of our knowledge, this work demonstrates for the first time a hybrid-type induced-coherence interferometer that incorporates a Mach-Zehnder-type interferometer for visible photons and a Michelson-type interferometer for infrared photons, based on double-pass pumped spontaneous parametric down-conversion. This configuration enables infrared optical measurements via the detection of near-visible photons and provides methods for characterizing the quality of measurements by identifying photon pairs of different origins. The results verify that the induced-coherence interference visibility is approximately the same as the heralding efficiencies between twin photons along the relevant spatial modes. Applications to both time-domain and frequency-domain quantum-optical induced-coherence tomography for three-dimensional test structures are demonstrated. The results prove the feasibility of practical undetected-photon sensing and imaging techniques based on the presented structure.

Authors:Zhao-Min Gao, Jia-Qi Li, Zi-Wen Li, Wen-Xiao Liu, Xin Wang

Abstract: Giant atoms, where the dipole approximation ceases to be valid, allow us to observe unconventional quantum optical phenomena arising from interference and time-delay effects. Most previous studies consider giant atoms coupling to conventional materials with right-handed dispersion. In this study, we first investigate the quantum dynamics of a giant atom interacting with left-handed superlattice metamaterials. Different from those right-handed counterparts, the left-handed superlattices exhibit an asymmetric band gap generated by anomalous dispersive bands and Bragg scattering bands. First, by assuming that the giant atom is in resonance with the continuous dispersive energy band, spontaneous emission will undergo periodic enhancement or suppression due to the interference effect. At the resonant position, there is a significant discrepancy in the spontaneous decay rates between the upper and lower bands, which arises from the differences in group velocity. Second, we explore the non-Markovian dynamics of the giant atom by considering the frequency of the emitter outside the energy band, where bound states will be induced by the interference between two coupling points. By employing both analytical and numerical methods, we demonstrate that the steady atomic population will be periodically modulated, driven by variations in the size of the giant atom. The presence of asymmetric band edges leads to diverse interference dynamics. Finally, we consider the case of two identical emitters coupling to the waveguide and find that the energy within the two emitters undergoes exchange through the mechanism Rabi oscillations.

Authors:Shun Umekawa, Jaeha Lee, Naomichi Hatano

Abstract: We investigate features of the quasi-joint-probability distribution for finite-state quantum systems, especially the two-state and three-state quantum systems, comparing different types of quasi-joint-probability distributions based on the general framework of quasi-classicalization. We show from two perspectives that the Kirkwood-Dirac distribution is the quasi-joint-probability distribution that behaves nicely for the finite-state quantum systems. One is the similarity to the genuine probability and the other is the information that we can obtain from the quasi-probability. By introducing the concept of the possible values of observables, we show for the finite-state quantum systems that the Kirkwood-Dirac distribution behaves more similarly to the genuine probability distribution in contrast to most of the other quasi-probabilities including the Wigner function. We also prove that the states of the two-state and three-state quantum systems can be completely distinguished by the Kirkwood-Dirac distribution of only two directions of the spin and point out for the two-state system that the imaginary part of the quasi-probability is essential for the distinguishability of the state.

Authors:Danilo Triggiani, Vincenzo Tamma

Abstract: We present a quantum sensing scheme achieving the ultimate quantum sensitivity in the estimation of the transverse displacement between two photons interfering at a balanced beam splitter, based on transverse-momentum sampling measurements at the output. This scheme can possibly lead to enhanced high-precision nanoscopic techniques, such as super-resolved single-molecule localization microscopy with quantum dots, by circumventing the requirements in standard direct imaging of cameras resolution at the diffraction limit, and of highly magnifying objectives. Interestingly, the ultimate spatial precision in nature is achieved irrespectively of the overlap of the two displaced photonic wavepackets. This opens a new research paradigm based on the interface between spatially resolved quantum interference and quantum-enhanced spatial sensitivity.

Authors:Aneirin Baker

Abstract: Parity measurements are central to quantum error correction (QEC). In current implementations measurements of stabilizers are performed using a number of Controlled Not (CNOT) gates. This implementation suffers from an exponential decrease in fidelity as the number of CNOT gates increases thus the stabilizer measurements also suffer a severe decrease in fidelity and increase in gate time. Speeding up and improving the fidelity of this process will improve error rates of these stabilizer measurements thus increasing the coherence times of logical qubits. We propose a single shot method useful for stabilizer readout based on dispersive shifts. We show a possible set up for this method and simulate a 4 qubit system showing that this method is an improvement over the previous CNOT circuit in both fidelity and gate time. We find a fidelity of 99.8% and gate time of 600 ns using our method and investigate the effects of higher order Z interactions on the system.

Authors:J. Mornhinweg Department of Physics, University of Regensburg, Germany Department of Physics, TU Dortmund University, Germany, L. Diebel Department of Physics, University of Regensburg, Germany, M. Halbhuber Department of Physics, University of Regensburg, Germany, M. Prager Department of Physics, University of Regensburg, Germany, J. Riepl Department of Physics, University of Regensburg, Germany, T. Inzenhofer Department of Physics, University of Regensburg, Germany, D. Bougeard Department of Physics, University of Regensburg, Germany, R. Huber Department of Physics, University of Regensburg, Germany, C. Lange Department of Physics, TU Dortmund University, Germany

Abstract: Dressing quantum states of matter with virtual photons can create exotic effects ranging from vacuum-field modified transport to polaritonic chemistry, and may drive strong squeezing or entanglement of light and matter modes. The established paradigm of cavity quantum electrodynamics focuses on resonant light-matter interaction to maximize the coupling strength $\Omega_\mathrm{R}/\omega_\mathrm{c}$, defined as the ratio of the vacuum Rabi frequency and the carrier frequency of light. Yet, the finite oscillator strength of a single electronic excitation sets a natural limit to $\Omega_\mathrm{R}/\omega_\mathrm{c}$. Here, we demonstrate a new regime of record-strong light-matter interaction which exploits the cooperative dipole moments of multiple, highly non-resonant magnetoplasmon modes specifically tailored by our metasurface. This multi-mode coupling creates an ultrabroadband spectrum of over 20 polaritons spanning 6 optical octaves, vacuum ground state populations exceeding 1 virtual excitation quantum for electronic and optical modes, and record coupling strengths equivalent to $\Omega_\mathrm{R}/\omega_\mathrm{c}=3.19$. The extreme interaction drives strongly subcycle exchange of vacuum energy between multiple bosonic modes akin to high-order nonlinearities otherwise reserved to strong-field physics, and entangles previously orthogonal electronic excitations solely via vacuum fluctuations of the common cavity mode. This offers avenues towards tailoring phase transitions by coupling otherwise non-interacting modes, merely by shaping the dielectric environment.

Authors:Shamminuj Aktar, Andreas Bärtschi, Abdel-Hameed A. Badawy, Diane Oyen, Stephan Eidenbenz

Abstract: Parameterized Quantum Circuits (PQCs) are essential to quantum machine learning and optimization algorithms. The expressibility of PQCs, which measures their ability to represent a wide range of quantum states, is a critical factor influencing their efficacy in solving quantum problems. However, the existing technique for computing expressibility relies on statistically estimating it through classical simulations, which requires many samples. In this work, we propose a novel method based on Graph Neural Networks (GNNs) for predicting the expressibility of PQCs. By leveraging the graph-based representation of PQCs, our GNN-based model captures intricate relationships between circuit parameters and their resulting expressibility. We train the GNN model on a comprehensive dataset of PQCs annotated with their expressibility values. Experimental evaluation on a four thousand random PQC dataset and IBM Qiskit's hardware efficient ansatz sets demonstrates the superior performance of our approach, achieving a root mean square error (RMSE) of 0.03 and 0.06, respectively.

Authors:M. Li, Y. N. Wang, Z. Y. Song, Y. M. Zhao, X. L. Zhao, H. Y. Ma

Abstract: As the simplest and most fundamental model describing the interaction between light and matter, a breakdown in the rotating wave approximation leads to phase-transition-like behavior versus coupling strength when the frequency of the qubit greatly surpasses that of the oscillator. We show that the dynamics can reflect the phase transition of the Rabi model. In addition to the excitation of the qubit and bosonic field in the ground state, we show that the witness of inseparability, mutual information, quantum Fisher information, and the variance of cavity quadrature can be employed to detect the phase transition in quench. We also reveal the negative impact of temperature on checking the phase transition by quench. This model can be implemented using trapped ions, where the coupling strength can be flexibly adjusted from weak to ultrastrong regime. By reflecting the phase transition in a fundamental quantum optics model without imposing the thermodynamic limit, we propose a method to explore phase transition in non-equilibrium process.

Authors:Yaohua Li, Yong-Chun Liu

Abstract: Quantum entanglement and classical topology are two distinct phenomena that are difficult to be connected together. Here we discover that an open bosonic quadratic chain exhibits topology-induced entanglement effect. When the system is in the topological phase, the edge modes can be entangled in the steady state, while no entanglement appears in the trivial phase. This finding is verified through the covariance approach based on the quantum master equations, which provide exact numerical results without truncation process. We also obtain concise approximate analytical results through the quantum Langevin equations, which perfectly agree with the exact numerical results. We show the topological edge states exhibit near-zero eigenenergies located in the band gap and are separated from the bulk eigenenergies, which match the system-environment coupling (denoted by the dissipation rate) and thus the squeezing correlations can be enhanced. Our work reveals that the stationary entanglement can be a quantum signature of the topological phase in bosonic systems, and inversely the topological quadratic systems can be powerful platforms to generate robust entanglement.

Authors:Gregory Morse, Tomasz Rybotycki, Ágoston Kaposi, Zoltán Kolarovszki, Uros Stojic, Tamás Kozsik, Oskar Mencer, Michał Oszmaniec, Zoltán Zimborás, Péter Rakyta

Abstract: In this work, we generalize the Balasubramanian-Bax-Franklin-Glynn (BB/FG) permanent formula to account for row multiplicities during the permanent evaluation and reduce the complexity of permanent evaluation in scenarios where such multiplicities occur. This is achieved by incorporating n-ary Gray code ordering of the addends during the evaluation. We implemented the designed algorithm on FPGA-based data-flow engines and utilized the developed accessory to speed up boson sampling simulations up to $40$ photons, by drawing samples from a $60$ mode interferometer at an averaged rate of $\sim80$ seconds per sample utilizing $4$ FPGA chips. We also show that the performance of our BS simulator is in line with the theoretical estimation of Clifford \& Clifford \cite{clifford2020faster} providing a way to define a single parameter to characterize the performance of the BS simulator in a portable way. The developed design can be used to simulate both ideal and lossy boson sampling experiments.

Authors:Jun-Hui Zheng, Arijit Dutta, Monika Aidelsburger, Walter Hofstetter

Abstract: The impact of weak disorder and its spatial correlation on the topology of a Floquet system is not well understood so far. In this study, we investigate a model closely related to a two-dimensional Floquet system that has been realized in experiments. In the absence of disorder, we determine the phase diagram and identify a new phase characterized by edge states with alternating chirality in adjacent gaps. When weak disorder is introduced, we examine the disorder-averaged Bott index and analyze why the anomalous Floquet topological insulator is favored by both uncorrelated and correlated disorder, with the latter having a stronger effect. For a system with a ring-shaped gap, the Born approximation fails to explain the topological phase transition, unlike for a system with a point-like gap.

Authors:Tareq Jaouni, Sören Arlt, Carlos Ruiz-Gonzalez, Ebrahim Karimi, Xuemei Gu, Mario Krenn

Abstract: Despite their promise to facilitate new scientific discoveries, the opaqueness of neural networks presents a challenge in interpreting the logic behind their findings. Here, we use a eXplainable-AI (XAI) technique called $inception$ or $deep$ $dreaming$, which has been invented in machine learning for computer vision. We use this techniques to explore what neural networks learn about quantum optics experiments. Our story begins by training a deep neural networks on the properties of quantum systems. Once trained, we "invert" the neural network -- effectively asking how it imagines a quantum system with a specific property, and how it would continuously modify the quantum system to change a property. We find that the network can shift the initial distribution of properties of the quantum system, and we can conceptualize the learned strategies of the neural network. Interestingly, we find that, in the first layers, the neural network identifies simple properties, while in the deeper ones, it can identify complex quantum structures and even quantum entanglement. This is in reminiscence of long-understood properties known in computer vision, which we now identify in a complex natural science task. Our approach could be useful in a more interpretable way to develop new advanced AI-based scientific discovery techniques in quantum physics.

Authors:Nathan Lacroix, Luca Hofele, Ants Remm, Othmane Benhayoune-Khadraoui, Alexander McDonald, Ross Shillito, Stefania Lazar, Christoph Hellings, Francois Swiadek, Dante Colao-Zanuz, Alexander Flasby, Mohsen Bahrami Panah, Michael Kerschbaum, Graham J. Norris, Alexandre Blais, Andreas Wallraff, Sebastian Krinner

Abstract: Quantum computers will require quantum error correction to reach the low error rates necessary for solving problems that surpass the capabilities of conventional computers. One of the dominant errors limiting the performance of quantum error correction codes across multiple technology platforms is leakage out of the computational subspace arising from the multi-level structure of qubit implementations. Here, we present a resource-efficient universal leakage reduction unit for superconducting qubits using parametric flux modulation. This operation removes leakage down to our measurement accuracy of $7\cdot 10^{-4}$ in approximately $50\, \mathrm{ns}$ with a low error of $2.5(1)\cdot 10^{-3}$ on the computational subspace, thereby reaching durations and fidelities comparable to those of single-qubit gates. We demonstrate that using the leakage reduction unit in repeated weight-two stabilizer measurements reduces the total number of detected errors in a scalable fashion to close to what can be achieved using leakage-rejection methods which do not scale. Our approach does neither require additional control electronics nor on-chip components and is applicable to both auxiliary and data qubits. These benefits make our method particularly attractive for mitigating leakage in large-scale quantum error correction circuits, a crucial requirement for the practical implementation of fault-tolerant quantum computation.

Authors:Jannes Nys, Zakari Denis, Giuseppe Carleo

Abstract: Solving the time-dependent quantum many-body Schr\"odinger equation is a challenging task, especially for states at a finite temperature, where the environment affects the dynamics. Most existing approximating methods are designed to represent static thermal density matrices, 1D systems, and/or zero-temperature states. In this work, we propose a method to study the real-time dynamics of thermal states in two dimensions, based on thermofield dynamics, variational Monte Carlo, and neural-network quantum states. To this aim, we introduce two novel tools: (i) a procedure to accurately simulate the cooling down of arbitrary quantum variational states from infinite temperature, and (ii) a generic thermal (autoregressive) recurrent neural-network (ARNNO) Ansatz that allows for direct sampling from the density matrix using thermofield basis rotations. We apply our technique to the transverse-field Ising model subject to an additional longitudinal field and demonstrate that the time-dependent observables, including correlation operators, can be accurately reproduced for a 4x4 spin lattice. We provide predictions of the real-time dynamics on a 6x6 lattice that lies outside the reach of exact simulations.

Authors:Edoardo Ballini, Giuseppe Clemente, Massimo D'Elia, Lorenzo Maio, Kevin Zambello

Abstract: In this paper, we show the application of the Quantum Metropolis Sampling (QMS) algorithm to a toy gauge theory with discrete non-Abelian gauge group $D_4$ in (2+1)-dimensions, discussing in general how some components of hybrid quantum-classical algorithms should be adapted in the case of gauge theories. In particular, we discuss the construction of random unitary operators which preserve gauge invariance and act transitively on the physical Hilbert space, constituting an ergodic set of quantum Metropolis moves between gauge invariant eigenspaces, and introduce a protocol for gauge invariant measurements. Furthermore, we show how a finite resolution in the energy measurements distorts the energy and plaquette distribution measured via QMS, and propose a heuristic model that takes into account part of the deviations between numerical results and exact analytical results, whose discrepancy tends to vanish by increasing the number of qubits used for the energy measurements.

Authors:Alexander Schnell

Abstract: This work makes progress on the issue of global- vs. local- master equations. Global master equations like the Redfield master equation (following from standard Born- and Markov- approximation) require a full diagonalization of the system Hamiltonian. This is especially challenging for interacting quantum many-body systems. We discuss a short-bath-correlation-time expansion in reciprocal (energy) space, leading to a series expansion of the jump operator, which avoids a diagonalization of the Hamiltonian. For a bath that is coupled locally to one site, this typically leads to an expansion of the global Redfield jump operator in terms of local operators. We additionally map the local Redfield master equation to an approximate Lindblad form, giving an equation which has the same conceptual advantages of traditional local Lindblad approaches, while being applicable in a much broader class of systems. Our ideas give rise to a non-heuristic foundation of local master equations, which can be combined with established many-body methods.

Authors:Debasmita Bhoumik, Ritajit Majumdar, Amit Saha, Susmita Sur-Kolay

Abstract: Quantum computers are noisy at present in the absence of error correction and fault tolerance. Interim methods such as error suppression and mitigation find wide applicability. Another method, which is independent of other error suppression and mitigation, and can be applied in conjunction with them to further lower the noise in the system, is circuit cutting. In this paper, we propose a scheduler that finds the optimum schedule for the subcircuits obtained by circuit cutting on the available set of hardware to (i) maximize the overall fidelity, and (ii) ensure that the predefined maximum execution time for each hardware is not exceeded. The fidelity obtained by this method on various benchmark circuits is significantly better than that of the uncut circuit executed on the least noisy device. The average increase in the fidelity obtained by our method are respectively ~12.3% and ~21% for 10-qubit benchmark circuits without and with measurement error mitigation, even when each hardware was allowed the minimum possible execution time. This noise and time optimized distributed scheduler is an initial step towards providing the optimal performance in the current scenario where the users may have limited access to quantum hardware.

Authors:Wan-Xia Li, Jin-Lei Wu, Shi-Lei Su, Jing Qian

Abstract: Position error is treated as the leading obstacle that prevents Rydberg antiblockade gates from being experimentally realizable, because of the inevitable fluctuations in the relative motion between two atoms invalidating the antiblockade condition. In this work we report progress towards a high-tolerance antiblockade-based Rydberg SWAP gate enabled by the use of modified antiblockade condition combined with carefully-optimized laser pulses. Depending on the optimization of diverse pulse shapes our protocol shows that the time-spent in the double Rydberg state can be shortened by a factor of > 70%, which significantly reduces the position error. Moreover, we benchmark the robustness of the gate via taking account of the technical noises, such as the Doppler dephasing due to atomic thermal motion, the fluctuations in laser intensity and laser phase and the intensity inhomogeneity. As compared with other existing antiblockade-gate schemes the predicted gate fidelity is able to maintain at above 0.91 after a very conservative estimation of various experimental imperfections,ns, especially considered for realistic interaction deviation of $\delta$V /V $\approx$ 5.92% at T $\sim$ 20$\mu$K. Our work paves the way to the experimental demonstration of Rydberg antiblockade gates in the near future.

Authors:Mehdi Aslani, Vahid Salari, Mehdi Abdi

Abstract: A shortcut to adiabatic scheme is proposed for preparing a massive object in a macroscopic spatial superposition state. In this scheme we propose to employ counterdiabatic driving to maintain the system in the groundstate of its instantaneous Hamiltonian while the trap potential is tuned from a parabola to a double well. This, in turn, is performed by properly ramping a control parameter. We show that a few counterdiabatic drives are enough for most practical cases. A hybrid electromechanical setup in superconducting circuits is proposed for the implementation. The efficiency of our scheme is benchmarked by numerically solving the system dynamics in the presence of noises and imperfections. The results show that very high fidelity cat states with distinguishable spatial separations can be prepared with our protocol. Furthermore, the protocol is robust against noises and imperfections. We also discuss a method for verifying the final state via spectroscopy of a coupled circuit electrodynamical cavity mode.

Authors:Jukka Kiukas, Pekka Lahti, Juha-Pekka Pellonpää

Abstract: We study joint measurability of quantum observables in open systems governed by a master equation of Lindblad form. We briefly review the historical perspective of open systems and conceptual aspects of quantum measurements, focusing subsequently on describing emergent classicality under quantum decoherence. While decoherence in quantum states has been studied extensively in the past, the measurement side is much less understood - here we present and extend some recent results on this topic.

Authors:Jai Lalita, Subhashish Banerjee

Abstract: The weak measurement (WM) and quantum measurement reversal (QMR) are crucial in protecting the collapse of quantum states. Recently, the idea of WM and QMR has been used to protect and enhance quantum correlations and universal quantum teleportation (UQT) protocol. Here, we study the quantum correlations, maximal fidelity, and fidelity deviation of the two-qubit negative quantum states developed using discrete Wigner functions (DWFs) with (without) WM and QMR. To take into account the effect of a noisy environment, we evolve the states via non-Markovian amplitude damping (AD) and random telegraph noise (RTN) quantum channels. To benchmark the performance of negative quantum states, we compare our results with the two-qubit maximally entangled Bell state. Interestingly, we observe that some of the negative quantum states perform better with WM and QMR than the Bell state for different cases under evolution via noisy quantum channels.

Authors:László B. Szabados

Abstract: Penrose's Spin Geometry Theorem is extended further, from $SU(2)$ and $E(3)$ (Euclidean) to $E(1,3)$ (Poincar\'e) invariant elementary quantum mechanical systems. The Lorentzian spatial distance between any two non-parallel timelike straight lines of Minkowski space, considered to be the centre-of-mass world lines of $E(1,3)$-invariant elementary classical mechanical systems with positive rest mass, is expressed in terms of \emph{$E(1,3)$-invariant basic observables}, viz. the 4-momentum and the Pauli--Lubanski spin vectors of the systems. An analogous expression for \emph{$E(1,3)$-invariant elementary quantum mechanical systems} in terms of the \emph{basic quantum observables} in an abstract, algebraic formulation of quantum mechanics is given, and it is shown that, in the classical limit, it reproduces the Lorentzian spatial distance between the timelike straight lines of Minkowski space with asymptotically vanishing uncertainty. Thus, the \emph{metric structure} of Minkowski space can be recovered from quantum mechanics in the classical limit using only the observables of abstract quantum systems.

Authors:Qi Zhang

Abstract: For a linear non-Hermitian system, I demonstrate that a Hamiltonian can be constructed such that the non-Hermitian equations can be expressed exactly in the form of Hamilton's canonical equations. This is first shown for discrete systems and then extended to continuous systems. With this Hamiltonian formulation, I am able to identify a conserved charge by applying Noether's theorem and recognize adiabatic invariants. When applied to Hermitian systems, all the results reduce to the familiar ones associated with the Schr\"odinger equation.

Authors:J. H. Busnaina, Z. Shi, A. McDonald, D. Dubyna, I. Nsanzineza, Jimmy S. C. Hung, C. W. Sandbo Chang, A. A. Clerk, C. M. Wilson

Abstract: Superconducting quantum circuits are a natural platform for quantum simulations of a wide variety of important lattice models describing topological phenomena, spanning condensed matter and high-energy physics. One such model is the bosonic analogue of the well-known fermionic Kitaev chain, a 1D tight-binding model with both nearest-neighbor hopping and pairing terms. Despite being fully Hermitian, the bosonic Kitaev chain exhibits a number of striking features associated with non-Hermitian systems, including chiral transport and a dramatic sensitivity to boundary conditions known as the non-Hermitian skin effect. Here, using a multimode superconducting parametric cavity, we implement the bosonic Kitaev chain in synthetic dimensions. The lattice sites are mapped to frequency modes of the cavity, and the $\textit{in situ}$ tunable complex hopping and pairing terms are created by parametric pumping at the mode-difference and mode-sum frequencies, respectively. We experimentally demonstrate important precursors of nontrivial topology and the non-Hermitian skin effect in the bosonic Kitaev chain, including chiral transport, quadrature wavefunction localization, and sensitivity to boundary conditions. Our experiment is an important first step towards exploring genuine many-body non-Hermitian quantum dynamics.

Authors:Chung-Yun Hsieh, Huan-Yu Ku, Costantino Budroni

Abstract: Steering resources, central for quantum advantages in one-sided device-independent quantum information tasks, can be enhanced via local filters. Recently, reversible steering conversion under local filters has been fully characterised. Here, we solve the problem in the irreversible scenario, which leads to a complete understanding of stochastic steering distillation. This result also provides an operational interpretation of the max-relative entropy as the optimal filter success probability. We further show that all steering measures can be used to quantify measurement incompatibility in certain stochastic steering distillation scenarios. Finally, for a broad class of steering robustness measures, we show that their maximally achievable values in stochastic steering distillation are always upper bounded by different types of incompatibility robustness measures. Hence, measurement incompatibility sets the fundamental limitations for stochastic steering distillation.

Authors:Simone Magaletti, Ludovic Mayer, Jean-François Roch, Thierry Debuisschert

Abstract: In this paper we study the dynamics of an ensemble of nitrogen-vacancy centers in diamond when its photoluminescence is detected by means of a widefield imaging system. We develop a seven-level model and use it to simulate the widefield detection of nitrogen-vacancy centers Rabi oscillations. The simulation results are compared with experimental measurements showing a good agreement. In particular, we use the model to explain the asymmetric shape of the detected Rabi oscillations due to an incomplete repolarization of the nitrogen-vacancy center during the pulse sequence implemented for the detection of Rabi oscillations.

Authors:Yi Zuo, Qinghong Yang, Banggui Liu, Dong E Liu

Abstract: Keldysh field theory, based on adiabatic assumptions, serves as an widely used framework for addressing nonequilibrium many-body systems. Nonetheless, the validity of such adiabatic assumptions when addressing interacting Gibbs states remains a topic of contention. We use the knowledge of work statistics developed in nonequilibrium thermodynamics to study this problem. Consequently, we deduce a universal theorem delineating the characteristics of evolutions that transition an initial Gibbs state to another. Based on this theorem, we analytically ascertain that adiabatic evolutions fail to transition a non-interacting Gibbs state to its interacting counterpart. However, this adiabatic approach remains a superior approximation relative to its non-adiabatic counterpart. Numerics verifying our theory and predictions are also provided. Furthermore, our findings render insights into the preparation of Gibbs states within the domain of quantum computation.

Authors:X. Gutiérrez de la Cal, M. Pons, D. Sokolovksi

Abstract: Eisenbud-Wigner-Smith delay and the Larmor time give different estimates for the duration of a quantum scattering event. The difference is most pronounced in the case where de-Broglie wavelength is large compared to the size of the scatterer. We use the methods of quantum measurement theory to analyse both approaches, and to decide which one of them, if any, describes the duration a particle spends in the region which contains the scattering potential. The cases of transmission, reflection and three-dimensional elastic scattering are discussed in some detail.

Authors:Adrià Labay-Mora, Roberta Zambrini, Gian Luca Giorgi

Abstract: Quantum oscillators with nonlinear driving and dissipative terms have gained significant attention due to their ability to stabilize cat-states for universal quantum computation. Recently, superconducting circuits have been employed to realize such long-lived qubits stored in coherent states. We present a generalization of these oscillators, which are not limited to coherent states, in the presence of different nonlinearities in driving and dissipation, exploring different degrees. Specifically, we present an extensive analysis of the asymptotic dynamical features and of the storage of squeezed states. We demonstrate that coherent superpositions of squeezed states are achievable in the presence of a strong symmetry, thereby allowing for the storage of squeezed cat-states. In the weak symmetry regime, accounting for linear dissipation, we investigate the potential application of these nonlinear driven-dissipative resonators for quantum computing and quantum associative memory and analyze the impact of squeezing on their performance.

Authors:Sharareh Sayyad, Jose L. Lado

Abstract: Identifying phase boundaries of interacting systems is one of the key steps to understanding quantum many-body models. The development of various numerical and analytical methods has allowed exploring the phase diagrams of many Hermitian interacting systems. However, numerical challenges and scarcity of analytical solutions hinder obtaining phase boundaries in non-Hermitian many-body models. Recent machine learning methods have emerged as a potential strategy to learn phase boundaries from various observables without having access to the full many-body wavefunction. Here, we show that a machine learning methodology trained solely on Hermitian correlation functions allows identifying phase boundaries of non-Hermitian interacting models. These results demonstrate that Hermitian machine learning algorithms can be redeployed to non-Hermitian models without requiring further training to reveal non-Hermitian phase diagrams. Our findings establish transfer learning as a versatile strategy to leverage Hermitian physics to machine learning non-Hermitian phenomena.

Authors:Yuan Liu, Ho Yiu Chung, Ravishankar Ramanathan

Abstract: The set of correlations between measurement outcomes observed by separated parties in a Bell test is of vital importance in Device-Independent (DI) information processing. However, characterising this set of quantum correlations is a hard problem, with a number of open questions. Here, we present families of quantum Bell inequalities that approximate this set in Bell scenarios with an arbitrary number of players, settings and outcomes, and study their applications to device-independent information processing. Firstly, it is known that quantum correlations on the non-signaling boundary are of crucial importance in the task of DI randomness extraction from weak sources. In the Bell scenario of two players with two $k$-outcome measurements, we derive inequalities that show a separation of the quantum boundary from classes of non-local faces of the non-signaling polytope of dimension $\leq 4k-4$, extending previous results from nonlocality distillation and the collapse of communication complexity. Secondly, in the scenario of two players with $m$ binary measurements, we consider a non-trivial portion of the quantum boundary that generalizes the boundary that for $m=2$ discovered by Tsirelson-Landau-Masanes. We prove that all points on this generalized boundary serve to self-test the two-qubit singlet and the corresponding $m$ measurements. In this scenario, we also derive a low-dimensional region of the quantum boundary that coincides with the boundary of the set of classical correlations. Finally, we conclude our investigation of the quantum boundary by answering the open quantum whether there exists a bipartite $(3,3)$-inputs, $(2,3)$-outputs pseudo-telepathy game in the negative.

Authors:Zhirui Hu, Robert Wolle, Mingzhen Tian, Qiang Guan, Travis Humble, Weiwen Jiang

Abstract: In the near-term noisy intermediate-scale quantum (NISQ) era, high noise will significantly reduce the fidelity of quantum computing. Besides, the noise on quantum devices is not stable. This leads to a challenging problem: At run-time, is there a way to efficiently achieve a consistent high-fidelity quantum system on unstable devices? To study this problem, we take quantum learning (a.k.a., variational quantum algorithm) as a vehicle, such as combinatorial optimization and machine learning. A straightforward approach is to optimize a Circuit with a parameter-shift approach on the target quantum device before using it; however, the optimization has an extremely high time cost, which is not practical at run-time. To address the pressing issue, in this paper, we proposed a novel quantum pulse-based noise adaptation framework, namely QuPAD. In the proposed framework, first, we identify that the CNOT gate is the fidelity bottleneck of the conventional VQC, and we employ a more robust parameterized multi-quit gate (i.e., Rzx gate) to replace the CNOT gate. Second, by benchmarking the Rzx gate with different parameters, we build a fitting function for each coupling qubit pair, such that the deviation between the theoretic output of the Rzx gate and its on-device output under a given pulse amplitude and duration can be efficiently predicted. On top of this, an evolutionary algorithm is devised to identify the pulse amplitude and duration of each Rzx gate (i.e., calibration) and find the quantum circuits with high fidelity. Experiments show that the runtime on quantum devices of QuPAD with 8-10 qubits is less than 15 minutes, which is up to 270x faster than the parameter-shift approach. In addition, compared to the vanilla VQC as a baseline, QuPAD can achieve 59.33% accuracy gain on a classification task, and average 66.34% closer to ground state energy for molecular simulation.

Authors:Renjan Rajan John, Krishna Priya R

Abstract: We employ the recently developed technique of null bootstrap to obtain the energy eigenvalues and the ladder operators of the sextic anharmonic oscillator up to second order in the coupling. We confirm our results by deriving the same from traditional perturbation theory. We further extend the analysis to non-Hermitian PT symmetric Hamiltonians, focusing on the shifted harmonic oscillator and the cubic theory.

Authors:Xiang Cheng, Kai-Chi Chang, Murat Can Sarihan, Andrew Mueller, Maria Spiropulu, Matthew D. Shaw, Boris Korzh, Andrei Faraon, Franco N. C. Wong, Jeffrey H. Shapiro, Chee Wei Wong

Abstract: High-dimensional quantum entanglement is a cornerstone for advanced technology enabling large-scale noise-tolerant quantum systems, fault-tolerant quantum computing, and distributed quantum networks. The recently developed biphoton frequency comb (BFC) provides a powerful platform for high-dimensional quantum information processing in its spectral and temporal quantum modes. Here we propose and generate a singly-filtered high-dimensional BFC via spontaneous parametric down-conversion by spectrally shaping only the signal photons with a Fabry-Perot cavity. High-dimensional energy-time entanglement is verified through Franson-interference recurrences and temporal correlation with low-jitter detectors. Frequency- and temporal- entanglement of our singly-filtered BFC is then quantified by Schmidt mode decomposition. Subsequently, we distribute the high-dimensional singly-filtered BFC state over a 10 km fiber link with a post-distribution time-bin dimension lower bounded to be at least 168. Our demonstrations of high-dimensional entanglement and entanglement distribution show the capability of the singly-filtered quantum frequency comb for high-efficiency quantum information processing and high-capacity quantum networks.

Authors:Rahul Arvind, Kishor Bharti, Jun Yong Khoo, Dax Enshan Koh, Jian Feng Kong

Abstract: We explore the interplay between symmetry and randomness in quantum information. Adopting a geometric approach, we consider states as $H$-equivalent if related by a symmetry transformation characterized by the group $H$. We then introduce the Haar measure on the homogeneous space $\mathbb{U}/H$, characterizing true randomness for $H$-equivalent systems. While this mathematical machinery is well-studied by mathematicians, it has seen limited application in quantum information: we believe our work to be the first instance of utilizing homogeneous spaces to characterize symmetry in quantum information. This is followed by a discussion of approximations of true randomness, commencing with $t$-wise independent approximations and defining $t$-designs on $\mathbb{U}/H$ and $H$-equivalent states. Transitioning further, we explore pseudorandomness, defining pseudorandom unitaries and states within homogeneous spaces. Finally, as a practical demonstration of our findings, we study the expressibility of quantum machine learning ansatze in homogeneous spaces. Our work provides a fresh perspective on the relationship between randomness and symmetry in the quantum world.

Authors:Alejandro Mata Ali, Iñigo Perez Delgado, Marina Ristol Roura, Aitor Moreno Fdez. de Leceta, Sebastián V. Romero

Abstract: We present an algorithm for solving systems of linear equations based on the HHL algorithm with a novel qudits methodology, a generalization of the qubits with more states, to reduce the number of gates to be applied and the amount of resources. Based on this idea, we will perform a quantum-inspired version on tensor networks, taking advantage of their ability to perform non-unitary operations such as projection. Finally, we will use this algorithm to obtain a solution for the harmonic oscillator with an external force, the forced damped oscillator and the 2D static heat equation differential equations.

Authors:Abhishek Yadav, Sandeep Mishra, Anirban Pathak

Abstract: Random numbers form an intrinsic part of modern day computing with applications in a wide variety of fields. But due to their limitations, the use of pseudo random number generators (PRNGs) is certainly not desirable for sensitive applications. Quantum systems due to their intrinsic randomness form a suitable candidate for generation of true random numbers that can also be certified. In this work, the violation of CHSH inequality has been used to propose a scheme by which one can generate device independent quantum random numbers by use of IBM quantum computers that are available on the cloud. The generated random numbers have been tested for their source of origin through experiments based on the testing of CHSH inequality through available IBM quantum computers. The performance of each quantum computer against the CHSH test has been plotted and characterized. Further, efforts have been made to close as many loopholes as possible to produce device independent quantum random number generators. This study will provide new directions for the development of self-testing and semi-self-testing random number generators using quantum computers.

Authors:Xi Cao, Jiangyu Cui, Man Hong Yung, Re-Bing Wu

Abstract: Fastness and robustness are both critical in the implementation of high-fidelity gates for quantum computation, but in practice, a trade-off has to be made between them. In this paper, we investigate the underlying robust time-optimal control problem so as to make the best balance. Based on the Taylor expansion of the system's unitary propagator, we formulate the design problem as the optimal control of an augmented finite-dimensional system at its quantum speed limit (QSL), where the robustness is graded by the degree of series truncation. The gradient-descent algorithm is then introduced to sequentially seek QSLs corresponding to different orders of robustness. Numerical simulations for single-qubit systems show that the obtained time-optimal control pulses can effectively suppress gate errors (to the prescribed robustness order) caused by qubit frequency and field amplitude uncertainties. These results provide a practical guide for selecting pulse lengths in the pulse-level compilation of quantum circuits.

Authors:Youssef Trifa, Tommaso Roscilde

Abstract: Controlling the quantum many-body state of arrays of qudits, possessing a large local Hilbert space, opens the path to a broad range of possibilities for many-particle entanglement, interesting both for fundamental quantum science, as well as for potential metrological applications. In this work we theoretically show that the spin-spin interactions realized in two-dimensional Mott insulators of large-spin magnetic atoms (such as Cr, Er or Dy) lead to scalable spin squeezing along the non-equilibrium unitary evolution initialized in a coherent spin state. An experimentally relevant perturbation to the collective squeezing dynamics is offered by a quadratic Zeeman shift, which leads instead to squeezing of individual spins. Making use of a truncated cumulant expansion for the quantum fluctuations of the spin array, we show that, for sufficiently small quadratic shifts, the spin squeezing dynamics is akin to that produced by the paradigmatic one-axis-twisting (OAT) model -- as expected from an effective separation between collective spin and spin-wave variables. Spin squeezing with OAT-like scaling is shown to be protected by the robustness of long-range ferromagnetic order to quadratic shifts in the equilibrium phase diagram of the system, that we reconstruct via quantum Monte Carlo and mean-field theory.

Authors:Chan Roh, Geunhee Gwak, Young-Do Yoon, Young-Sik Ra

Abstract: Measurement-based quantum computing is a promising paradigm of quantum computation, where universal computing is achieved through a sequence of local measurements. The backbone of this approach is the preparation of multipartite entanglement, known as cluster states. While a cluster state with two-dimensional (2D) connectivity is required for universality, a three-dimensional (3D) cluster state is necessary for additionally achieving fault tolerance. However, the challenge of making 3D connectivity has limited cluster state generation up to 2D. Here we experimentally generate a 3D cluster state in the continuous-variable optical platform. To realize 3D connectivity, we harness a crucial advantage of time-frequency modes of ultrafast quantum light: an arbitrary complex mode basis can be accessed directly, enabling connectivity as desired. We demonstrate the versatility of our method by generating cluster states with 1D, 2D, and 3D connectivities. For their complete characterization, we develop a quantum state tomography method for multimode Gaussian states. Moreover, we verify the cluster state generation by nullifier measurements, as well as full inseparability and steering tests. Finally, we highlight the usefulness of 3D cluster state by demonstrating quantum error detection in topological quantum computation. Our work paves the way toward fault-tolerant and universal measurement-based quantum computing.

Authors:Kourosh Sayar Dogahe, Tamara Sarac, Delphine De Smedt, Koen Bertels

Abstract: The article summarizes the study performed in the context of the Deloitte Quantum Climate Challenge in 2023. We present a hybrid quantum-classical method for calculating Potential Energy Surface scans, which are essential for designing Metal-Organic Frameworks for Direct Air Capture applications. The primary objective of this challenge was to highlight the potential advantages of employing quantum computing. To evaluate the performance of the model, we conducted total energy calculations using various computing frameworks and methods. The results demonstrate, at a small scale, the potential advantage of quantum computing-based models. We aimed to define relevant classical computing model references for method benchmarking. The most important benefits of using the PISQ approach for hybrid quantum-classical computational model development and assessment are demonstrated.

Authors:Hilario Espinós, Loris Maria Cangemi, Amikam Levy, Ricardo Puebla, Erik Torrontegui

Abstract: Quantum many-body systems are emerging as key elements in the quest for quantum-based technologies and in the study of fundamental physics. In this context, finding control protocols that allow for fast and high fidelity evolutions across quantum phase transitions is of particular interest. Ideally, such controls should be scalable with the system size and not require controllable and unwanted extra interactions. In addition, its performance should be robust against potential imperfections. Here we design an invariant-based control technique that ensures perfect adiabatic-like evolution in the lowest energy subspace of the many-body system, and is able to meet all these requirements -- tuning the controllable parameter according to the analytical control results in high-fidelity evolutions operating close to the speed limit, valid for any number particles. As such, Kibble-Zurek scaling laws break down, leading to tunable and much better time scaling behavior. We illustrate our findings by means of detailed numerical simulations in the transverse-field Ising and long-range Kitaev models and demonstrate the robustness against noisy controls and disorder.

Authors:Jayakrishnan M. P. Nair, Marlan O. Scully, Girish S. Agarwal

Abstract: Non-Hermitian topological phenomena have gained much interest among physicists in recent years. In this paper, we expound on the physics of dissipatively coupled Su-Schrieffer-Heeger (SSH) lattices, specifically in systems with bosonic and electrical constituents. In the context of electrical circuits, we demonstrate that a series of resistively coupled LCR circuits mimics the topology of a dissipatively coupled SSH model. In addition, we foreground a scheme to construct dissipatively coupled SSH lattices involving a set of non-interacting bosonic oscillators weakly coupled to engineered reservoirs of modes possessing substantially small lifetimes when compared to other system timescales. Further, by activating the coherent coupling between bosonic oscillators, we elucidate the emergence of non-reciprocal dissipative coupling which can be controlled by the phase of the coherent interaction strength precipitating in phase-dependent topological transitions and skin effect. Our analyses are generic, apropos of a large class of systems involving, for instance, optical and microwave settings, while the circuit implementation represents the most straightforward of them.

Authors:David Bucher, Jonas Nüßlein, Corey O'Meara, Ivan Angelov, Benedikt Wimmer, Kumar Ghosh, Giorgio Cortiana, Claudia Linnhoff-Popien

Abstract: Demand Side Response (DSR) is a strategy that enables consumers to actively participate in managing electricity demand. It aims to alleviate strain on the grid during high demand and promote a more balanced and efficient use of electricity resources. We implement DSR through discount scheduling, which involves offering discrete price incentives to consumers to adjust their electricity consumption patterns. Since we tailor the discounts to individual customers' consumption, the Discount Scheduling Problem (DSP) becomes a large combinatorial optimization task. Consequently, we adopt a hybrid quantum computing approach, using D-Wave's Leap Hybrid Cloud. We observe an indication that Leap performs better compared to Gurobi, a classical general-purpose optimizer, in our test setup. Furthermore, we propose a specialized decomposition algorithm for the DSP that significantly reduces the problem size, while maintaining an exceptional solution quality. We use a mix of synthetic data, generated based on real-world data, and real data to benchmark the performance of the different approaches.

Authors:Colin Benjamin, Aditya Dash

Abstract: A quantum many-body state built on a classical 1D Ising model with locally entangled qubits is considered. This setup can model an infinite-player quantum Prisoner's dilemma game with each site representing two entangled players (or qubits). The local entanglement $\gamma$ between two qubits placed on a site in the 1D Ising model and classical coupling between adjacent sites of the Ising model has an apposite influence on qubits. It points to a counter-intuitive situation wherein local entanglement at a site can exactly cancel global correlations, signaling an artificial quantum many-body state wherein, by locally tuning the entanglement at a particular site, one can transition from a strongly correlated quantum state to an uncorrelated quantum state and then to a correlated classical state. In other words, we can simulate a state similar to a Type II superconducting state via local tuning of entanglement in a 1D Ising chain with entangled qubits.

Authors:Adam Wills, Ting-Chun Lin, Min-Hsiu Hsieh

Abstract: In this work, we continue the search for quantum locally testable codes (qLTCs) of new parameters by presenting three constructions that can make new qLTCs from old. The first analyses the soundness of a quantum code under Hastings' weight reduction construction for qLDPC codes arXiv:2102.10030 to give a weight reduction procedure for qLTCs. Secondly, we exhibit the `identity product': the first product construction that is known to preserve both the soundness and locality of the inputted quantum code. This can be used to grow the dimension of a quantum code, where now the tradeoff is put onto the distance. Finally, we apply the AEL distance amplification construction to the case of qLTCs for the first time which could, in future, be used to convert high-distance qLTCs into linear distance qLTCs. These constructions can be used on as-yet undiscovered qLTCs to obtain new parameters, but we are able to apply these presently to the hypersphere product code arXiv:1608.05089 and the hemicubic code arXiv:1911.03069 to obtain many previously unknown parameters. In particular, the only previously known codes to have inverse polylogarithmic soundness, polynomial distance and polynomial dimension have polynomial locality. We obtain such codes with constant locality.

Authors:Ben Barber, Kenton M. Barnes, Tomasz Bialas, Okan Buğdaycı, Earl T. Campbell, Neil I. Gillespie, Kauser Johar, Ram Rajan, Adam W. Richardson, Luka Skoric, Canberk Topal, Mark L. Turner, Abbas B. Ziad

Abstract: Quantum computers promise to solve computing problems that are currently intractable using traditional approaches. This can only be achieved if the noise inevitably present in quantum computers can be efficiently managed at scale. A key component in this process is a classical decoder, which diagnoses the errors occurring in the system. If the decoder does not operate fast enough, an exponential slowdown in the logical clock rate of the quantum computer occurs. Additionally, the decoder must be resource efficient to enable scaling to larger systems and potentially operate in cryogenic environments. Here we introduce the Collision Clustering decoder, which overcomes both challenges. We implement our decoder on both an FPGA and ASIC, the latter ultimately being necessary for any cost-effective scalable solution. We simulate a logical memory experiment on large instances of the leading quantum error correction scheme, the surface code, assuming a circuit-level noise model. The FPGA decoding frequency is above a megahertz, a stringent requirement on decoders needed for e.g. superconducting quantum computers. To decode an 881 qubit surface code it uses only $4.5\%$ of the available logical computation elements. The ASIC decoding frequency is also above a megahertz on a 1057 qubit surface code, and occupies 0.06 mm$^2$ area and consumes 8 mW of power. Our decoder is optimised to be both highly performant and resource efficient, while its implementation on hardware constitutes a viable path to practically realising fault-tolerant quantum computers.

Authors:Salvatore Sinno, Thomas Groß, Alan Mott, Arati Sahoo, Deepak Honnalli, Shruthi Thuravakkath, Bhavika Bhalgamiya

Abstract: Quantum annealing (QA) is a heuristic search algorithm that can run on Adiabatic Quantum Computation (AQC) processors to solve combinatorial optimization problems. Although theoretical studies and simulations on classic hardware have shown encouraging results, these analyses often assume that the computation occurs in adiabatically closed systems without environmental interference. This is not a realistic assumption for real systems; therefore, without extensive empirical measurements on real quantum platforms, theory-based predictions, simulations on classical hardware or limited tests do not accurately assess the current commercial capabilities. This study has assessed the quality of the solution provided by a commercial quantum annealing platform compared to known solutions for the Capacitated Vehicle Routing Problem (CVRP). The study has conducted extensive analysis over more than 30 hours of access to QA commercial platforms to investigate how the size of the problem and its complexity impact the solution accuracy and the time used to find a solution. Our results have found that the absolute error is between 0.12 and 0.55, and the quantum processor unit (QPU) time is between 30 and 46 micro seconds. Our results show that as the constraint density increases, the quality of the solution degrades. Therefore, more than the problem size, the model complexity plays a critical role, and practical applications should select formulations that minimize the constraint density.

Authors:Jakob Bätge, Yu Wang, Amikam Levy, Wenjie Dou, Michael Thoss

Abstract: Periodic driving and Floquet engineering have emerged as invaluable tools for controlling and uncovering novel phenomena in quantum systems. In this study, we adopt these methods to manipulate nonequilibrium processes within electronic-vibronic open quantum systems. Through resonance mechanisms and by focusing on the limit-cycle dynamics and quantum thermodynamic properties, we illustrate the intricate interplay between the driving field and vibronic states and its overall influence on the electronic system. Specifically, we observe an effective decoupling of the electronic system from the periodic driving at specific frequencies, a phenomenon that is mediated by the vibrational mode interaction. Additionally, we engineer the driving field to obtain a partial removal of the Franck-Condon blockade. These insights hold promise for efficient charge current control. Our results are obtained from numerically exact calculations of the hierarchical equations of motion and further analyzed by a time-periodic master equation approach.

Authors:Han Xu, Benran Wang, Haidong Yuan, Xin Wang

Abstract: Quantum hypothesis testing plays a pivotal role in quantum technologies, making decisions or drawing conclusions about quantum systems based on observed data. Recently, quantum control techniques have been successfully applied to quantum hypothesis testing, enabling the reduction of error probabilities in the task of distinguishing magnetic fields in presence of environmental noise. In real-world physical systems, such control is prone to various channels of inaccuracies. Therefore improving the robustness of quantum control in the context of quantum hypothesis testing is crucial. In this work, we utilize optimal control methods to compare scenarios with and without accounting for the effects of signal frequency inaccuracies. For parallel dephasing and spontaneous emission, the optimal control inherently demonstrates a certain level of robustness, while in the case of transverse dephasing with an imperfect signal, it may result in a higher error probability compared to the uncontrolled scheme. To overcome these limitations, we introduce a robust control approach optimized for a range of signal noise, demonstrating superior robustness beyond the predefined tolerance window. On average, both the optimal control and robust control show improvements over the uncontrolled schemes for various dephasing or decay rates, with the robust control yielding the lowest error probability.

Authors:Simone Chicco, Giuseppe Allodi, Alessandro Chiesa, Elena Garlatti, Christian D. Buch, Paolo Santini, Roberto De Renzi, Stergios Piligkos, Stefano Carretta

Abstract: The use of $d$-level qudits instead of two-level qubits can largely increase the power of quantum logic for many applications, ranging from quantum simulations to quantum error correction. Molecular Nanomagnets are ideal spin systems to realize these large-dimensional qudits. Indeed, their Hamiltonian can be engineered to an unparalleled extent and can yield a spectrum with many low-energy states. In particular, in the last decade intense theoretical, experimental and synthesis efforts have been devoted to develop quantum simulators based on Molecular Nanomagnets. However, this remarkable potential is practically unexpressed, because no quantum simulation has ever been experimentally demonstrated with these systems. Here we show the first prototype quantum simulator based on an ensemble of molecular qudits and a radiofrequency broadband spectrometer. To demonstrate the operativity of the device, we have simulated quantum tunneling of the magnetization and the transverse-field Ising model, representative of two different classes of problems. These results represent an important step towards the actual use of molecular spin qudits in quantum technologies.

Authors:Yichen Huang

Abstract: A Floquet quantum system is governed by a Hamiltonian that is periodic in time. Consider the space of piecewise time-independent Floquet systems with (geometrically) local interactions. We prove that for all but a measure zero set of systems in this space, starting from a random product state, many properties (including expectation values of observables and the entanglement entropy of a macroscopically large subsystem) at long times approximately oscillate (with possibly zero amplitude) at the same frequency as the Hamiltonian. Thus, in almost every Floquet system of arbitrarily large but finite size, discrete time-crystalline behavior does not persist to strictly infinite time.

Authors:Youning Li, Chao Zhang, Shi-Yao Hou, Zipeng Wu, Xuanran Zhu, Bei Zeng

Abstract: Symmetric extensions are essential in quantum mechanics, providing a lens to investigate the correlations of entangled quantum systems and to address challenges like the quantum marginal problem. Though semi-definite programming (SDP) is a recognized method for handling symmetric extensions, it grapples with computational constraints, especially due to the large real parameters in generalized qudit systems. In this study, we introduce an approach that adeptly leverages permutation symmetry. By fine-tuning the SDP problem for detecting \( k \)-symmetric extensions, our method markedly diminishes the searching space dimensionality and trims the number of parameters essential for positive definiteness tests. This leads to an algorithmic enhancement, reducing the complexity from \( O(d^{2k}) \) to \( O(k^{d^2}) \) in the qudit \( k \)-symmetric extension scenario. Additionally, our approach streamlines the process of verifying the positive definiteness of the results. These advancements pave the way for deeper insights into quantum correlations, highlighting potential avenues for refined research and innovations in quantum information theory.

Authors:Adam Kinos, Andreas Walther, Stefan Kröll, Lars Rippe

Abstract: We present a quantum repeater protocol for distributing entanglement over long distances, where each repeater node contains several qubits that can couple to one single-photon emitter. Photons from the emitters perform heralded entanglement generation between qubits in neighboring nodes. The protocol leaves the emitters disentangled from the qubits and photons, thus allowing them to be reused to entangle other qubits. The protocol can therefore be time multiplexed, which increases the rate of generated EPR pairs. Deterministic entanglement swapping and heralded entanglement purification are used to extend the distance of the entanglement and reduce the error of the entangled qubits, respectively. We perform a complete protocol analysis by considering all relevant error sources, such as initialization, two-qubit gate, and qubit measurement errors, as well as the exponential decoherence of the qubits with time. The latter is particularly important since we analyze the protocol performance for a broad range of experimental parameters and obtain secret key rates ranging from $1 \rightarrow 1000$ Hz at a distance of $1000$ km. Our results suggest that it is important to reach a qubit memory coherence time of around one second, and two-qubit gate and measurement errors in the order of $10^{-3}$ to obtain reasonable secret key rates over distances longer than achievable with direct transmission.

Authors:Philipp Strasberg

Abstract: Recently, Elouard and Lombard Latune [PRX Quantum 4, 020309 (2023)] claimed to extend the laws of thermodynamics to "arbitrary quantum systems" valid "at any scale" using "consistent" definitions allowing them to "recover known results" from the literature. I show that their definitions are in conflict with textbook thermodynamics and over- or underestimate the real entropy production by orders of magnitude. The cause of this problem is traced back to problematic definitions of entropy and temperature, the latter, for instance, violates the zeroth law. It is pointed out that another framework presented in PRX Quantum 2, 030202 (2021) does not suffer from these problems, while Elouard and Lombard Latune falsely claim that it only provides a positive entropy production for a smaller class of initial states. A simple way to unify both approaches is also presented.

Authors:Nicolò Piccione, Benedetto Militello, Anna Napoli, Bruno Bellomo

Abstract: This extended abstract contains an outline of the work reported at the conference IQIS2018. We show that it is possible to exploit a thermalization process to extract work from a resource system $R$ to a bipartite system $S$. To do this, we propose a simple protocol in a general setting in the presence of a single bath at temperature $T$ and then examine it when $S$ is described by the quantum Rabi model at $T=0$. We find the theoretical bounds of the protocol in the general case and we show that when applied to the Rabi model it gives rise to a satisfactory extraction of work and efficiency.

Authors:Fabio Di Pumpo, Alexander Friedrich, Enno Giese

Abstract: Several terrestrial detectors for gravitational waves and dark matter based on long-baseline atom interferometry are currently in the final planning stages or already under construction. These upcoming vertical sensors are inherently subject to gravity and thus feature gradiometer or multi-gradiometer configurations using single-photon transitions for large momentum transfer. While there has been significant progress on optimizing these experiments against detrimental noise sources and for deployment at their projected sites, finding optimal configurations that make the best use of the available resources are still an open issue. Even more, the fundamental limit of the device's sensitivity is still missing. Here we fill this gap and show that (a) resonant-mode detectors based on multi-diamond fountain gradiometers achieve the optimal, shot-noise limited, sensitivity if their height constitutes 20% of the available baseline; (b) this limit is independent of the dark-matter oscillation frequency; and (c) doubling the baseline decreases the ultimate measurement uncertainty by approximately 65%.

Authors:Sang Min Lee

Abstract: The evaluation of a photon-pair source employs metrics like photon-pair generation rate, heralding efficiency, and second-order correlation function, all of which are determined by the photon number distribution of the source. These metrics, however, can be altered due to spectral or spatial filtering and optical losses, leading to changes in the metric characteristics. In this paper, we theoretically describe these changes in the photon number distribution and the effect of noise counts. We also review the previous methods used for estimating these characteristics and the photon number distribution. Moreover, we introduce an improved methodology for estimating the photon number distribution, focusing on photon-pair sources, and discuss the accuracy of the calculated characteristics from the estimated (or reconstructed) photon number distribution through simulations and experiments.

Authors:Tobias Heindel, Je-Hyung Kim, Niels Gregersen, Armando Rastelli, Stephan Reitzenstein

Abstract: The generation, manipulation, storage, and detection of single photons play a central role in emerging photonic quantum information technology. Individual photons serve as flying qubits and transmit the quantum information at high speed and with low losses, for example between individual nodes of quantum networks. Due to the laws of quantum mechanics, quantum communication is fundamentally tap-proof, which explains the enormous interest in this modern information technology. On the other hand, stationary qubits or photonic states in quantum computers can potentially lead to enormous increases in performance through parallel data processing, to outperform classical computers in specific tasks when quantum advantage is achieved. Here, we discuss in depth the great potential of quantum dots (QDs) in photonic quantum information technology. In this context, QDs form a key resource for the implementation of quantum communication networks and photonic quantum computers because they can generate single photons on-demand. Moreover, QDs are compatible with the mature semiconductor technology, so that they can be integrated comparatively easily into nanophotonic structures, which form the basis for quantum light sources and integrated photonic quantum circuits. After a thematic introduction, we present modern numerical methods and theoretical approaches to device design and the physical description of quantum dot devices. We then present modern methods and technical solutions for the epitaxial growth and for the deterministic nanoprocessing of quantum devices based on QDs. Furthermore, we present the most promising concepts for quantum light sources and photonic quantum circuits that include single QDs as active elements and discuss applications of these novel devices in photonic quantum information technology. We close with an overview of open issues and an outlook on future developments.

Authors:Rahul Gupta, Manan Jain, Sudhir R. Jain

Abstract: Quantum dynamics of a collection of atoms subjected to phase modulation has been carefully revisited. We present an exact analysis of the evolution of a two-level system (represented by a spinor) under the action of a time-dependent matrix Hamiltonian. The dynamics is shown to evolve on two coupled potential energy surfaces, one of them binding while the other one scattering type. The dynamics is shown to be quasi-integrable with nonlinear resonances. The bounded dynamics with intermittent scattering at random moments presents the scenario reminiscent to Anderson and dynamical localization. We believe that a careful analytical investigation of a multi-component system which is classically non-integrable is relevant to many other fields, including quantum computation with multi-qubit system.

Authors:William Davis, Cameron McGarry, Tabijah Wasawo, Peter J Mosley, Joshua Nunn

Abstract: High-speed switching with low loss would be a versatile tool for photonic quantum technologies, with applications in state generation, multiplexing, and the implementation of quantum gates. Phase modulation is one method of achieving this switching, but existing optical phase modulators either achieve high bandwidth or low loss, but not both. We demonstrate fast ($100\,\mathrm{MHz}$) bandwidth), low-loss ($74(2)\,\%$) transmission) phase shifting ($\Delta\phi = (0.90(5))\pi$) in a signal field, induced by a control field, and mediated by the two-photon $5S_{1/2} \rightarrow{} 5P_{3/2} \rightarrow{} 5D_{5/2}$ transition in rubidium-87 vapour. We discuss routes to enhance both performance and scalability for application to a range of quantum and classical technologies.

Authors:Yoshiki Sunada, Kenshi Yuki, Zhiling Wang, Takeaki Miyamura, Jesper Ilves, Kohei Matsuura, Peter A. Spring, Shuhei Tamate, Shingo Kono, Yasunobu Nakamura

Abstract: Residual noise photons in a readout resonator become a major source of dephasing for a superconducting qubit when the resonator is optimized for a fast, high-fidelity dispersive readout. Here, we propose and demonstrate a nonlinear Purcell filter that suppresses such an undesired dephasing process without sacrificing the readout performance. When a readout pulse is applied, the filter automatically reduces the effective linewidth of the readout resonator, increasing the sensitivity of the qubit to the input field. The noise tolerance of the device we fabricated is shown to be enhanced by a factor of three relative to a device with a linear filter. The measurement rate is enhanced by another factor of three by utilizing the bifurcation of the nonlinear filter. A readout fidelity of 99.4% and a QND fidelity of 99.2% are achieved using a 40-ns readout pulse. The nonlinear Purcell filter will be an effective tool for realizing a fast, high-fidelity readout without compromising the coherence time of the qubit.

Authors:Muhammad Asaduzzaman, Simon Catterall, Yannick Meurice, Goksu Can Toga

Abstract: This paper investigates the transverse Ising model on a discretization of two-dimensional anti-de Sitter space. We use classical and quantum algorithms to simulate real-time evolution and measure out-of-time-ordered correlators (OTOC). The latter can probe thermalization and scrambling of quantum information under time evolution. We compared tensor network-based methods both with simulation on gated-based superconducting quantum devices and analog quantum simulation using Rydberg arrays. While studying this system's thermalization properties, we observed different regimes depending on the radius of curvature of the space. In particular, we find a region of parameter space where the thermalization time depends only logarithmically on the number of degrees of freedom.

Authors:Nik O. Gjonbalaj, Stefan Ostermann, Susanne F. Yelin

Abstract: Atomic arrays can exhibit collective light emission when the transition wavelength exceeds their lattice spacing. Subradiant states take advantage of this phenomenon to drastically reduce their overall decay rate, allowing for long-lived states in dissipative open systems. We build on previous work to investigate whether or not disorder can further decrease the decay rate of a singly-excited atomic array. More specifically, we consider spatial disorder of varying strengths in a 1D half waveguide and in 1D, 2D, and 3D atomic arrays in free space and analyze the effect on the most subradiant modes. While we confirm that the dilute half waveguide exhibits an analog of Anderson localization, the dense half waveguide and free space systems can be understood through the creation of close-packed, few-body subradiant states similar to those found in the Dicke limit. In general, we find that disorder provides little advantage in generating darker subradiant states in free space on average and will often accelerate decay. However, one could potentially change interatomic spacing within the array to engineer specific subradiant states.

Authors:Steve Campbell

Abstract: We use the quantum work statistics to characterize the controlled dynamics governed by a counterdiabatic driving field. Focusing on the Shannon entropy of the work probability distribution, $P(W)$, we demonstrate that the thermodynamics of a controlled evolution serves as an insightful tool for studying the non-equilibrium dynamics of complex quantum systems. In particular, we show that the entropy of $P(W)$ recovers the expected scaling according to the Kibble-Zurek mechanism for the Landau-Zener model. Furthermore, we propose that the entropy of the work distribution provides a useful summary statistic for characterizing the need and complexity of the control fields for many-body systems.

Authors:Antonio Ferrer-Sánchez, Carlos Flores-Garrigos, Carlos Hernani-Morales, José J. Orquín-Marqués, Narendra N. Hegade, Alejandro Gomez Cadavid, Iraitz Montalban, Enrique Solano, Yolanda Vives-Gilabert, José D. Martín-Guerrero

Abstract: We introduce a novel methodology that leverages the strength of Physics-Informed Neural Networks (PINNs) to address the counterdiabatic (CD) protocol in the optimization of quantum circuits comprised of systems with $N_{Q}$ qubits. The primary objective is to utilize physics-inspired deep learning techniques to accurately solve the time evolution of the different physical observables within the quantum system. To accomplish this objective, we embed the necessary physical information into an underlying neural network to effectively tackle the problem. In particular, we impose the hermiticity condition on all physical observables and make use of the principle of least action, guaranteeing the acquisition of the most appropriate counterdiabatic terms based on the underlying physics. The proposed approach offers a dependable alternative to address the CD driving problem, free from the constraints typically encountered in previous methodologies relying on classical numerical approximations. Our method provides a general framework to obtain optimal results from the physical observables relevant to the problem, including the external parameterization in time known as scheduling function, the gauge potential or operator involving the non-adiabatic terms, as well as the temporal evolution of the energy levels of the system, among others. The main applications of this methodology have been the $\mathrm{H_{2}}$ and $\mathrm{LiH}$ molecules, represented by a 2-qubit and 4-qubit systems employing the STO-3G basis. The presented results demonstrate the successful derivation of a desirable decomposition for the non-adiabatic terms, achieved through a linear combination utilizing Pauli operators. This attribute confers significant advantages to its practical implementation within quantum computing algorithms.

Authors:Hyeong-Gyu Kim, Siheon Park, June-Koo Kevin Rhee

Abstract: In quantum machine learning, algorithms with parameterized quantum circuits (PQC) based on a hardware-efficient ansatz (HEA) offer the potential for speed-ups over traditional classical algorithms. While much attention has been devoted to supervised learning tasks, unsupervised learning using PQC remains relatively unexplored. One promising approach within quantum machine learning involves optimizing fewer parameters in PQC than in its classical counterparts, under the assumption that a sub-optimal solution exists within the Hilbert space. In this paper, we introduce the Variational Quantum Approximate Spectral Clustering (VQASC) algorithm - a NISQ-compatible method that requires optimization of fewer parameters than the system size, N, traditionally required in classical problems. We present numerical results from both synthetic and real-world datasets. Furthermore, we propose a descriptor, complemented by numerical analysis, to identify an appropriate ansatz circuit tailored for VQASC.

Authors:Christopher W. Wächtler, Joel E. Moore

Abstract: The gapped symmetric phase of the Affleck-Kennedy-Lieb-Tasaki (AKLT) model exhibits fractionalized spins at the ends of an open chain. We show that breaking SU(2) symmetry and applying a global spin-lowering dissipator achieves synchronization of these fractionalized spins. Additional local dissipators ensure convergence to the ground state manifold. In order to understand which aspects of this synchronization are robust within the entire Haldane-gap phase, we reduce the biquadratic term which eliminates the need for an external field but destabilizes synchronization. Within the ground state subspace, stability is regained using only the global lowering dissipator. These results demonstrate that fractionalized degrees of freedom can be synchronized in extended systems with a significant degree of robustness arising from topological protection.

Authors:Xueshi Guo, Wen Zhao, Xiaoying Li, Z. Y. Ou

Abstract: Multi-mode entanglement is one of the critical resource in quantum information technology. Generating large scale multi-mode entanglement state by coherently combining time-delayed continuous variables Einstein-Podolsky-Rosen pairs with linear beam-splitters has been widely studied recently. Here we theoretically investigate the multi-mode quantum correlation property of the optical fields generated from an unbalanced SU(1,1) interferometer pumped ultra-short pulses, which generates multi-mode entangled state by using a non-degenerate parametric processes to coherently combine delayed Einstein-Podolsky-Rosen pairs in different frequency band. The covariance matrix of the generated multi-mode state is derived analytically for arbitrary mode number $M$ within adjacent timing slot, which shows a given mode is maximally correlated to 5 other modes. Based on the derived covariance matrix, both photon number correlation and quadrature amplitude correlation of the generated state is analyzed. We also extend our analyzing method to the scheme of generating entangled state by using linear beam splitter as a coherent combiner of delayed EPR pairs, and compare the states generated by the two coherently combining schemes. Our result provides a comprehensive theoretical description on the quantum correlations generated from an unbalanced SU(1,1) interferometer within Gaussian system range, and will offer more perspectives to quantum information technology.

Authors:Cheng Yang, Jiteng Sheng, Haibin Wu

Abstract: Clock synchronization is critically important in positioning, navigation and timing systems. While its performance has been intensively studied in a wide range of disciplines, much less is known for the fundamental thermodynamics of clock synchronization, what limits the precision and how to optimize the energy cost for clock synchronization. Here, we report the first experimental investigation of two stochastic clocks synchronization, unveiling the thermodynamic relation between the entropy cost and clock synchronization in an open cavity optomechanical system. Two autonomous clocks are synchronized spontaneously by engineering the controllable photon-mediated dissipative optomechanical coupling and the disparate decay rates of hybrid modes. The measured dependence of the degree of synchronization on entropy cost exhibits an unexpected non-monotonic characteristic, indicating that the perfect clock synchronization does not cost the maximum entropy and there exists an optimum. The investigation of transient dynamics of clock synchronization exposes a trade-off between energy and time consumption. Our results reveal the fundamental relation between clock synchronization and thermodynamics, and have a great potential for precision measurements, distributed quantum networks, and biological science.

Authors:Alexander Bott, Fabio Di Pumpo, Enno Giese

Abstract: Single-photon transitions are one of the key technologies for designing and operating very-long-baseline atom interferometers tailored for terrestrial gravitational-wave and dark-matter detection. Since such setups aim at the detection of relativistic and beyond-Standard-Model physics, the analysis of interferometric phases as well as of atomic diffraction must be performed to this precision and including these effects. In contrast, most treatments focused on idealized diffraction so far. Here, we study single-photon transitions, both magnetically-induced and direct ones, in gravity and Standard-Model extensions modeling dark matter as well as Einstein-equivalence-principle violations. We take into account relativistic effects like the coupling of internal to center-of-mass degrees of freedom, induced by the mass defect, as well as the gravitational redshift of the diffracting light pulse. To this end, we also include chirping of the light pulse required by terrestrial setups, as well as its associated modified momentum transfer for single-photon transitions.

Authors:Ulf Saalmann, Jan M. Rost

Abstract: Scattering properties and time delays for general (non-symmetric) potentials in terms of the respective S-matrices are discussed paradigmatically in one dimension and in comparison to symmetric potentials. Only for the latter the Wigner and Smith time delays coincide. Considering asymmetric potentials also reveals that only one version of S-matrices used in the literature (the one with reflection coefficients on the diagonal) generalizes to the asymmetric case. Finally, we give a criterion how to identify a potential with intrinsic symmetry which behaves like an asymmetric one if it is merely offset from the scattering center.

Authors:Vadim Petruhanov, Alexander Pechen

Abstract: An important problem in quantum computation is generation of single-qubit quantum gates such as Hadamard ($H$) and $\pi/8$ ($T$) gates which are components of a universal set of gates. Qubits in experimental realizations of quantum computing devices are interacting with their environment. While the environment is often considered as an obstacle leading to decrease of the gate fidelity, in some cases it can be used as a resource. Here we consider the problem of optimal generation of $H$ and $T$ gates using coherent control and the environment as a resource acting on the qubit via incoherent control. For this problem, we study quantum control landscape which represents the behaviour of the infidelity as a functional of the controls. We consider three landscapes, with infidelities defined by steering between two, three (via Goerz-Reich-Koch approach), and four matrices in the qubit Hilbert space. We observe that for the $H$ gate, which is Clifford gate, for all three infidelities the distributions of minimal values obtained with gradient search have a simple form with just one peak. However, for $T$ gate which is a non-Clifford gate, the situation is surprisingly different - this distribution for the infidelity defined by two matrices also has one peak, whereas distributions for the infidelities defined by three and four matrices have two peaks, that might indicate possible existence of two isolated minima in the control landscape. Important is that among these three infidelities only those defined with three and four matrices guarantee closeness of generated gate to a target and can be used as a good measure of closeness. We study sets of optimized solutions for this most general and not treated before case of coherent and incoherent controls acting together, and discover that they form submanifolds in the control space, and unexpected, in some cases two isolated submanifolds.

Authors:Saptak Bhattacharya

Abstract: In this paper, we prove log-convexity of some parametrized versions of the relative entropy and fidelity. We also look at a R\'enyi generalization of relative entropy difference introduced by Seshadreesan et. al. in J. Phys. A: Math. Theor. 48 (2015) and give a counterexample to one of their conjectures.

Authors:Scarlett Gauthier, Gayane Vardoyan, Stephanie Wehner

Abstract: Entanglement between quantum network nodes is often produced using intermediary devices - such as heralding stations - as a resource. When scaling quantum networks to many nodes, requiring a dedicated intermediary device for every pair of nodes introduces high costs. Here, we propose a cost-effective architecture to connect many quantum network nodes via a central quantum network hub called an Entanglement Generation Switch (EGS). The EGS allows multiple quantum nodes to be connected at a fixed resource cost, by sharing the resources needed to make entanglement. We propose an algorithm called the Rate Control Protocol (RCP) which moderates the level of competition for access to the hub's resources between sets of users. We proceed to prove a convergence theorem for rates yielded by the algorithm. To derive the algorithm we work in the framework of Network Utility Maximization (NUM) and make use of the theory of Lagrange multipliers and Lagrangian duality. Our EGS architecture lays the groundwork for developing control architectures compatible with other types of quantum network hubs as well as system models of greater complexity.

Authors:Giuseppe Bimonte, Thorsten Emig

Abstract: We develop a general multiple scattering expansion (MSE) for computing Casimir forces between magneto-dielectric bodies and Casimir-Polder forces between polarizable particles and magneto-dielectric bodies. The approach is based on fluctuating electric and magnetic surface currents and charges. The surface integral equations for these surface fields can be formulated in terms of surface scattering operators (SSO). We show that there exists an entire family of such operators. One particular member of this family is only weakly divergent and allows for a MSE that appears to be convergent for general magneto-dielectric bodies. We proof a number of properties of this operator, and demonstrate explicitly convergence for sufficiently low and high frequencies, and for perfect conductors. General expressions are derived for the Casimir interaction between macroscopic bodies and for the Casimir-Polder interaction between particles and macroscopic bodies in terms of the SSO, both at zero and finite temperatures. An advantage of our approach above previous scattering methods is that it does not require the knowledge of the scattering amplitude (T-operator) of the bodies. A number of simple examples are provided to demonstrate the use of the method. Some applications of our approach have appeared previously [T. Emig, G. Bimonte, Phys. Rev. Lett. 130, 200401 (2023)]. Here we provide additional technical aspects and details of our approach.

Authors:He-Guang Xu, V. Montenegro, Gao Xianlong, Jiasen Jin, G. D. de Moraes Neto

Abstract: Quantum correlations and nonclassical states are at the heart of emerging quantum technologies. Efforts to produce long-lived states of such quantum resources are a subject of tireless pursuit. Among several platforms useful for quantum technology, the mature quantum system of light-matter interactions offers unprecedented advantages due to current on-chip nanofabrication, efficient quantum control of its constituents, and its wide range of operational regimes. Recently, a continuous transition between the Jaynes-Cummings model and the Rabi model has been proposed by exploiting anisotropies in their light-matter interactions, known as the anisotropic quantum Rabi model. In this work, we study the long-lived quantum correlations and nonclassical states generated in the anisotropic Rabi model and how these indeed persist even at thermal equilibrium. To achieve this, we thoroughly analyze several quantumness quantifiers, where the long-lived quantum state is obtained from a dressed master equation that is valid for all coupling regimes and with the steady state ensured to be the canonical Gibbs state. Furthermore, we demonstrate a stark distinction between virtual excitations produced beyond the strong coupling regime and the quantumness quantifiers once the light-matter interaction has been switched off. This raises the key question about the nature of the equilibrium quantum features generated in the anisotropic quantum Rabi model and paves the way for future experimental investigations, without the need for challenging ground-state cooling.

Authors:M. C. Smith, A. D. Leu, M. F. Gely, D. M. Lucas

Abstract: In 1995, Cirac and Zoller proposed the first concrete implementation of a small-scale quantum computer, using laser beams focused to micron spot sizes to address individual trapped ions in a linear crystal. Here we propose a method to focus entangling gate interactions, but driven by microwave fields, to micron-sized zones, corresponding to $10^{-5}$ microwave wavelengths. We demonstrate the ability to suppress the spin-dependent force using a single ion, and find the required interaction introduces $3.7(4)\times 10^{-4}$ error per emulated gate in a single-qubit benchmarking sequence. We model the scheme for a 17-qubit ion crystal, and find that any pair of ions should be addressable with an average crosstalk error of $\sim 10^{-5}$.

Authors:Yassine Dakir, Abdallah Slaoui, Abdel-Baset A. Mohamed, Rachid Ahl Laamara, Hichem Eleuch

Abstract: We investigate the dynamics of non-classical correlations and quantum coherence in open quantum systems by employing metrics like local quantum Fisher information, local quantum uncertainty, and quantum Jensen-Shannon divergence. Our focus here is on a system of two qubits in two distinct physical situations: the first one when the two qubits are coupled to a single-mode cavity, while the second consists of two qubits immersed in dephasing reservoirs. Our study places significant emphasis on how the evolution of these quantum criterion is influenced by the initial state's purity (whether pure or mixed) and the nature of the environment (whether Markovian or non-Markovian). We observe that a decrease in the initial state's purity corresponds to a reduction in both quantum correlations and quantum coherence, whereas higher purity enhances these quantumness. Furthermore, we establish a quantum teleportation strategy based on the two different physical scenarios. In this approach, the resulting state of the two qubits functions as a quantum channel integrated into a quantum teleportation protocol. We also analyze how the purity of the initial state and the Markovian or non-Markovian regimes impact the quantum teleportation process.

Authors:Pablo Serra, Alejandro Ferrón, Omar Osenda

Abstract: The pretty good transmission scenario implies that the probability of sending one excitation from one extreme of a spin chain to the other can reach values arbitrarily close to the unity just by waiting a time long enough. The conditions that ensure the appearance of this scenario are known for chains with different interactions and lengths. Sufficient conditions for the presence of pretty good transmission depend on the spectrum of the Hamiltonian of the spin chain. Some works suggest that the time $t_{\varepsilon}$ at which the pretty good transmission takes place scales as $1/(|\varepsilon|)^{f(N)}$, where $\varepsilon$ is the difference between the probability that a single excitation propagates from one extreme of the chain to the other and the unity, while $f(N)$ is an unknown function of the chain length. In this paper, we show that the exponent is not a simple function of the chain length but a power law of the number of linearly independent irrational eigenvalues of the one-excitation block of the Hamiltonian that enter into the expression of the probability of transmission of one excitation. We explicitly provide examples of a chain showing that the exponent changes when the couplings between the spins change while the length remains fixed. For centrosymmetric spin chains the exponent is at most $N/2$.

Authors:Hanna Linn, Yu Zheng, Anton Frisk Kockum

Abstract: We explore the use of machine-learning techniques to detect quantum speedup in random walks on graphs. Specifically, we investigate the performance of three different neural-network architectures (variations on fully connected and convolutional neural networks) for identifying linear, cyclic, and random graphs that yield quantum speedups in terms of the hitting time for reaching a target node after starting in another node of the graph. Our results indicate that carefully building the data set for training can improve the performance of the neural networks, but all architectures we test struggle to classify large random graphs and generalize from training on one graph size to testing on another. If classification accuracy can be improved further, valuable insights about quantum advantage may be gleaned from these neural networks, not only for random walks, but more generally for quantum computing and quantum transport.

Authors:Arseni Goussev, Jaewoo Joo

Abstract: Quantum backflow is a counterintuitive effect in which the probability density of a free particle moves in the direction opposite to the particle's momentum. If the particle is electrically charged, then the effect can be viewed as the contrast between the direction of electric current and that of the momentum. To date, there has been no direct experimental observation of quantum backflow. However, the effect has been simulated numerically (using classical computers) and optically (using classical light). In this study, we present the first simulation of quantum backflow using a real quantum computer.

Authors:Ning Hu, Zhi-Xiang Tang, Xun-Wei Xu

Abstract: As a promising approach for optical nonreciprocity without magnetic materials, optomechanically induced nonreciprocity has great potential for all-optical controllable isolators and circulators on chips. However, as a very important issue in practical applications, the bandwidth for nonreciprocal transmission with high isolation has not been fully investigated yet. In this study we review the nonreciprocity in a Brillouin optomechanical system with single cavity and point out the challenge in achieving broad bandwidth with high isolation. To overcome this challenge, we propose a one dimensional optomechanical array to realize broadband optical nonreciprocity via nonreciprocal band structure. We exploit nonreciprocal band structure by the stimulated Brillouin scattering induced transparency with directional optical pumping, and show that it is possible to demonstrate optical nonreciprocity with both broad bandwidth and high isolation. Such Brillouin optomechanical lattices with nonreciprocal band structure, offer an avenue to explore nonreciprocal collective effects in different electromagnetic and mechanical frequency regimes, such as nonreciprocal topological photonic and phononic phases.

Authors:Martin Koppenhöfer, A. A. Clerk

Abstract: Spin squeezing can increase the sensitivity of interferometric measurements of small signals in large spin ensembles beyond the standard quantum limit. In many practical settings, the ideal metrological gain is limited by imperfect readout of the sensor. To overcome this issue, protocols based on time reversal of unitary one-axis-twist (OAT) spin-squeezing dynamics have been proposed. Such protocols mitigate readout noise and, when implemented using cavity feedback, have been argued to also be robust against dissipation as long as the collective cooperativity of the system is sufficiently large [Davis et al., PRL 116, 053601 (2016)]. Here, we perform a careful systematic study of dissipative effects on three different implementations of a OAT twist-untwist sensing scheme (based on symmetric as well as asymmetric cavity feedback and on a Tavis-Cummings interaction). Our full treatment shows that the three approaches have markedly different properties and resilience when subject to dissipation. Moreover, the metrological gain for an implementation using symmetric cavity feedback is more sensitive to undesired dissipation than was previously appreciated.

Authors:Luigi Santamaria Amato, Fabrizio Sgobba, Cosmo Lupo

Abstract: Interferometric methods have been recently investigated to achieve sub-Rayleigh imaging and precision measurements of faint incoherent sources up to the ultimate quantum limit. Here we consider single-photon imaging of two point-like emitters of unequal intensity. This is motivated by the fact that pairs of natural emitters will typically have unequal brightness, as for example binary star systems and exoplanets. We address the problems of estimating the transverse separation $d$ and the relative intensity $\epsilon$. Our theoretical analysis shows that the associated statistical errors are qualitatively different from the case of equal intensity. We employ multi-plane light conversion technology to experimentally implement Hermite-Gaussian (HG) spatial-mode demultiplexing (SPADE), and demonstrate sub-Rayleigh measurement of two emitters with Gaussian point-spread function. The experimental errors are comparable with the theoretical bounds. The latter are benchmarked against direct imaging, yielding a $\epsilon^{-1/2}$ improvement in the signal-to-noise ratio, which may be significant when the primary source is much brighter than the secondary one, as for example for imaging of exoplanets. However, achieving this improved scaling requires low noise in the implementation of SPADE, which is typically affected by crosstalk between HG modes.

Authors:Zhe Kan, Andrew A. Voitiv, Patrick C. Ford, Mark T. Lusk, Mark E. Siemens

Abstract: We demonstrate entangled-state swapping, within the Hermite-Gaussian basis of first-order modes, directly from the process of spontaneous parametric down-conversion within a nonlinear crystal. The method works by explicitly tailoring the spatial structure of the pump photon such that it resembles the product of the desired entangled spatial modes exiting the crystal. Importantly, the result is an entangled state of balanced HG modes, which may be beneficial in applications that depend on symmetric accumulations of geometric phase through optics or in applications of quantum sensing and imaging with azimuthal sensitivity. Furthermore, the methods are readily adaptable to other spatial mode bases.

Authors:Shunya Konno, Warit Asavanant, Fumiya Hanamura, Hironari Nagayoshi, Kosuke Fukui, Atsushi Sakaguchi, Ryuhoh Ide, Fumihiro China, Masahiro Yabuno, Shigehito Miki, Hirotaka Terai, Kan Takase, Mamoru Endo, Petr Marek, Radim Filip, Peter van Loock, Akira Furusawa

Abstract: A quantum computer with low-error, high-speed quantum operations and capability for interconnections is required for useful quantum computations. A logical qubit called Gottesman-Kitaev-Preskill (GKP) qubit in a single Bosonic harmonic oscillator is efficient for mitigating errors in a quantum computer. The particularly intriguing prospect of GKP qubits is that entangling gates as well as syndrome measurements for quantum error correction only require efficient, noise-robust linear operations. To date, however, GKP qubits have been only demonstrated at mechanical and microwave frequency in a highly nonlinear physical system. The physical platform that naturally provides the scalable linear toolbox is optics, including near-ideal loss-free beam splitters and near-unit efficiency homodyne detectors that allow to obtain the complete analog syndrome for optimized quantum error correction. Additional optical linear amplifiers and specifically designed GKP qubit states are then all that is needed for universal quantum computing. In this work, we realize a GKP state in propagating light at the telecommunication wavelength and demonstrate homodyne meausurements on the GKP states for the first time without any loss corrections. Our GKP states do not only show non-classicality and non-Gaussianity at room temperature and atmospheric pressure, but unlike the existing schemes with stationary qubits, they are realizable in a propagating wave system. This property permits large-scale quantum computation and interconnections, with strong compatibility to optical fibers and 5G telecommunication technology.

Authors:Arif Warsi Laskar, Pratik Adhikary, Niharika Singh, Saikat Ghosh

Abstract: Electromagnetically induced transparency (EIT) and Autler-Townes splitting (ATS) are generally characterized and distinguished by the width of the transparency created in the absorption profile of a weak probe in presence of a strong control field. This often leads to ambiguities, as both phenomena yield similar spectroscopic signature. However, an objective method based on the AIC test offers a quantitative way to discern the two regimes when applied on the probe absorption profile. The obtained transition value of control field strength was found to be higher than the value given by pole analysis of the corresponding off-diagonal density matrix element $\rho_{13}$. By contrast, we apply the test on ground state coherence $\rho_{12}$ and the measured coherence quantifier, which yielded a distinct transition point around the predicted value also in presence of noise. Our test accurately captures the transition between the two regimes, indicating that a proper measure of coherence is essential for making such distinctions.

Authors:Simone Cialdi Università degli Studi di Milano Istituto Nazionale di Fisica Nucleare, Edoardo Suerra Università degli Studi di Milano Istituto Nazionale di Fisica Nucleare, Samuele Altilia Università degli Studi di Milano Istituto Nazionale di Fisica Nucleare, Stefano Olivares Università degli Studi di Milano Istituto Nazionale di Fisica Nucleare, Bruno Paroli Università degli Studi di Milano Istituto Nazionale di Fisica Nucleare, Marco A. C. Potenza Università degli Studi di Milano Istituto Nazionale di Fisica Nucleare, Mirko Siano Università degli Studi di Milano Istituto Nazionale di Fisica Nucleare, Matteo G. A. Paris Università degli Studi di Milano Istituto Nazionale di Fisica Nucleare

Abstract: We address the use of a calcite crystal-based local detector to the discrimination of orbital angular momentum of quantum radiation produced by parametric down conversion. We demonstrate that a discrimination can be obtained exploiting the introduction of a fine and controlled spatial shift between two replicas of the state in the crystals. We believe that this technology could be used for future development of long-distance quantum communication techniques, where information encoding is based on orbital angular momentum.

Authors:Jing Zhou, D. L. Zhou

Abstract: As an important tomography technique in quantum computing, Hamiltonian Learning (HL) provides a significant method for verifying the accuracy of a quantum system. Often, learning a certain Hamiltonian requires the measurements from its steady states. However, not all the Hamiltonian can be uniquely determined from the steady state. It has been revealed that the success of HL depends on the Hamiltonian model and the rank of the state. Here, we analyze the HL with respect to a specific type of steady state that is decomposed by eigenstates with degeneracy, making the Hamiltonian's eigenstate unknown. To overcome this challenge, we extract information from the orthogonality relationship between the eigenstate space and its complement space, constructing the orthogonal space equation (OSE). The equation number of OSE can be utilized to determine whether a Hamiltonian can be recovered from a certain steady state. Finally, we investigate how symmetries in the Hamiltonian affect the feasibility of the HL method.

Authors:Hironobu Yoshida

Abstract: We present a simple proof of a sufficient condition for the uniqueness of non-equilibrium steady states of Gorini-Kossakowski-Sudarshan-Lindblad equations. We demonstrate the applications of the sufficient condition using examples of the transverse-field Ising model, the XYZ model, and the tight-binding model with dephasing.

Authors:Sergei S. Kuzmin, Varvara I. Mikhailova, Ivan V. Dyakonov, Stanislav S. Straupe

Abstract: The constantly increasing dimensionality of artificial quantum systems demands for highly efficient methods for their characterization and benchmarking. Conventional quantum tomography fails for larger systems due to the exponential growth of the required number of measurements. The conceptual solution for this dimensionality curse relies on a simple idea - a complete description of a quantum state is excessive and can be discarded in favor of experimentally accessible information about the system. The probably approximately correct (PAC) learning theory has been recently successfully applied to a problem of building accurate predictors for the measurement outcomes using a dataset which scales only linearly with the number of qubits. Here we present a constructive and numerically efficient protocol which learns a tensor network model of an unknown quantum system. We discuss the limitations and the scalability of the proposed method.

Authors:Jaskaran Singh, Adán Cabello

Abstract: There are bipartite quantum nonlocal correlations requiring very low detection efficiency to reach the loophole-free regime but that need too many measurement settings to be practical for actual experiments. This leads to the general problem of what can be concluded about loophole-free Bell nonlocality if only a random subset of these settings is tested. Here we develop a method to address this problem. We show that, in some cases, it is possible to detect loophole-free Bell nonlocality testing only a small random fraction of the settings. The prize to pay is a higher detection efficiency. The method allows for a novel approach to the design of loophole-free Bell tests in which, given the dimension of the local system, the visibility, and the detection efficiency available, one can calculate the fraction of the contexts needed to reach the detection-loophole-free regime. The results also enforce a different way of thinking about the costs of classically simulating quantum nonlocality, as it shows that the amount of resources that are needed can be made arbitrarily large simply by considering more contexts.

Authors:Galina L. Klimchitskaya, Vladimir M. Mostepanenko

Abstract: The Casimir-Polder force between atoms or nanoparticles and graphene-coated dielectric substrates is investigated in the region of large separations. Graphene coating with any value of the energy gap and chemical potential is described in the framework of the Dirac model using the formalism of the polarization tensor. It is shown that the Casimir-Polder force from a graphene-coated substrate reaches the limit of large separations at approximately 5.6 $\mu$m distance between an atom or a nanoparticle and graphene coating independently of the values of the energy gap and chemical potential. According to our results, however, the classical limit, where the Casimir-Polder force no longer depends on the Planck constant and the speed of light, may be attained at much larger separations depending on the values of the energy gap and chemical potential. In addition, we have found a simple analytic expression for the Casimir-Polder force from a graphene-coated substrate at large separations and determined the region of its applicability. It is demonstrated that the asymptotic results for the large-separation Casimir-Polder force from a graphene-coated substrate are in better agreement with the results of numerical computations for the graphene sheets with larger chemical potential and smaller energy gap. Possible applications of the obtained results in nanotechnology and bioelectronics are discussed.

Authors:David Gaspard, Jean-Marc Sparenberg

Abstract: In a previous paper, the distribution of resonance poles in the complex plane of the wavenumber $k$ associated to the multiple scattering of a quantum particle in a random point field was numerically discovered. This distribution presented two distinctive structures: a set of peaks at small $k$ when the wavelength is larger than the interscatterer distance, and a band almost parallel to the real axis at larger $k$. In this paper, a detailed theoretical study based on wave transport theory is proposed to explain the origin of these structures and to predict their location in the complex $k$ plane. First, it is shown that the peaks at small $k$ can be understood using an effective wave equation for the average wave function over the disorder. Then, that the band at large $k$ can be described by the Bethe-Salpeter equation for the square modulus of the wavefunction, which is derived from the diagrammatic method. This study is supported by careful comparisons with numerical simulations. The largest simulations revealed the presence of quantum scars in the bulk of the disordered medium.

Authors:Viet Pham Ngoc, Herbert Wiklicky

Abstract: In this paper, we study the tunable quantum neural network architecture in the quantum exact learning framework with access to a uniform quantum example oracle. We present an approach that uses amplitude amplification to correctly tune the network to the target concept. We applied our approach to the class of positive $k$-juntas and found that $O(n^22^k)$ quantum examples are sufficient with experimental results seemingly showing that a tighter upper bound is possible.

Authors:Joan J. Caceres, Daniel Dominguez, Maria Jose Sanchez

Abstract: Fast quantum gates are of paramount importance for enabling efficient and error-resilient quantum computations. In the present work we analyze Landau-Zener-St\"uckelberg-Majorana (LSZM) strong driving protocols, tailored to implement fast gates with particular emphasis on small gap qubits. We derive analytical equations to determine the specific set of driving parameters for the implementation of single qubit and two qubit gates employing single period sinusoidal pulses. Our approach circumvents the need to scan experimentally a wide range of parameters and instead it allows to focus in fine-tuning the device near the analytically predicted values. We analyze the dependence of relaxation and decoherence on the amplitude and frequency of the pulses, obtaining the optimal regime of driving parameters to mitigate the effects of the environment. Our results focus on the study of the single qubit $X_{\frac{\pi}{2}}$, $Y_{\frac{\pi}{2}}$ and identity gates. Also, we propose the $\sqrt{\rm{bSWAP}}$ as the simplest two-qubit gate attainable through a robust LZSM driving protocol.

Authors:Howard Barnum Institute for Quantum Computing, University of Waterloo, Matthew A. Graydon Institute for Quantum Computing, University of Waterloo, Alex Wilce Susquehanna University

Abstract: Given a monoidal probabilistic theory -- a symmetric monoidal category $\mathcal{C}$ of systems and processes, together with a functor $\mathbf{V}$ assigning concrete probabilistic models to objects of $\mathcal{C}$ -- we construct a locally tomographic probabilistic theory LT$(\mathcal{C},\mathbf{V})$ -- the locally tomographic shadow of $(\mathcal{C},\mathbf{V})$ -- describing phenomena observable by local agents controlling systems in $\mathcal{C}$, and able to pool information about joint measurements made on those systems. Some globally distinct states become locally indistinguishable in LT$(\mathcal{C},\mathbf{V})$, and we restrict the set of processes to those that respect this indistinguishability. This construction is investigated in some detail for real quantum theory.

Authors:Ewan Murphy University of Oxford, Aleks Kissinger University of Oxford

Abstract: A common approach to quantum circuit transformation is to use the properties of a specific gate set to create an efficient representation of a given circuit's unitary, such as a parity matrix or stabiliser tableau, and then resynthesise an improved circuit, e.g. with fewer gates or respecting connectivity constraints. Since these methods rely on a restricted gate set, generalisation to arbitrary circuits usually involves slicing the circuit into pieces that can be resynthesised and working with these separately. The choices made about what gates should go into each slice can have a major effect on the performance of the resynthesis. In this paper we propose an alternative approach to generalising these resynthesis algorithms to general quantum circuits. Instead of cutting the circuit into slices, we "cut out" the gates we can't resynthesise leaving holes in our quantum circuit. The result is a second-order process called a quantum comb, which can be resynthesised directly. We apply this idea to the RowCol algorithm, which resynthesises CNOT circuits for topologically constrained hardware, explaining how we were able to extend it to work for quantum combs. We then compare the generalisation of RowCol using our method to the naive "slice and build" method empirically on a variety of circuit sizes and hardware topologies. Finally, we outline how quantum combs could be used to help generalise other resynthesis algorithms.

Authors:L. A. Markovich, J. Borregaard

Abstract: The estimation of multi-qubit observables is a key task in quantum information science. The standard approach is to decompose a multi-qubit observable into a weighted sum of Pauli strings. The observable can then be estimated from projective single qubit measurements according to the Pauli strings followed by a classical summation. As the number of Pauli strings in the decomposition increases, shot-noise drastically builds up, and the accuracy of such estimation can be considerably compromised. Access to a single qubit quantum memory, where measurement data may be stored and accumulated can circumvent the build-up of shot noise. Here, we describe a many-qubit observable estimation approach to achieve this with a much lower number of interactions between the multi-qubit device and the single qubit memory compared to previous approaches. Our algorithm offers a reduction in the required number of measurements for a given target variance that scales $N^{\frac{2}{3}}$ with the number of Pauli strings $N$ in the observable decomposition. The low number of interactions between the multi-qubit device and the memory is desirable for noisy intermediate-scale quantum devices.

Authors:Chukwudubem Umeano, Annie E. Paine, Vincent E. Elfving, Oleksandr Kyriienko

Abstract: We can learn from analyzing quantum convolutional neural networks (QCNNs) that: 1) working with quantum data can be perceived as embedding physical system parameters through a hidden feature map; 2) their high performance for quantum phase recognition can be attributed to generation of a very suitable basis set during the ground state embedding, where quantum criticality of spin models leads to basis functions with rapidly changing features; 3) pooling layers of QCNNs are responsible for picking those basis functions that can contribute to forming a high-performing decision boundary, and the learning process corresponds to adapting the measurement such that few-qubit operators are mapped to full-register observables; 4) generalization of QCNN models strongly depends on the embedding type, and that rotation-based feature maps with the Fourier basis require careful feature engineering; 5) accuracy and generalization of QCNNs with readout based on a limited number of shots favor the ground state embeddings and associated physics-informed models. We demonstrate these points in simulation, where our results shed light on classification for physical processes, relevant for applications in sensing. Finally, we show that QCNNs with properly chosen ground state embeddings can be used for fluid dynamics problems, expressing shock wave solutions with good generalization and proven trainability.

Authors:Albert Much, Rainer Verch

Abstract: It is known that in quantum field theory, localized operations, e.g.\ given by unitary operators in local observable algebras, may lead to non-causal, or superluminal, state changes within their localization region. In this article, it is shown that both in quantum field theory as well as in classical relativistic field theory, there are localized operations which correspond to ``instantaneous'' spatial rotations (leaving the localization region invariant) leading to superluminal effects within the localization region. This shows that ``impossible measurement scenarios'' which have been investigated in the literature, and which rely on the presence of localized operations that feature superluminal effects within their localization region, do not only occur in quantum field theory, but also in classical field theory.

Authors:Ali Hadizadeh Moghadam, Payman Kazemikhah, Hossein Aghababa

Abstract: The clique problems, including $k$-CLIQUE and Triangle Finding, form an important class of computational problems; the former is an NP-complete problem, while the latter directly gives lower bounds for Matrix Multiplication. A number of previous efforts have approached these problems with Quantum Computing methods, such as Amplitude Amplification. In this paper, we provide new Quantum oracle designs based on the 1-factorization of complete graphs, all of which have depth $O(n)$ instead of the $O(n^2)$ presented in previous studies. Also, we discuss the usage of one of these oracles in bringing the Triangle Finding time complexity down to $O(n^{2.25} poly(log n))$, compared to the $O(n^{2.38})$ classical record. Finally, we benchmark the number of required Amplitude Amplification iterations for another presented oracle, for solving $k$-CLIQUE.

Authors:Jakob Günther, Alberto Baiardi, Markus Reiher, Matthias Christandl

Abstract: Quantum computation is the most promising new paradigm for the simulation of physical systems composed of electrons and atomic nuclei. An atomistic problem in chemistry, solid-state physics, materials science, or molecular biology can be mapped to a representation on a (digital) quantum computer. Any such representation will be reduced dimensional as, for instance, accomplished by active-orbital-space approaches. While it is, in principle, obvious how to improve on the representation by including more orbitals, this is usually unfeasible in practice (e.g., because of the limited number of qubits available on a quantum computer) and severely compromises the accuracy of the obtained results. Here, we propose a quantum algorithm that improves on the representation of the physical problem by virtue of second-order perturbation theory. In particular, our quantum algorithm evaluates the second-order energy correction through a series of time-evolution steps under the unperturbed Hamiltonian ($H$), which allows us to take advantage of an underlying structure that $H$ might have. For multireference perturbation theory, we exploit that $H$ is diagonal for virtual orbitals and show that the number of qubits is independent of the number of virtual orbitals. Moreover, our perturbation theory quantum algorithm can be applied to Symmetry-Adapted Perturbation Theory (SAPT). Here, we use the fact that $H$ is the sum of two commuting monomer Hamiltonians, which makes it possible to calculate the full second-order energy correction of SAPT while only having access to the state of one of the monomers at a time. As such, we reduce the quantum hardware requirements for quantum chemistry by leveraging perturbation theory.

Authors:Haruki Mitarai, Yoshihiko Hasegawa

Abstract: Following the groundbreaking observation of the dynamical Casimir effect in 2011, the exploration of the dynamical Casimir effect in superconducting circuits has garnered significant attention. In this paper, we investigate the synchronization of qubits induced by the dynamical Casimir effect. Our investigation revolves around a pragmatic configuration of a quantum system, where superconducting qubits coupled with a shared coplanar waveguide resonator, terminated by a SQUID at one end. We identify the sufficient condition required for achieving in-phase synchronization, which is expected to be accomplished by generating photons in the resonator. Furthermore, we numerically simulate the time evolution of the system and verify that photon generation via the dynamical Casimir effect arguably induces the synchronization of two qubits. Our result suggests that photon generation by the dynamical Casimir effect affects both the qubits and the resonator, and is a fruitful resource for the control of quantum systems. In addition, we unveil a remarkable feature that is probably unique to the dynamical Casimir effect: The differences in initial states and coupling strengths affect the synchronization independently with no overlap between them.

Authors:Pei-Zhe Li, Josephine Dias, William J. Munro, Peter van Loock, Kae Nemoto, Nicoló Lo Piparo

Abstract: Quantum error correction codes based on continuous variables play an important role for the implementation of quantum communication systems. A natural application of such codes occurs within quantum repeater systems which are used to combat severe channel losses and local gate errors. In particular, channel loss drastically reduces the distance of communication between remote users. Here we consider a cavity-QED based repeater scheme to address the losses in the quantum channel. This repeater scheme relies on the transmission of a specific class of rotationally invariant error-correcting codes. We compare several rotation-symmetric bosonic codes (RSBCs) being used to encode the initial states of two remote users connected by a quantum repeater network against the convention of the cat codes and we quantify the performance of the system using the secret key rate. In particular, we determine the number of stations required to exchange a secret key over a fixed distance and establish the resource overhead.

Authors:Ji-kun Xie, Sheng-li Ma, Ya-long Ren, Shao-yan Gao, Fu-li Li

Abstract: Unidirectional photon emission is crucial for constructing quantum networks and realizing scalable quantum information processing. In the present work an efficient scheme is developed for the unidirectional emission of a tunable squeezed microwave field. Our scheme is based on a chiral cavity magnonic system, where a magnon mode in a single-crystalline yttrium iron garnet (YIG) sphere is selectively coupled to one of the two degenerate rotating microwave modes in a torus-shaped cavity with the same chirality. With the YIG sphere driven by a two-color Floquet field to induce sidebands in the magnon-photon coupling, we show that the unidirectional emission of a tunable squeezed microwave field can be generated via the assistance of the dissipative magnon mode and a waveguide. Moreover, the direction of the proposed one-way emitter can be controlled on demand by reversing the biased magnetic field. Our work opens up an avenue to create and manipulate one-way nonclassical microwave radiation field and could find potential quantum technological applications.

Authors:F. Khorasani, Reza Pirmoradian, Mohammad Reza Tanhayi

Abstract: In this paper, we study the computational complexity of Gaussian states for a harmonic oscillator subjected to an external electric field. We use Nielsen's geometric approach to obtain the so-called complexity of the thermofield double state for a harmonic oscillator, and then by numerical analysis, we investigate the effect of the appeared parameters on the complexity. Precisely, by numerical analysis, we consider the effect of an external electric field on the dynamics of complexity. Our results indicate that turning on the electric field may reduce the system's complexity.

Authors:Lucie Pepe, Vincent Pouthier, Saad Yalouz

Abstract: In an extended star with peripheral defects and a core occupied by a trap, it has been shown that exciton-mediated energy transport from the periphery to the core can be optimized [S. Yalouz et al. Phys. Rev. E 106, 064313 (2022)]. If the defects are judiciously chosen, the exciton dynamics is isomorphic to that of an asymmetric chain and a speedup of the excitonic propagation is observed. Here, we extend this previous work by considering that the exciton in both an extended star and an asymmetric chain, is perturbed by the presence of a dephasing environment. Simulating the dynamics using a Lindblad master equation, two questions are addressed: how does the environment affect the energy transport on these two networks? And, do the two systems still behave equivalently in the presence of dephasing? Our results reveal that the time-scale for the exciton dynamics strongly depends on the nature of the network. But quite surprisingly, the two networks behave similarly regarding the survival of their optimization law. In both cases, the energy transport can be improved using the same original optimal tuning of energy defects as long as the dephasing remains weak. However, for moderate/strong dephasing, the optimization law is lost due to quantum Zeno effect.

Authors:Yeow Meng Chee, Hoang Ta, Van Khu Vu

Abstract: Determining the optimal fidelity for the transmission of quantum information over noisy quantum channels is one of the central problems in quantum information theory. Recently, [Berta \& et al., Mathematical Programming, 2021] introduced an asymptotically converging semidefinite programming hierarchy of outer bounds for this quantity. However, the size of the semidefinite program (SDP) grows exponentially with respect to the level of the hierarchy, and thus computing the SDP directly is inefficient. In this work, by exploiting the symmetries in the SDP, we show that, for fixed input and output dimensions, we can compute the SDP in polynomial time in term of level of the hierarchy. As a direct consequence of our result, the optimal fidelity can be approximated with an accuracy of $\epsilon$ in a time that is polynomial in $1/\epsilon$.

Authors:Patryk Lipka-Bartosik, Martí Perarnau-Llobet, Nicolas Brunner

Abstract: We develop a physics-based model for classical computation based on autonomous quantum thermal machines. These machines consist of few interacting quantum bits (qubits) connected to several environments at different temperatures. Heat flows through the machine are here exploited for computing. The process starts by setting the temperatures of the environments according to the logical input. The machine evolves, eventually reaching a non-equilibrium steady state, from which the output of the computation can be determined via the temperature of an auxilliary finite-size reservoir. Such a machine, which we term a "thermodynamic neuron", can implement any linearly-separable function, and we discuss explicitly the cases of NOT, 3-majority and NOR gates. In turn, we show that a network of thermodynamic neurons can perform any desired function. We discuss the close connection between our model and artificial neurons (perceptrons), and argue that our model provides an alternative physics-based analogue implementation of neural networks, and more generally a platform for thermodynamic computing.

Authors:Jingzhong Yang, Zenghui Jiang, Frederik Benthin, Joscha Hanel, Tom Fandrich, Raphael Joos, Stephanie Bauer, Sascha Kolatschek, Ali Hreibi, Eddy Patrick Rugeramigabo, Michael Jetter, Simone Luca Portalupi, Michael Zopf, Peter Michler, Stefan Kück, Fei Ding

Abstract: Quantum key distribution (QKD) enables the transmission of information that is secure against general attacks by eavesdroppers. The use of on-demand quantum light sources in QKD protocols is expected to help improve security and maximum tolerable loss. Semiconductor quantum dots (QDs) are a promising building block for quantum communication applications because of the deterministic emission of single photons with high brightness and low multiphoton contribution. Here we report on the first intercity QKD experiment using a bright deterministic single photon source. A BB84 protocol based on polarisation encoding is realised using the high-rate single photons in the telecommunication C-band emitted from a semiconductor QD embedded in a circular Bragg grating structure. Utilising the 79 km long link with 25.49 dB loss (equivalent to 130 km for the direct-connected optical fibre) between the German cities of Hannover and Braunschweig, a record-high secret key bits per pulse of 4.8e-5 with an average quantum bit error ratio of 0.65 % are demonstrated. An asymptotic maximum tolerable loss of 28.11 dB is found, corresponding to a length of 144 km of standard telecommunication fibre. Deterministic semiconductor sources therefore compete with state-of-the-art decoy state QKD with weak coherent pulses with respect to high secret key rate and have the potential to excel in measurement device independent protocols and quantum repeater applications.

Authors:Mingrui Shi, Haozhen Situ, Cai Zhang

Abstract: Image classification is a fundamental computer vision problem, and neural networks offer efficient solutions. With advancing quantum technology, quantum neural networks have gained attention. However, they work only for low-dimensional data and demand dimensionality reduction and quantum encoding. Two recent image classification methods have emerged: one employs PCA dimensionality reduction and angle encoding, the other integrates QNNs into CNNs to boost performance. Despite numerous algorithms, comparing PCA reduction with angle encoding against the latter remains unclear. This study explores these algorithms' performance in multi-class image classification and proposes an optimized hybrid quantum neural network suitable for the current environment. Investigating PCA-based quantum algorithms unveils a barren plateau issue for QNNs as categories increase, unsuitable for multi-class in the hybrid setup. Simultaneously, the combined CNN-QNN model partly overcomes QNN's multi-class training challenges but lags in accuracy to superior traditional CNN models. Additionally, this work explores transfer learning in the hybrid quantum neural network model. In conclusion, quantum neural networks show promise but require further research and optimization, facing challenges ahead.

Authors:Youngrong Lim, Minki Hhan, Hyukjoon Kwon

Abstract: We demonstrate a substantial gap between local and nonlocal strategies in a quantum state discrimination task under a non-destructiveness condition. The condition imposes additional constraints to conventional state discrimination that the initial state should be returned without disturbance. For a set of maximally entangled states, the success probability of the proposed task using local operations and classical communications is completely suppressed; it cannot beat random guessing. We also show that a local strategy that efficiently exploits pre-shared entanglement for this task can be essentially different from the conventional approaches. We construct a non-destructive and adaptive strategy to achieve perfect discrimination of maximally entangled states which has a strictly lower entanglement cost than the best-known method based on teleportation. Our approach can be generalized to multipartite scenarios, offering an application in entanglement certification of a quantum network.

Authors:Yotam Shapira, Jovan Markov, Nitzan Akerman, Ady Stern, Roee Ozeri

Abstract: Simulation of quantum systems is notoriously challenging for classical computers, while quantum computers are naturally well-suited for this task. However, the imperfections of contemporary quantum computers pose a considerable challenge in carrying out accurate simulations over long evolution times. Here we experimentally demonstrate a method for quantum simulations on a small-scale trapped ions-based quantum computer. Our method enables quantum simulations of programmable spin-Hamiltonians, using only simple global fields, driving all qubits homogeneously and simultaneously. We measure the evolution of a quantum Ising ring and accurately reconstruct the Hamiltonian parameters, showcasing an accurate and high-fidelity simulation. Our method enables a significant reduction in the required control and depth of quantum simulations, thus generating longer evolution times with higher accuracy.

Authors:Xizheng Ma, Gengyan Zhang, Feng Wu, Feng Bao, Xu Chang, Jianjun Chen, Hao Deng, Ran Gao, Xun Gao, Lijuan Hu, Honghong Ji, Hsiang-Sheng Ku, Kannan Lu, Lu Ma, Liyong Mao, Zhijun Song, Hantao Sun, Chengchun Tang, Fei Wang, Hongcheng Wang, Tenghui Wang, Tian Xia, Make Ying, Huijuan Zhan, Tao Zhou, Mengyu Zhu, Qingbin Zhu, Yaoyun Shi, Hui-Hai Zhao, Chunqing Deng

Abstract: The fluxonium qubits have emerged as a promising platform for gate-based quantum information processing. However, their extraordinary protection against charge fluctuations comes at a cost: when coupled capacitively, the qubit-qubit interactions are restricted to XX-interactions. Consequently, effective XX- or XZ-interactions are only constructed either by temporarily populating higher-energy states, or by exploiting perturbative effects under microwave driving. Instead, we propose and demonstrate an inductive coupling scheme, which offers a wide selection of native qubit-qubit interactions for fluxonium. In particular, we leverage a built-in, flux-controlled ZZ-interaction to perform qubit entanglement. To combat the increased flux-noise-induced dephasing away from the flux-insensitive position, we use a continuous version of the dynamical decoupling scheme to perform noise filtering. Combining these, we demonstrate a 20 ns controlled-Z (CZ) gate with a mean fidelity of 99.53%. More than confirming the efficacy of our gate scheme, this high-fidelity result also reveals a promising but rarely explored parameter space uniquely suitable for gate operations between fluxonium qubits.

Authors:Yi-Xi Zhang, Zhen-Tao Zhang, Xiao-Zhi Wei, Bao-Long Liang, Feng Mei, Zhen-Shan Yang

Abstract: The evolution of entanglement in a non-Hermitian quantum system may behave differently compared to its Hermitian counterpart. In this paper, we investigate the entanglement dynamics of two coupled and driven non-Hermitian qubits. Through calculating the concurrence of the system, we find that the evolution of the bipartite entanglement manifests two distinct patterns in the parameter space. In the low non-Hermiticity regime, the concurrence oscillates significantly, while in the opposite regime the same quantity would trend to a stable value. We attribute this phenomenon to parity-time ($ \mathcal{PT}$) symmetry phase transition. In addition, we have also studied the effect of decoherence on the entanglement dynamics. Our research provides a method to stabilize entanglement by exploiting non-Hermiticity.

Authors:Kenza Hammam, Gonzalo Manzano, Gabriele De Chiara

Abstract: Non-equilibrium effects may have a profound impact on the performance of thermal devices performing thermodynamic tasks such as refrigeration or heat pumping. The possibility of enhancing the performance of thermodynamic operations by means of quantum coherence is of particular interest but requires an adequate characterization of heat and work at the quantum level. In this work, we demonstrate that the presence of even small amounts of coherence in the thermal reservoirs powering a three-terminal machine, enables the appearance of combined and hybrid modes of operation, where either different resources are combined to perform a single thermodynamic task, or more than one task is performed at the same time. We determine the performance of such coherence-enabled modes of operation obtaining their power and efficiency and discussing the beneficial or detrimental roles of coherence.

Authors:Ju-Yeon Gyhm, Dario Rosa, Dominik Šafránek

Abstract: We introduce a quantum charging distance as the minimal time that it takes to reach one state (charged state) from another state (depleted state) via a unitary evolution, assuming limits on the resources invested into the charging. We show that for pure states it is equal to the Bures angle, while for mixed states, its computation leads to an optimization problem. Thus, we also derive easily computable bounds on this quantity. The charging distance tightens the known bound on the mean charging power of a quantum battery, it quantifies the quantum charging advantage, and it leads to an always achievable quantum speed limit. In contrast with other similar quantities, the charging distance does not depend on the eigenvalues of the density matrix, it depends only on the corresponding eigenspaces. This research formalizes and interprets quantum charging in a geometric way, and provides a measurable quantity that one can optimize for to maximize the speed of charging of future quantum batteries.

Authors:Giovanni Scala, Seyed Arash Ghoreishi, Marcin Pawłowski

Abstract: Paw\l owski and Winter's Hyperbit Theory, proposed in 2012, presented itself as a captivating alternative to quantum theory, suggesting novel ways of redefining entanglement and classical communication paradigms. This research undertakes a meticulous reevaluation of Hyperbit Theory, uncovering significant operational constraints that question its equivalence with quantum mechanics. Crucially, the supposition that Hyperbit Theory and Quantum Theory are equivalent relies on the receiver having unattainable additional knowledge about the sender's laboratory, indicating that the work by Paw\l owski and Winter is incorrect. This study accentuates the constraints of hyperbits in information processing and sheds light on the superiority of quantum communication, thereby advancing the investigation at the intersection of classical and quantum communication.

Authors:Javier Gonzalez-Conde, Andrew T. Sornborger

Abstract: We study the quantum simulation of mixed quantum-semiclassical (MQS) systems, of fundamental interest in many areas of physics, such as molecular scattering and gravitational backreaction. A basic question for these systems is whether quantum algorithms of MQS systems would be valuable at all, when one could instead study the full quantum-quantum system. We study MQS simulations in the context where a semiclassical system is encoded in a Koopman-von Neumann (KvN) Hamiltonian and a standard quantum Hamiltonian describes the quantum system. In this case, because KvN and quantum Hamiltonians are constructed with the same operators on a Hilbert space, standard theorems guaranteeing simulation efficiency apply. We show that, in this context, $\textit{many-body}$ MQS particle simulations give only nominal improvements in qubit resources over quantum-quantum simulations due to logarithmic scaling in the ratio, $S_q/S_c$, of actions between quantum and semiclassical systems. However, $\textit{field}$ simulations can give improvements proportional to the ratio of quantum to semiclassical actions, $S_q/S_c$. Of particular note, due to the ratio $S_q/S_c \sim 10^{-18}$ of particle and gravitational fields, this approach could be important for semiclassical gravity. We demonstrate our approach in a model of gravitational interaction, where a harmonic oscillator mediates the interaction between two spins. In particular, we demonstrate a lack of distillable entanglement generation between spins due to classical mediators, a distinct difference in dynamics relative to the fully quantum case.

Authors:Lei Du, Lingzhen Guo, Yan Zhang, Anton Frisk Kockum

Abstract: Giant emitters derive their name from nonlocal field-emitter interactions and feature diverse self-interference effects. Most of the existing works on giant emitters have considered Hermitian waveguides or photonic lattices. In this work, we unveil how giant emitters behave if they are coupled to a non-Hermitian bath, i.e., a Hatano-Nelson (HN) model which features a non-Hermitian skin effect due to the asymmetric inter-site tunneling rates. We show that the behaviors of the giant emitters are closely related to the stability of the bath. In the convectively unstable regime, where the HN model can be mapped to a pseudo-Hermitian lattice, a giant emitter can either behave as in a Hermitian bath or undergo excitation amplification, depending on the relative strength of different emitter-bath coupling paths. Based on this mechanism, we can realize protected nonreciprocal interactions between giant emitters, with nonreciprocity opposite to that of the bath. Such giant-emitter effects are not allowed, however, if the HN model enters the absolutely unstable regime, where the coupled emitters always show secular energy growth. Our proposal provides a new paradigm of non-Hermitian quantum optics, which may be useful for, e.g., engineering effective interactions between quantum emitters and performing many-body simulations in the non-Hermitian framework.

Authors:Wai-Keong Mok, Hui Zhang, Tobias Haug, Xianshu Luo, Guo-Qiang Lo, Hong Cai, M. S. Kim, Ai Qun Liu, Leong-Chuan Kwek

Abstract: Reducing noise in quantum systems is a major challenge towards the application of quantum technologies. Here, we propose and demonstrate a scheme to reduce noise using a quantum autoencoder with rigorous performance guarantees. The quantum autoencoder learns to compresses noisy quantum states into a latent subspace and removes noise via projective measurements. We find various noise models where we can perfectly reconstruct the original state even for high noise levels. We apply the autoencoder to cool thermal states to the ground state and reduce the cost of magic state distillation by several orders of magnitude. Our autoencoder can be implemented using only unitary transformations without ancillas, making it immediately compatible with the state of the art. We experimentally demonstrate our methods to reduce noise in a photonic integrated circuit. Our results can be directly applied to make quantum technologies more robust to noise.

Authors:Ke Huang, Xiao Li, David A. Huse, Amos Chan

Abstract: We study out-of-time-order correlators (OTOCs) of local operators in spatial-temporal invariant or random quantum circuits using light-like generators (LLG) -- many-body operators that exist in and act along the light-like directions. We demonstrate that the OTOC can be approximated by the leading singular value of the LLG, which, for the case of generic many-body chaotic circuits, is increasingly accurate as the size of the LLG, $w$, increases. We analytically show that the OTOC has a decay with a universal form in the light-like direction near the causal light cone, as dictated by the sub-leading eigenvalues of LLG, $z_2$, and their degeneracies. Further, we analytically derive and numerically verify that the sub-leading eigenvalues of LLG of any size can be accessibly extracted from those of LLG of the smallest size, i.e., $z_2(w)= z_2(w=1)$. Using symmetries and recursive structures of LLG, we propose two conjectures on the universal aspects of generic many-body quantum chaotic circuits, one on the algebraic degeneracy of eigenvalues of LLG, and another on the geometric degeneracy of the sub-leading eigenvalues of LLG. As corollaries of the conjectures, we analytically derive the asymptotic form of the leading singular state, which in turn allows us to postulate and efficiently compute a product-state variational ansatz away from the asymptotic limit. We numerically test the claims with four generic circuit models of many-body quantum chaos, and contrast these statements against the cases of a dual unitary system and an integrable system.

Authors:Fumiya Hanamura, Warit Asavanant, Seigo Kikura, Moeto Mishima, Shigehito Miki, Hirotaka Terai, Masahiro Yabuno, Fumihiro China, Kosuke Fukui, Mamoru Endo, Akira Furusawa

Abstract: Uncertainty principle prohibits the precise measurement of both components of displacement parameters in phase space. We have theoretically shown that this limit can be beaten using single-photon states, in a single-shot and single-mode setting [F. Hanamura et al., Phys. Rev. A 104, 062601 (2021)]. In this paper, we validate this by experimentally beating the classical limit. In optics, this is the first experiment to estimate both parameters of displacement using non-Gaussian states. This result is related to many important applications, such as quantum error correction.

Authors:Emily A Van Milligen, Eliana Jacobson, Ashlesha Patil, Gayane Vardoyan, Don Towsley, Saikat Guha

Abstract: Quantum networks will be able to service consumers with long distance entanglement by use of repeater nodes that can both generate external Bell pairs with their neighbors, iid with probability $p$, as well as perform internal Bell State Measurements (BSMs) which succeed with some probability $q$. The actual values of these probabilities is dependent upon the experimental parameters of the network in question. While global link state knowledge is needed to maximize the rate of entanglement generation between any two consumers, this may be an unreasonable request due to the dynamic nature of the network. This work evaluates a local link state knowledge, multi-path routing protocol that works with time multiplexed repeaters that are able to perform BSMs across different time steps. This study shows that the average rate increases with the time multiplexing block length, $k$, although the initial latency also increases. When a step function memory decoherence model is introduced so that qubits are held in the quantum memory for a time exponentially distributed with mean $\mu$, an optimal $k$ ($k_\text{opt}$) value appears. As $p$ decreases or $\mu$ increases the value of $k_\text{opt}$ increases. This value is such that the benefits from time multiplexing are balanced with the increased risk of losing a previously established entangled pair.

Authors:Olaf Lechtenfeld, Miloslav Znojil

Abstract: In the conventional Schr\"{o}dinger's formulation of quantum mechanics the unitary evolution of a state $\psi$ is controlled, in Hilbert space ${\cal L}$, by a Hamiltonian $\mathfrak{h}$ which must be self-adjoint. In the recent, ``quasi-Hermitian'' reformulation of the theory one replaces $\mathfrak{h}$ by its isospectral but non-Hermitian avatar $H = \Omega^{-1}\mathfrak{h}\Omega$ with $\Omega^\dagger\Omega = \Theta \neq I$. Although acting in another, manifestly unphysical Hilbert space ${\cal H}$, the amended Hamiltonian $H \neq H^\dagger$ can be perceived as self-adjoint with respect to the amended inner-product metric $\Theta$. In our paper motivated by a generic technical ``user-unfriendliness'' of the non-Hermiticity of $H$ we introduce and describe a specific new family of Hamiltonians $H$ for which the metrics $\Theta$ become available in closed form.

Authors:Kazuto Oshima

Abstract: We propose a procedure to extract vacuum expectation values from approximate vacuum prepared by the adiabatic quantum computation. We use plural ancilla bits with hierarchical structure, intending to gradually put up approximate precision. We exhibit simulation results for a typical one-qubit system and a two-qubits system based on the (1+1)-dimensional Schwinger model using classically emulated digital quantum simulator.

Authors:Philipp Stammer

Abstract: The recent progress in the quantum optical formulation of the process of high harmonic generation has reached a point where the successful semi-classical model reaches its limitations. Until recently the light source which drives the process was considered to be provided by a laser, in agreement with the classical picture. However, quantum optics allows to consider light fields beyond the classical realm, such as bright squeezed vacuum or Fock states. Both field states have vanishing electric field amplitudes, but can still lead to the generation of high harmonic radiation for sufficiently high intensities. This poses new questions about the range of validity of the semi-classical picture, which is the matter discussed here.

Authors:Takuya Hatomura

Abstract: The Trotter decomposition is a basic approach to Hamiltonian simulation (digital quantum simulation). The first-order Trotter decomposition is the simplest one, whose deviations from target dynamics are of the first order of a small coefficient in terms of the infidelity. In this paper, we consider the first-order Trotter decomposition in the dynamical-invariant basis. By using a state-dependent inequality, we point out that deviations of this decomposition are of the second order of a small coefficient. Moreover, we also show that this decomposition includes a useful example, i.e., digital implementation of shortcuts to adiabaticity by counterdiabatic driving.

Authors:Leonid P. Pryadko, Vadim A. Shabashov, Valerii K. Kozin

Abstract: The GAP package QDistRnd implements a probabilistic algorithm for finding the minimum distance of a quantum low-density parity-check code linear over a finite field GF(q). At each step several codewords are randomly drawn from a distribution biased toward smaller weights. The corresponding weights are used to update the upper bound on the distance, which eventually converges to the minimum distance of the code. While there is no performance guarantee, an empirical convergence criterion is given to estimate the probability that a minimum weight codeword has been found. In addition, a format for storing matrices associated with q-ary quantum codes is introduced and implemented via the provided import/export functions. The format, MTXE, is based on the well established MaTrix market eXchange (MTX) Coordinate format developed at NIST, and is designed for full backward compatibility with this format. Thus, MTXE files are readable by any software package which supports MTX.

Authors:Abhishek Banerjee, Pratapaditya Bej, Somshubhro Bandyopadhyay

Abstract: We study the quantum change point problem within the paradigm of local operations and classical communication (LOCC). Specifically, we consider a source that emits entangled pairs in a default state but undergoes mutation at some stage and begins producing an orthogonal entangled state. A sequence of entangled pairs prepared from such a source and shared between distant observers cannot be used for quantum information processing tasks as the identity of each entangled pair remains unknown. Assuming every point of a given sequence is equally likely to be the change point, including the possibility that no change occurs, we present a pretty-good LOCC protocol that identifies the change point and distills free entangled pairs. Next, we consider a variation of this problem where the source switches to an unknown entangled state that belongs to a known set. Here we show the local distinguishability of the collection of states, containing the default and all possible mutations, plays a crucial role: if they are locally distinguishable, the problem reduces to the previous one, but if not, one may still identify the mutated state, the change point, and distill entanglement, as we illustrate with a concrete example.

Authors:Ilya A. Simakov, Grigoriy S. Mazhorin, Ilya N. Moskalenko, Seidali S. Seidov, Ilya S. Besedin

Abstract: The Toffoli gate takes a special place in the quantum information theory. It opens up a path for efficient implementation of complex quantum algorithms. Despite tremendous progress of the quantum processors based on the superconducting qubits, realization of a high-fidelity three-qubit operation is still a challenging problem. Here, we propose a novel way to perform a high-fidelity CCZ gate on fluxoniums capacitively connected via a transmon qubit, activated by a microwave pulse on the coupler. The main advantages of the approach are relative quickness, simplicity of calibration and significant suppression of the unwanted longitudinal ZZ interaction. We provide numerical simulation of 95-ns long gate of higher than 99.99% fidelity with realistic circuit parameters in the noiseless model and estimate an error of about 0.25% under the conventional decoherence rates.

Authors:Bo Wang, Franco Nori, Ze-Liang Xiang

Abstract: In this letter, we investigate the ground state properties of an optomechanical system consisting of a coupled cavity and mechanical modes. An exact solution is given when the ratio $\eta$ between the cavity and mechanical frequencies tends to infinity. This solution reveals a coherent photon occupation in the ground state by breaking continuous or discrete symmetries, exhibiting an equilibrium quantum phase transition (QPT). In the $U(1)$-broken phase, an unstable Goldstone mode can be excited. In the model featuring $Z_2$ symmetry, we discover the mutually (in the finite $\eta$) or unidirectionally (in $\eta \rightarrow \infty$) dependent relation between the squeezed vacuum of the cavity and mechanical modes. In particular, when the cavity is driven by a squeezed field along the required squeezing parameter, it enables modifying the region of $Z_2$-broken phase and significantly reducing the coupling strength to reach QPTs. Furthermore, by coupling atoms to the cavity mode, the hybrid system can undergo a QPT at a hybrid critical point, which is cooperatively determined by the optomechanical and light-atom systems. These results suggest that this optomechanical system complements other phase transition models for exploring novel critical phenomena.

Authors:Axel M. Eriksson, Théo Sépulcre, Mikael Kervinen, Timo Hillmann, Marina Kudra, Simon Dupouy, Yong Lu, Maryam Khanahmadi, Jiaying Yang, Claudia Castillo Moreno, Per Delsing, Simone Gasparinetti

Abstract: Linear bosonic modes offer a hardware-efficient alternative for quantum information processing but require access to some nonlinearity for universal control. The lack of nonlinearity in photonics has led to encoded measurement-based quantum computing, which rely on linear operations but requires access to resourceful ('nonlinear') quantum states, such as cubic phase states. In contrast, superconducting microwave circuits offer engineerable nonlinearities but suffer from static Kerr nonlinearity. Here, we demonstrate universal control of a bosonic mode composed of a superconducting nonlinear asymmetric inductive element (SNAIL) resonator, enabled by native nonlinearities in the SNAIL element. We suppress static nonlinearities by operating the SNAIL in the vicinity of its Kerr-free point and dynamically activate nonlinearities up to third order by fast flux pulses. We experimentally realize a universal set of generalized squeezing operations, as well as the cubic phase gate, and exploit them to deterministically prepare a cubic phase state in 60 ns. Our results initiate the experimental field of universal continuous-variables quantum computing.

Authors:Felipe Mahlow, Barış Çakmak, Göktuğ Karpat, İskender Yalçınkaya, Felipe Fanchini

Abstract: We have applied a machine learning algorithm to predict the emergence of environment-induced spontaneous synchronization between two qubits in an open system setting. In particular, we have considered three different models, encompassing global and local dissipation regimes, to describe the open system dynamics of the qubits. We have utilized the $k$-nearest neighbors algorithm to estimate the long time synchronization behavior of the qubits only using the early time expectation values of qubit observables in these three distinct models. Our findings clearly demonstrate the possibility of determining the occurrence of different synchronization phenomena with high precision even at the early stages of the dynamics using a machine learning-based approach. Moreover, we show the robustness of our approach against potential measurement errors in experiments by considering random errors in qubit expectation values. We believe that the presented results can prove to be useful in experimental studies on the determination of quantum synchronization.

Authors:Shubhayan Sarkar

Abstract: Quantum steering is an asymmetric form of quantum nonlocality where one can trust the measurements of one of the parties. In this work, inspired by practical considerations we investigate the scenario if one can not fully trust their measurement devices but only up to some precision. We first find the effect of such an imprecision on standard device-dependent quantum tomography. We then utilise this result to compute the variation in the local bound of any general steering inequality depending on the amount of trust one puts in one of the party's measurement devices. This is particularly important as we show that even a small distrust on Alice might cause the parties to observe steerability even if the quantum state is unsteerable. Furthermore, this effect becomes more relevant when observing higher dimensional quantum steering.

Authors:Armin Tavakoli

Abstract: We study quantum steering experiments without assuming that the trusted party can perfectly control their measurement device. Instead, we introduce a scenario in which these measurements are subject to small imprecision. We show that small measurement imprecision can have a large detrimental influence in terms of false positives for steering inequalities, and that this effect can become even more relevant for high-dimensional systems. We then introduce a method for taking generic measurement imprecision into account in tests of bipartite steering inequalities. The revised steering bounds returned by this method are analytical, easily computable, and are even optimal for well-known families of arbitrary-dimensional steering tests. Furthermore, it applies equally well to generalised quantum steering scenarios, where the shared quantum state does not need to be separable, but is instead limited by some other entanglement property.

Authors:Zhi-Xiang Tang, Xun-Wei Xu

Abstract: Optomechanical systems with parametric coupling between optical (photon) and mechanical (phonon) modes provide a useful platform to realize various magnetic-free nonreciprocal devices, such as isolators, circulators, and directional amplifiers. However, nonreciprocal router with multiaccess channels has not been extensively studied yet. Here, we propose a nonreciprocal router with one transmitter, one receiver, and two output terminals, based on an optomechanical plaquette composing of two optical modes and two mechanical modes. The time-reversal symmetry of the system is broken via synthetic magnetism induced by driving the two optical modes with phase-correlated laser fields. The prerequisites for nonreciprocal routing are obtained both analytically and numerically, and the robustness of the nonreciprocity is demonstrated numerically. Multi-terminal nonreciprocal router in optomechanical plaquette provides a useful quantum node for development of quantum network information security and realization of quantum secure communication.

Authors:Frederik vom Ende

Abstract: Given any pair of completely positive, trace-preserving maps $\Phi_1,\Phi_2$ such that at least one of them has Kraus rank one, as well as any respective Stinespring isometries $V_1,V_2$, we prove that there exists a unitary $U$ on the environment such that $\|V_1-({\bf1}\otimes U)V_2\|_\infty\leq\sqrt{2\|\Phi_1-\Phi_2\|_\diamond}$. Moreover, we provide a simple example which shows that the factor $\sqrt2$ on the right-hand side is optimal, and we conjecture that this inequality holds for every pair of channels.

Authors:Lifeng Zhang, Zhihua Chen, Shao-Ming Fei

Abstract: Quantum entanglement lies at the heart in quantum information processing tasks. Although many criteria have been proposed, efficient and scalable methods to detect the entanglement of generally given quantum states are still not available yet, particularly for high-dimensional and multipartite quantum systems. Based on FixMatch and Pseudo-Label method, we propose a deep semi-supervised learning model with a small portion of labeled data and a large portion of unlabeled data. The data augmentation strategies are applied in this model by using the convexity of separable states and performing local unitary operations on the training data. We verify that our model has good generalization ability and gives rise to better accuracies compared to traditional supervised learning models by detailed examples.

Authors:Gurpahul Singh, Ritesh K. Singh, Soumitro Banerjee

Abstract: The problem of how measurement in quantum mechanics takes place has existed since its formulation. Von Neumann proposed a scheme where he treated measurement as a two-part process -- a unitary evolution in the full system-ancilla space and then a projection onto one of the pointer states of the ancilla (representing the "collapse" of the wavefunction). The Lindblad master equation, which has been extensively used to explain dissipative quantum phenomena in the presence of an environment, can effectively describe the first part of the von Neumann measurement scheme when the jump operators in the master equation are Hermitian. We have proposed a non-Hermitian Hamiltonian formalism to emulate the first part of the von Neumann measurement scheme. We have used the embedding protocol to dilate a non-Hermitian Hamiltonian that governs the dynamics in the system subspace into a higher-dimensional Hermitian Hamiltonian that evolves the full space unitarily. We have obtained the various constraints and the required dimensionality of the ancilla Hilbert space in order to achieve the required embedding. Using this particular embedding and a specific projection operator, one obtains non-Hermitian dynamics in the system subspace that closely follow the Lindblad master equation. This work lends a new perspective to the measurement problem by employing non-Hermitian Hamiltonians.

Authors:Nhat A. Nghiem

Abstract: Geometry and topology have generated impacts far beyond their pure mathematical primitive, providing a solid foundation for many applicable tools. Typically, real-world data are represented as vectors, forming a linear subspace for a given data collection. Computing distances between different subspaces is generally a computationally challenging problem with both theoretical and applicable consequences, as, for example, the results can be used to classify data from different categories. Fueled by the fast-growing development of quantum algorithms, we consider such problems in the quantum context and provide a quantum algorithm for estimating two kinds of distance: Grassmann distance and ellipsoid distance. Under appropriate assumptions and conditions, the speedup of our quantum algorithm is exponential with respect to both the dimension of the given data and the number of data points. Some extensions regarding estimating different kinds of distance are then discussed as a corollary of our main quantum algorithmic method.

Authors:Jhih-Yuan Kao, Hsi-Sheng Goan

Abstract: The Pauli stabilizer formalism is perhaps the most thoroughly studied means of procuring quantum error-correcting codes, whereby the code is obtained through commutative Pauli operators and ``stabilized'' by them. In this work we will show that every quantum error-correcting code, including Pauli stabilizer codes and subsystem codes, has a similar structure, in that the code can be stabilized by commutative ``Paulian'' operators which share many features with Pauli operators and which form a \textbf{Paulian stabilizer group}. By facilitating a controlled gate we can measure these Paulian operators to acquire the error syndrome. Examples concerning codeword stabilized codes and bosonic codes will be presented; specifically, one of the examples has been demonstrated experimentally and the observable for detecting the error turns out to be Paulian, thereby showing the potential utility of this approach. This work provides a possible approach to implement error-correcting codes and to find new codes.

Authors:Germain Tobar, Sreenath K. Manikandan, Thomas Beitel, Igor Pikovski

Abstract: The quantization of gravity is widely believed to result in gravitons -- particles of discrete energy that form gravitational waves. But their detection has so far been considered impossible. Here we show that signatures of single gravitons can be observed in laboratory experiments. We show that stimulated and spontaneous single-graviton processes can become relevant for massive quantum acoustic resonators and that stimulated absorption can be resolved through continuous sensing of quantum jumps. We analyze the feasibility of observing the exchange of single energy quanta between matter and gravitational waves. Our results show that single graviton signatures are within reach of experiments. In analogy to the discovery of the photo-electric effect for photons, such signatures can provide the first experimental evidence of the quantization of gravity.

Authors:Naphan Benchasattabuse, Andreas Bärtschi, Luis Pedro García-Pintos, John Golden, Nathan Lemons, Stephan Eidenbenz

Abstract: The quantum alternating operator ansatz (QAOA) is a heuristic hybrid quantum-classical algorithm for finding high-quality approximate solutions to combinatorial optimization problems, such as Maximum Satisfiability. While QAOA is well-studied, theoretical results as to its runtime or approximation ratio guarantees are still relatively sparse. We provide some of the first lower bounds for the number of rounds (the dominant component of QAOA runtimes) required for QAOA. For our main result, (i) we leverage a connection between quantum annealing times and the angles of QAOA to derive a lower bound on the number of rounds of QAOA with respect to the guaranteed approximation ratio. We apply and calculate this bound with Grover-style mixing unitaries and (ii) show that this type of QAOA requires at least a polynomial number of rounds to guarantee any constant approximation ratios for most problems. We also (iii) show that the bound depends only on the statistical values of the objective functions, and when the problem can be modeled as a $k$-local Hamiltonian, can be easily estimated from the coefficients of the Hamiltonians. For the conventional transverse field mixer, (iv) our framework gives a trivial lower bound to all bounded occurrence local cost problems and all strictly $k$-local cost Hamiltonians matching known results that constant approximation ratio is obtainable with constant round QAOA for a few optimization problems from these classes. Using our novel proof framework, (v) we recover the Grover lower bound for unstructured search and -- with small modification -- show that our bound applies to any QAOA-style search protocol that starts in the ground state of the mixing unitaries.

Authors:Michele Avalle, Alessio Serafini

Abstract: We consider scattering processes where a quantum system is comprised of an inner subsystem and of a boundary, and is subject to Haar-averaged random unitaries acting on the boundary-environment Hilbert space only. We show that, regardless of the initial state, a single scattering event will disentangle the unconditional state (i.e., the scattered state when no information about the applied unitary is available) across the inner subsystem-boundary partition. Also, we apply Levy's lemma to constrain the trace norm fluctuations around the unconditional state. Finally, we derive analytical formulae for the mean scattered purity for initial globally pure states, and provide one with numerical evidence of the reduction of fluctuations around such mean values with increasing environmental dimension.

Authors:Alejandro Gomez Cadavid, Iraitz Montalban, Archismita Dalal, Enrique Solano, Narendra N. Hegade

Abstract: We propose a faster digital quantum algorithm for portfolio optimization using the digitized-counterdiabatic quantum optimization (DCQO) paradigm in the impulse regime, that is, where the counterdiabatic terms are dominant. Our approach notably reduces the circuit depth requirement of the algorithm and enhances the solution accuracy, making it suitable for current quantum processors. We apply this protocol to a real-case scenario of portfolio optimization with 20 assets, using purely quantum and hybrid classical-quantum paradigms. We experimentally demonstrate the advantages of our protocol using up to 20 qubits on an IonQ trapped-ion quantum computer. By benchmarking our method against the standard quantum approximate optimization algorithm and finite-time digitized-adiabatic algorithms, we obtain a significant reduction in the circuit depth by factors of 2.5 to 40, while minimizing the dependence on the classical optimization subroutine. Besides portfolio optimization, the proposed method is applicable to a large class of combinatorial optimization problems.

Authors:Chunfeng Huang, Ye Chen, Tingting Luo, Wenjie He, Xin Liu, Zhenrong Zhang, Kejin Wei

Abstract: Quantum Key Distribution (QKD) is a physical layer encryption technique that enables two distant parties to exchange secure keys with information-theoretic security. In the last two decades, QKD has transitioned from laboratory research to real-world applications, including multi-user quantum access networks (QANs). This network structure allows users to share single-photon detectors at a network node through time-division multiplexing, thereby significantly reducing the network cost. However, current QAN implementations require additional hardware for auxiliary tasks such as time synchronization. To address this issue, we propose a cost-efficient QAN that uses qubit-based synchronization. In this approach, the transmitted qubits facilitate time synchronization, eliminating the need for synchronization hardware. We tested our scheme by implementing a network for two users and successfully achieved average secure key rates of $53.84$ kbps and $71.90$ kbps for each user over a 50-km commercial fiber spool. In addition, we investigated the capacity of the access network under cross-talk and loss conditions. The simulation results demonstrate that this scheme can support a QAN with 64 users with key rates up to 1070~bps. Our work provides a feasible and cost-effective way to implement a multi-user QKD network, further promoting the widespread application of QKD.

Authors:Tanya Sharma, Ayan Biswas, Jayanth Ramakrishnan, Pooja Chandravanshi, Ravindra P. Singh

Abstract: Quantum key distribution (QKD) theoretically offers unconditional security. Unfortunately, the gap between theory and practice threatens side-channel attacks on practical QKD systems. Many well-known QKD protocols use weak coherent laser pulses to encode the quantum information. These sources differ from ideal single photon sources and follow Poisson statistics. Many protocols, such as decoy state and coincidence detection protocols, rely on monitoring the photon statistics to detect any information leakage. The accurate measurement and characterization of photon statistics enable the detection of adversarial attacks and the estimation of secure key rates, strengthening the overall security of the QKD system. We have rigorously characterized our source to estimate the mean photon number employing multiple detectors for comparison against measurements made with a single detector. Furthermore, we have also studied intensity fluctuations to help identify and mitigate any potential information leakage due to state preparation flaws. We aim to bridge the gap between theory and practice to achieve information-theoretic security.

Authors:J. L. Alonso, C. Bouthelier-Madre, J. Clemente-Gallardo, D. Martínez-Crespo, J. Pomar

Abstract: The definition of a consistent evolution equation for statistical hybrid quantum-classical systems is still an open problem. In this paper we analyze the case of Ehrenfest dynamics on systems defined by a probability density and identify the relations of the non-linearity of the dynamics with the obstructions to define a consistent dynamics for the first quantum moment of the distribution. This first quantum moment represents the physical states as a family of classically-parametrized density matrices $\hat \rho(\xi)$, for $\xi$ a classical point; and it is the most common representation of hybrid systems in the literature. Due to this obstruction, we consider higher order quantum moments, and argue that only a finite number of them are physically measurable. Because of this, we propose an effective solution for the hybrid dynamics problem based on approximating the distribution by those moments and representing the states by them.

Authors:Reo Shikanai, Masayuki Ohzeki, Kazuyuki Tanaka

Abstract: The quantum annealing machine manufactured by D-Wave Systems is expected to find the optimal solution for QUBO (Quadratic Unconstrained Binary Optimization) accurately and quickly. This would be useful in future applications where real-time calculation is needed. One such application is traffic signal optimization. Some studies use quantum annealing for this. However, they are formulated in unrealistic settings, such as only crossroads on the map. Therefore, we suggest a QUBO, which can deal with T-junctions and multi-forked roads. To validate the efficiency of our approach, SUMO (Simulation of Urban MObility) is used. This enables us to experiment with geographic information data very close to the real world. We compared results with those using the Gurobi Optimizer in the experiment to confirm that quantum annealing can find a ground state. The results show that the quantum annealing cannot find the ground state, but our model can reduce the time that vehicles wait at a red light. It is also inferior to the Gurobi Optimizer in calculation time. This seems to be due to the D-Wave machine's hardware limitations and noise effects, such as ambient temperature. If these problems are solved, and the number of qubits is increased, the use of quantum annealing is likely to be superior in terms of the speed of calculating an optimal solution.

Authors:Tomohiro Fujita, Youka Kaku, Akira Matumura, Yuta Michimura

Abstract: In the quest for quantum gravity, we have lacked experimental verification, hampered by the weakness of gravity and decoherence. Recently, various experiments have been proposed to verify quantum entanglement induced by Newtonian gravitational interactions. However, they are not yet certainly feasible with existing techniques. To search for a new setup, we compute the logarithmic negativity of two oscillators with arbitrary quadratic potential coupled by gravity. We find that unstable inverted oscillators generate gravity-induced entanglement most quickly and are most resistant to decoherence from environmental fluctuations. As an experimental realization, we propose a setup of the optical levitation of mirrors with the anti-spring effect. To avoid decoherence due to photon shot noise, a sandwich configuration that geometrically creates the anti-spring is promising.

Authors:Steffen Wilksen, Frederik Lohof, Isabell Willmann, Frederik Bopp, Michelle Lienhart, Christopher Thalacker, Jonathan Finley, Matthias Florian, Christopher Gies

Abstract: Electrically controllable quantum-dot molecules (QDMs) are a promising platform for deterministic entanglement generation and, as such, a resource for quantum-repeater networks. We develop a microscopic open-quantum-systems approach based on a time-dependent Bloch-Redfield equation to model the generation of entangled spin states with high fidelity. The state preparation is a crucial step in a protocol for deterministic entangled-photon-pair generation that we propose for quantum repeater applications. Our theory takes into account the quantum-dot molecules' electronic properties that are controlled by time-dependent electric fields as well as dissipation due to electron-phonon interaction. We quantify the transition between adiabatic and non-adiabatic regimes, which provides insights into the dynamics of adiabatic control of QDM charge states in the presence of dissipative processes. From this, we infer the maximum speed of entangled-state preparation under different experimental conditions, which serves as a first step towards simulation of attainable entangled photon-pair generation rates. The developed formalism opens the possibility for device-realistic descriptions of repeater protocol implementations.

Authors:Gourab Bhanja, Devvrat Tiwari, Subhashish Banerjee

Abstract: Quantum Brownian motion serves as a fundamental paradigm for investigations in open quantum systems, where a harmonic oscillator interacts with a bosonic thermal bath. A generalized coupling of the environment to the harmonic oscillator system via both its position and momentum was developed in recent times. To this end, we take up this generalized model of quantum Brownian motion and study it from the perspective of quantum thermodynamics. The system of interest is envisaged as a quantum battery interacting with the bath acting as a charger (dissipation) mechanism. We probe into the problem of maximum work that can be extracted through such a system using ergotropy and its (in)-coherent parts along with the instantaneous and average powers of the battery. We examine the effect of bath temperature and momentum coupling on the charging-discharging behavior of the battery. A connection between the memory effects of the system with charging-discharging behavior is further explored.

Authors:Victor Bittencourt, Massimo Blasone, Gennaro Zanfardino

Abstract: By means of the Dirac bispinor formalism, we show that the state of a (massive) oscillating neutrino produced by a weak interaction process, is an hyperentangled state of flavor, chirality, and spin. Since chirality is not a conserved quantity, chiral oscillations take place and affects the flavor transition probability. By means of the complete complementarity relations, we analyze how correlations and coherence are redistributed in time between the different degrees-of-freedoms of the system. In a similar way, we consider a spin entangled lepton-antineutrino pair and describe the redistribution of the spin-spin entanglement into correlations between the other degrees-of-freedom. In both cases the effects of chiral oscillations are relevant in the non-relativistic regime. Our analysis provides a complete characterization of the quantum correlations involved in lepton-antineutrino pairs and in single particle neutrino evolution, and provides a further insight on possible routes to interpret and measure chiral oscillations.

Authors:Nicolas Maring, Andreas Fyrillas, Mathias Pont, Edouard Ivanov, Eric Bertasi, Mario Valdivia, Jean Senellart

Abstract: The integration of Quantum Computers (QC) within High-Performance Computing (HPC) environments holds significant promise for solving real-world problems by leveraging the strengths of both computational paradigms. However, the integration of a complex QC platform in an HPC infrastructure poses several challenges, such as operation stability in non-laboratory like environments, and scarce access for maintenance. Currently, only a few fully-assembled QCs currently exist worldwide, employing highly heterogeneous and cutting-edge technologies. These platforms are mostly used for research purposes, and often bear closer resemblance to laboratory assemblies rather than production-ready, stable, and consistently-performing turnkey machines. Moreover, public cloud services with access to such quantum computers are scarce and their availability is generally limited to few days per week. In November 2022, we introduced the first cloud-accessible general-purpose quantum computer based on single photons. One of the key objectives was to maintain the platform's availability as high as possible while anticipating seamless compatibility with HPC hosting environment. In this article, we describe the design and implementation of our cloud-accessible quantum computing platform, and demonstrate one nine availability (92 %) for external users during a six-month period, higher than most online services. This work lay the foundation for advancing quantum computing accessibility and usability in hybrid HPC-QC infrastructures.

Authors:Vjosa Blakaj, Michael M. Wolf

Abstract: The set of two-body reduced states of translation invariant, infinite quantum spin chains can be approximated from inside and outside using matrix product states and marginals of finite systems, respectively. These lead to hierarchies of algebraic approximations that become tight only in the limit of infinitely many auxiliary variables. We show that this is necessarily so for any algebraic ansatz by proving that the set of reduced states is not semialgebraic. We also provide evidence that additional elementary transcendental functions cannot lead to a finitary description.

Authors:Minjie Wang, Haole Jiao, Jiajin Lu, Wenxin Fan, Zhifang Yang, Mengqi Xi, Shujing Li, Hai Wang

Abstract: Duan-Lukin-Cirac-Zoller quantum repeater protocol provides a feasible scheme to implement long-distance quantum communication and large-scale quantum networks. The elementary link, namely the entanglement between two atomic ensembles, is a fundamental component of quantum repeater. For practical quantum repeater, it is required that the elementary link can be prepared with high yield and the spin waves stored in atoms can be efficiently converted into photons on demand. However, so far, such quantum repeater link has not been demonstrated in experiments. Here, we demonstrate a proof-of-principle multiplexed quantum repeater link by entangling two temporally multiplexed quantum memory. Compared with a single-mode link, the successful preparation rate of the multiplexed link is increased by one order of magnitude. By using the cavity-enhanced scheme, the on-demand retrieval efficiency of atomic spin waves is improved to 70%, which is beneficial for the subsequent entanglement swapping between adjacent links. The realization of temporally multiplexed quantum repeater link with high retrieval efficiency lays a foundation for the development of practical quantum networks.

Authors:Gideon Lee, Tony Jin, Yu-Xin Wang, Alexander McDonald, Aashish Clerk

Abstract: Despite its fully unitary dynamics, the bosonic Kitaev chain (BKC) displays key hallmarks of non-Hermitian physics including non-reciprocal transport and the non-Hermitian skin effect. Here we demonstrate another remarkable phenomena: the existence of an entanglement phase transition (EPT) in a variant of the BKC that occurs as a function of a Hamiltonian parameter g, and which coincides with a transition from a reciprocal to a non-reciprocal phase. As g is reduced below a critical value, the post-quench entanglement entropy of a subsystem of size l goes from a volume-law phase where it scales as l to a super-volume law phase where it scales like lN with N the total system size. This EPT occurs for a system undergoing purely unitary evolution and does not involve measurements, post-selection, disorder or dissipation. We derive analytically the entanglement entropy out of and at the critical point for the $l=1$ and $l/N \ll 1$ case.

Authors:Arkady Fedorov, N. Pradeep Kumar, Dat Thanh Le, Rohit Navarathna, Prasanna Pakkiam, Thomas M. Stace

Abstract: Building large-scale superconducting quantum circuits will require miniaturisation and integration of supporting devices including microwave circulators, which are currently bulky, stand-alone components. Here we report the realisation of a passive on-chip circulator which is made from a loop consisting of three tunnel-coupled superconducting islands, with DC-only control fields. We observe the effect of quasiparticle tunnelling, and we dynamically classify the system into different quasiparticle sectors. When tuned for circulation, the device exhibits strongly non-reciprocal 3-port scattering, with average on-resonance insertion loss of 2 dB, isolation of 14 dB, power reflectance of -11 dB, and a bandwidth of 200 MHz.

Authors:Wencheng Zhao, Tingting Chen, Ruyu Yang

Abstract: Quantum computing stands at the vanguard of science, focused on exploiting quantum mechanical phenomena like superposition and entanglement. Its goal is to create innovative computational models that address intricate problems beyond classical computers' capabilities. In the Noisy Intermediate-Scale Quantum (NISQ) era, developing algorithms for nonlinear function calculations on density matrices is of paramount importance. This project endeavors to design scalable algorithms for calculating power functions of mixed quantum states. This study introduces two algorithms based on the Hadamard Test and Gate Set Tomography. Additionally, a comparison of their computational outcomes is offered, accompanied by a meticulous assessment of errors inherent in the Gate Set Tomography-based approach.

Authors:X. L. He, Yong Lu, D. Q. Bao, Hang Xue, W. B. Jiang, Zhen Wang, A. F. Roudsari, Per Delsing, J. S. Tsai, Z. R. Lin

Abstract: Schr\"odinger cat states, quantum superpositions of macroscopically distinct classical states, are an important resource for quantum communication, quantum metrology and quantum computation. Especially, cat states in a phase space protected against phase-flip errors can be used as a logical qubit. However, cat states, normally generated in three-dimensional cavities, are facing the challenges of scalability and controllability. Here, we present a novel strategy to generate and store cat states in a coplanar superconducting circuit by the fast modulation of Kerr nonlinearity. At the Kerr-free work point, our cat states are passively preserved due to the vanishing Kerr effect. We are able to prepare a 2-component cat state in our chip-based device with a fidelity reaching 89.1% under a 96 ns gate time. Our scheme shows an excellent route to constructing a chip-based bosonic quantum processor.

Authors:Lei Chen, Isabella L. Giovannelli, Nadav Shaibe, Steven M. Anlage

Abstract: We explore the physics of quantum systems that suffer from partial decoherence, in the intermediate range between coherent quantum evolution and incoherent classical physics. It has been predicted that new physics and technology are enabled in this intermediate regime. In particular we explore the asymmetric transmission through an Aharonov-Bohm (AB) ring that supports a 3:1 asymmetry in transmission times, augmented with de-phasing features that act preferentially on the longer-lingering quantum waves. Such a device is realized as a microwave analogue quantum graph utilizing a gyrator to create the 3:1 transmission time delay asymmetry, along with both homogeneous and localized losses to mimic the effects of de-phasing in the analogous mesoscopic electron system. Measurements and simulations of this device demonstrate the required non-reciprocal transmission time delay, as well as an asymmetry in transmission probability. The measurements and simulations are performed in both the frequency domain, and in the time domain using wave packets. We demonstrate asymmetric transmission through the AB-ring graph as a function of loss/de-phasing in both simulation and experiment, in both the frequency- and time-domains, and compare to expectations for the corresponding quantum system. The results are consistent with the hypothesis that the transmission asymmetry and loss of detailed balance is an equilibrium property of the analogous mesoscopic quantum graph.

Authors:Maria Papageorgiou, Jose de Ramon, Charis Anastopoulos

Abstract: Pointlike systems coupled to quantum fields are often employed as toy models for measurements in quantum field theory. In this paper, we identify the field observables recorded by such models. We show that in models that work in the strong coupling regime, the apparatus is correlated with smeared field amplitudes, while in models that work in weak coupling the apparatus records particle aspects of the field, such as the existence of a particle-like time of arrival and resonant absorption. Then, we develop an improved field-detector interaction model, adapting the formalism of Quantum Brownian motion, that is exactly solvable. This model confirms the association of field and particle properties in the strong and weak coupling regimes, respectively. Further, it can also describe the intermediate regime, in which the field-particle characteristics `merge'. In contrast to standard perturbation techniques, this model also recovers the relativistic Breit-Wigner resonant behavior in the weak coupling regime. The modulation of field-particle-duality by a single tunable parameter is a novel feature that is, in principle, experimentally accessible.

Authors:Dragan Marković, Mihailo Čubrović

Abstract: We study chaotic dynamics and anomalous transport in a Bose-Hubbard chain in the semiclassical regime (the limit when the number of particles goes to infinity). We find that the system has mixed phase space with both regular and chaotic dynamics, even for long chains with up to hundred wells. The consequence of the mixed phase space is strongly anomalous diffusion in the space of occupation numbers, with a discrete set of transport exponents. After very long times the system crosses over to the hydrodynamic regime with normal diffusion. Anomalous transport is quite universal, almost completely independent of the parameters of the model (Coulomb interaction, chemical potential): it is mainly determined by the initial distribution of particles along the chain. We corroborate our findings by analytical arguments: scaling analysis for the anomalous regime and the Langevin equation for the normal diffusion regime.

Authors:Jayadev Vijayan, Johannes Piotrowski, Carlos Gonzalez-Ballestero, Kevin Weber, Oriol Romero-Isart, Lukas Novotny

Abstract: The ability to engineer cavity-mediated interactions has emerged as a powerful tool for the generation of non-local correlations and the investigation of non-equilibrium phenomena in many-body systems. Levitated optomechanical systems have recently entered the multi-particle regime, with promise for using arrays of massive strongly coupled oscillators for exploring complex interacting systems and sensing. Here, by combining advances in multi-particle optical levitation and cavity-based quantum control, we demonstrate, for the first time, programmable cavity-mediated interactions between nanoparticles in vacuum. The interaction is mediated by photons scattered by spatially separated particles in a cavity, resulting in strong coupling ($G_\text{zz}/\Omega_\text{z} = 0.238\pm0.005$) that does not decay with distance within the cavity mode volume. We investigate the scaling of the interaction strength with cavity detuning and inter-particle separation, and demonstrate the tunability of interactions between different mechanical modes. Our work paves the way towards exploring many-body effects in nanoparticle arrays with programmable cavity-mediated interactions, generating entanglement of motion, and using interacting particle arrays for optomechanical sensing.

Authors:Yunya Liu, Jiakun Liu, Jordan R. Raney, Pai Wang

Abstract: Variational quantum algorithms exploit the features of superposition and entanglement to optimize a cost function efficiently by manipulating the quantum states. They are suitable for noisy intermediate-scale quantum (NISQ) computers that recently became accessible to the worldwide research community. Here, we implement and demonstrate the numerical processes on the 5-qubit and 7-qubit quantum processors on the IBM Qiskit Runtime platform. We combine the commercial finite-element-method (FEM) software ABAQUS with the implementation of Variational Quantum Eigensolver (VQE) to establish an integrated pipeline. Three examples are used to investigate the performance: a hexagonal truss, a Timoshenko beam, and a plane-strain continuum. We conduct parametric studies on the convergence of fundamental natural frequency estimation using this hybrid quantum-classical approach. Our findings can be extended to problems with many more degrees of freedom when quantum computers with hundreds of qubits become available in the near future.

Authors:Emilio Annoni, Massimo Frigerio, Matteo G. A. Paris

Abstract: Quantum transport across discrete structures is a relevant topic of solid state physics and quantum information science, which can be suitably studied in the context of continuous-time quantum walks. The addition of phases degrees of freedom, leading to chiral quantum walks, can also account for directional transport on graphs with loops. We discuss criteria for quantum transport and study the enhancement that can be achieved with chiral quantum walks on chain-like graphs, exploring different topologies for the chain units and optimizing over the phases. We select three candidate structures with optimal performance and investigate their transport behaviour with Krylov reduction. While one of them can be reduced to a weighted line with minor couplings modulation, the other two are truly chiral quantum walks, with enhanced transport probability over long chain structures.

Authors:Yang Wang, Sven Ahrens

Abstract: We demonstrate that spin-dependent electron diffraction is possible for a smooth range transverse electron momenta in a two-photon Bragg scattering scenario of the Kapitza-Dirac effect. Our analysis is rendered possible by introducing a generalized specification for quantifying spin-dependent diffraction, yielding an optimization problem which is solved by making use of a Newton gradient iteration scheme. With this procedure, we investigate the spin-dependent effect for different transverse electron momenta and different laser polarizations of the standing light wave the Kapitza-Dirac scattering. The possibility for using arbitrary low transverse electron momenta, when setting up a spin-dependent Kapitza-Dirac experiment allows longer interaction times of the electron with the laser and therefore enables less constraining parameters for an implementation of the effect.

Authors:Li Li, Tong Liu, Xue-Yi Guo, He Zhang, Silu Zhao, Zhongcheng Xiang, Xiaohui Song, Yu-Xiang Zhang, Kai Xu, Heng Fan, Dongning Zheng

Abstract: Simulating the dynamics of open quantum systems is essential in achieving practical quantum computation and understanding novel nonequilibrium behaviors. However, quantum simulation of a many-body system coupled to an engineered reservoir has yet to be fully explored in present-day experiment platforms. In this work, we introduce engineered noise into a one-dimensional ten-qubit superconducting quantum processor to emulate a generic many-body open quantum system. Our approach originates from the stochastic unravellings of the master equation. By measuring the end-to-end correlation, we identify multiple steady states stemmed from a strong symmetry, which is established on the modified Hamiltonian via Floquet engineering. Furthermore, we find that the information saved in the initial state maintains in the steady state driven by the continuous dissipation on a five-qubit chain. Our work provides a manageable and hardware-efficient strategy for the open-system quantum simulation.

Authors:Nilakantha Meher, Eilon Poem, Tomáš Opatrný, Ofer Firstenberg, Gershon Kurizki

Abstract: By consensus, estimation of phase delay between interferometer arms may exhibit an error below the standard quantum (shot-noise) limit if the input is an entangled two-mode state, e.g., a N00N state. We show, by contrast, that such super-sensitive phase estimation is achievable by incoherent, e.g., thermal, input in an interferometer with Kerr-nonlinear two-mode coupler. Not less remarkably, the Heisenberg precision bound is attainable and even surpassed in such nonlinear interferometers even for small nonlinear phase-shifts per photon pair or for significant photon loss. Feasible mode couplers with giant Kerr nonlinearity that stems either from dipole-dipole interactions of Rydberg polaritons in a cold atomic gas, or from cavity-enhanced dispersive atom-field interactions, may exploit such effects to substantially advance interferometric phase microscopy using incoherent, faint light sources.

Authors:Harriet Apel, Nouédyn Baspin

Abstract: While LDPC codes have been demonstrated with desirable error correcting properties, this has come at a cost of diverging from the geometrical constraints of many hardware platforms. Viewing codes as the groundspace of a Hamiltonian, we consider engineering a simulation Hamiltonian reproducing some relevant features of the code. Techniques from Hamiltonian simulation theory are used to build a simulation of LDPC codes using only 2D nearest-neighbour interactions at the cost of an energy penalty polynomial in the system size. We derive guarantees for the simulation that allows us to approximately reproduce the ground state of the code Hamiltonian, approximating a $[[N, \Omega(\sqrt{N}), \Omega(\sqrt{N})]]$ code in 2D. The key ingredient is a new constructive tool to simulate an $l$-long interaction between two qubits by a 1D chain of $l$ nearest-neighbour interacting qubits using $\mathrm{poly}( l)$ interaction strengths. This is an exponential advantage over the existing gadgets for this routine which facilitates the first $\epsilon$-simulation of \emph{arbitrary sparse} Hamiltonian on $n$ qubits with a Hamiltonian on a 2D lattice of $O(n^2)$ qubits with interaction strengths scaling as $O\left(\mathrm{poly}(n,1/\epsilon)\right)$.

Authors:Gabriele Bressanini, Hyukjoon Kwon, M. S. Kim

Abstract: Gaussian boson sampling (GBS) is a promising candidate for an experimental demonstration of quantum advantage using photons. However, sufficiently large noise might hinder a GBS implementation from entering the regime where quantum speedup is achievable. Here, we investigate how thermal noise affects the classical intractability of generic quantum optical sampling experiments, GBS being a particular instance of the latter. We do so by establishing sufficient conditions for an efficient simulation to be feasible, expressed in the form of inequalities between the relevant parameters that characterize the system and its imperfections. We demonstrate that the addition of thermal noise has the effect of tightening the constraints on the remaining noise parameters, required to show quantum advantage. Furthermore, we show that there exist a threshold temperature at which any quantum sampling experiment becomes classically simulable, and provide an intuitive physical interpretation by relating this occurrence with the disappearance of the quantum state's non-classical properties.

Authors:Xiao Zheng, Ai-Ling Ji, Guo-Feng Zhang

Abstract: Recently,D.Mondal et.al[Phys. Rev. A. 95, 052117(2017)]creatively introduce a new interesting concept of reverse uncertainty relation which indicates that one cannot only prepare quantum states with joint small uncertainty, but also with joint great uncertainty for incompatible observables. However, the uncertainty upper bound they constructed cannot express the essence of this concept well, i.e., the upper bound will go to infinity in some cases even for incompatible observables. Here, we construct a new reverse uncertainty relation and successfully fix this "infinity" problem. Also, it is found that the reverse uncertainty relation and the normal uncertainty relation are the same in essential, and they both can be unified by the same theoretical framework. Moreover, taking advantage of this unified framework, one can construct a reverse uncertainty relation for multiple observables with any tightness required. Meanwhile, the application of the new uncertainty relation in purity detection is discussed.

Authors:Jie Xu, Xiao Zheng, Ai-Ling Ji, Guo-Feng Zhang

Abstract: Recently, Zheng constructs a quantum-control-assisted multipartite variance-based uncertainty relation, which successfully extends the conditional uncertainty relation to the multipartite case [Annalen der physik, 533, 2100014 (2021)]. We here investigate the dynamics of the new uncertainty relation in the Heisenberg system with the Dzyaloshinski-Moriya interaction. It is found that, different from entanglement, the mixedness of the system has an interesting single-valued relationship with the tightness and lower bound of the uncertainty relation. This single-valued relationship indicates that the tightness and lower bound of the uncertainty relation can be written as the functional form of the mixedness. Moreover, the single-valued relationship with the mixedness is the common nature of conditional uncertainty relations, and has no relationship with the form of the uncertainty relations. Also, the comparison between the new conditional variance-based uncertainty relation and the existing entropic one has been made.

Authors:Sergey R. Usmanov, Gleb V. Salakhov, Anton A. Bozhedarov, Evgeniy O. Kiktenko, Aleksey K. Fedorov

Abstract: Operation management of nuclear power plants consists of several computationally hard problems. Searching for an in-core fuel loading pattern is among them. The main challenge of this combinatorial optimization problem is the exponential growth of the search space with a number of loading elements. Here we study a reloading problem in a Quadratic Unconstrained Binary Optimization (QUBO) form. Such a form allows us to apply various techniques, including quantum annealing, classical simulated annealing, and quantum-inspired algorithms in order to find fuel reloading patterns for several realistic configurations of nuclear reactors. We present the results of benchmarking the in-core fuel management problem in the QUBO form using the aforementioned computational techniques. This work demonstrates potential applications of quantum computers and quantum-inspired algorithms in the energy industry.

Authors:Yong-Hong Yu, Rui-Zhi Zhang, Yue Xu, Xiu-Qi Chen, Huijie Zheng, Quan Li, Ren-Bao Liu, Xin-Yu Pan, Dmitry Budker, Gang-Qin Liu

Abstract: As promising quantum sensors, nitrogen-vacancy (NV) centers in diamond have been widely used in frontier studies in condensed matter physics, material sciences, and life sciences. In practical applications, weak laser excitation is favorable as it reduces the side effects of laser irradiation, for example, phototoxicity and heating. Here we report a combined theoretical and experimental study of optically detected magnetic resonance (ODMR) of NV-center ensembles under weak 532-nm laser excitation. In this regime, both the width and splitting of ODMR spectra decrease with increasing laser power. This power dependence is reproduced with a model considering laser-induced charge neutralization of NV--N+ pairs in the diamond lattice. These results are important for understanding and designing NV-based quantum sensing in light-sensitive applications.

Authors:Marko Kuzmanović, Isak Björkman, John J. McCord, Shruti Dogra, Gheorghe Sorin Paraoanu

Abstract: We present a set of robust and high-fidelity pulses that realize paradigmatic operations such as the transfer of the ground state population into the excited state and arbitrary $X/Y$ rotations on the Bloch sphere. These pulses are based on the phase modulation of the control field. We implement these operations on a transmon qubit, demonstrating resilience against deviations in the drive amplitude of more than $\approx 20\%$ and/or detuning from the qubit transition frequency in the order of $10~\mathrm{MHz}$. The concept and modulation scheme is straightforward to implement and it is compatible with other quantum-technology experimental platforms.

Authors:Julien Du Crest, Francisco Garcia-Herrero, Mehdi Mhalla, Valentin Savin, Javier Valls

Abstract: We address the problem of performing message-passing-based decoding of quantum LDPC codes under hardware latency limitations. We propose a novel way to do layered decoding that suits quantum constraints and outperforms flooded scheduling, the usual scheduling on parallel architectures. A generic construction is given to construct layers of hypergraph product codes. In the process, we introduce two new notions, t-covering layers which is a generalization of the usual layer decomposition, and a new scheduling called random order scheduling. Numerical simulations show that the random ordering is of independent interest as it helps relieve the high error floor typical of message-passing decoders on quantum codes for both layered and serial decoding without the need for post-processing.

Authors:Nicolás F. Del Grosso, Fernando C. Lombardo, Francisco D. Mazzitelli, Paula I. Villar

Abstract: Shortcuts to adiabaticity (STA) are relevant in the context of quantum systems, particularly regarding their control when they are subjected to time-dependent external conditions. In this paper, we investigate the completion of a nonadiabatic evolution into a shortcut to adiabaticity for a quantum field confined within a one-dimensional cavity containing two movable mirrors. Expanding upon our prior research, we characterize the field's state using two Moore functions that enables us to apply reverse engineering techniques in constructing the STA. Regardless of the initial evolution, we achieve a smooth extension of the Moore functions that implements the STA. This extension facilitates the computation of the mirrors' trajectories based on the aforementioned functions. Additionally, we draw attention to the existence of a comparable problem within nonrelativistic quantum mechanics.

Authors:Sofia Ribeiro, Javier Aizpurua, Ruben Esteban

Abstract: A two-dimensional (2D) material, formed for example by a self-assembled molecular monolayer or by a single layer of a Van der Walls material, can couple efficiently with photonic nanocavities, potentially reaching the strong coupling regime. The coupling can be modelled using classical harmonic oscillator models or cavity quantum electrodynamics Hamiltonians that often neglect the direct dipole-dipole interactions within the monolayer. Here, we diagonalize the full Hamiltonian of the system, including these direct dipole-dipole interactions. The main effect on the optical properties of a typical 2D system is simply to renormalize the effective energy of the bright collective excitation of the monolayer that couples with the nanophotonic mode. On the other hand, we show that for situations of extreme field confinement, large transition dipole moments and low losses, fully including the direct dipole-dipole interactions is critical to correctly capture the optical response, with many collective states participating in it. To quantify this result, we propose a simple equation that indicates the condition for which the direct interactions strongly modify the optical response.

Authors:Ren-Xin Zhao, Jinjing Shi, Xuelong Li

Abstract: Self-Attention Mechanism (SAM) is skilled at extracting important information from the interior of data to improve the computational efficiency of models. Nevertheless, many Quantum Machine Learning (QML) models lack the ability to distinguish the intrinsic connections of information like SAM, which limits their effectiveness on massive high-dimensional quantum data. To address this issue, a Quantum Kernel Self-Attention Mechanism (QKSAM) is introduced, which combines the data representation benefit of Quantum Kernel Methods (QKM) with the efficient information extraction capability of SAM. A Quantum Kernel Self-Attention Network (QKSAN) framework is built based on QKSAM, with Deferred Measurement Principle (DMP) and conditional measurement techniques, which releases half of the quantum resources with probabilistic measurements during computation. The Quantum Kernel Self-Attention Score (QKSAS) determines the measurement conditions and reflects the probabilistic nature of quantum systems. Finally, four QKSAN models are deployed on the Pennylane platform to perform binary classification on MNIST images. The best-performing among the four models is assessed for noise immunity and learning ability. Remarkably, the potential learning benefit of partial QKSAN models over classical deep learning is that they require few parameters for a high return of 98\% $\pm$ 1\% test and train accuracy, even with highly compressed images. QKSAN lays the foundation for future quantum computers to perform machine learning on massive amounts of data, while driving advances in areas such as quantum Natural Language Processing (NLP).

Authors:Pierre Wulles, Sergey E. Skipetrov

Abstract: The spectrum of excitations a two-dimensional, planar honeycomb lattice of two-level atoms coupled by the in-plane electromagnetic field may exhibit band gaps that can be opened either by applying an external magnetic field or by breaking the symmetry between the two triangular sublattices of which the honeycomb one is a superposition. We establish the conditions of band gap opening, compute the width of the gap, and characterize its topological property by a topological index (Chern number). The topological nature of the band gap leads to inversion of the population imbalance between the two triangular sublattices for modes with frequencies near band edges. It also prohibits a transition to the trivial limit of infinitely spaced, noninteracting atoms without closing the spectral gap. Surrounding the lattice by a Fabry-P\'erot cavity with small intermirror spacing $d < {\pi}/k_0$ , where $k_0$ is the free-space wave number at the atomic resonance frequency, renders the system Hermitian by suppressing the leakage of energy out of the atomic plane without modifying its topological properties. In contrast, a larger $d$ allows for propagating optical modes that are built up due to reflections at the cavity mirrors and have frequencies inside the band gap of the free-standing lattice, thus closing the latter.

Authors:Ilya V. Kondratyev, Veronika V. Ivanova, Sergey A. Zhuravitskii, Artem S. Argenchiev, Nikolay N. Skryabin, Ivan V. Dyakonov, Suren A. Fldzhyan, Mikhail Yu. Saygin, Stanislav S. Straupe, Alexander A. Korneev, Sergei P. Kulik

Abstract: We introduce a programmable 8-port interferometer with the recently proposed error-tolerant architecture capable of performing a broad class of transformations. The interferometer has been fabricated with femtosecond laser writing and it is the largest programmable interferometer of this kind to date. We have demonstrated its advantageous error tolerance by showing an operation in a broad wavelength range from $920$ to $980$ nm, which is particularly relevant for quantum photonics due to efficient photon sources. Our work highlights the importance of developing novel architectures of programmable photonics for information processing.

Authors:Hideaki Hakoshima, Suguru Endo, Kaoru Yamamoto, Yuichiro Matsuzaki, Nobuyuki Yoshioka

Abstract: Analog and digital quantum simulators can efficiently simulate quantum many-body systems that appear in natural phenomena. However, experimental limitations of near-term devices still make it challenging to perform the entire process of quantum simulation. The purification-based quantum simulation methods can alleviate the limitations in experiments such as the cooling temperature and noise from the environment, while this method has the drawback that it requires global entangled measurement with a prohibitively large number of measurements that scales exponentially with the system size. In this Letter, we propose that we can overcome these problems by restricting the entangled measurements to the vicinity of the local observables to be measured, when the locality of the system can be exploited. We provide theoretical guarantees that the global purification operation can be replaced with local operations under some conditions, in particular for the task of cooling and error mitigation. We furthermore give a numerical verification that the localized purification is valid even when conditions are not satisfied. Our method bridges the fundamental concept of locality with quantum simulators, and therefore expected to open a path to unexplored quantum many-body phenomena.

Authors:Janne V. Kujala, Ehtibar N. Dzhafarov

Abstract: Counterfactual definiteness (CFD) means that if some property is measured in some context, then the outcome of the measurement would have been the same had this property been measured in a different context. A context includes all other measurements made together with the one in question, and the spatiotemporal relations among them. The proviso for CFD is non-disturbance: any physical influence of the contexts on the property being measured is excluded by the laws of nature, so that no one measuring this property has a way of ascertaining its context. It is usually claimed that in quantum mechanics CFD does not hold, because if one assigns the same value to a property in all contexts it is measured in, one runs into a logical contradiction, or at least contravenes quantum theory and experimental evidence. We show that this claim is not substantiated if one takes into account that only one of the possible contexts can be a factual context, all other contexts being counterfactual. With this in mind, any system of random variables can be viewed as satisfying CFD. The concept of CFD is closely related to but distinct from that of noncontextuality, and it is the latter property that may or may not hold for a system, in particular being contravened by some quantum systems.

Authors:E. E. Perepelkin, B. I. Sadovnikov, N. G. Inozemtseva, P. V. Afonin

Abstract: Using the Wigner-Vlasov formalism, an exact 3D solution of the Schr\"odinger equation for a scalar particle in an electromagnetic field is constructed. Electric and magnetic fields are non-uniform. According to the exact expression for the wave function, the search for two types of the Wigner functions is conducted. The first function is the usual Wigner function with a modified momentum. The second Wigner function is constructed on the basis of the Weyl-Stratonovich transform in papers [Phys. Rev. A 35 2791 (1987)] or [Phys. Rev. B 99 014423 (2019)]. It turns out that the second function, unlike the first one, has areas of negative values for wave functions with the Gaussian distribution (Hudson's theorem). On the one hand, knowing the Wigner functions allows one to find the distribution of the mean momentum vector field and the energy spectrum of the quantum system. On the other hand, within the framework of the Wigner-Vlasov formalism, the mean momentum distribution and the magnitude of the energy are initially known. Consequently, the mean momentum distributions and energy values obtained according to the Wigner functions can be compared with the exact momentum distribution and energy values. This paper presents this comparison and describes the differences. For the first Wigner function, an analog of the Moyal equation with an electromagnetic part and the Hamilton-Jacobi operator equation are obtained. An operator analogue of the {\guillemotleft}motion equation{\guillemotright} with electromagnetic interaction is constructed. For the second Vlasov equation, an operator expression for the Vlasov-Moyal approximation for systems with electromagnetic interaction is obtained.

Authors:Jingqi Chen, Wei Liu, Wenjie Dou

Abstract: In our recent paper [Mosallanejad et al., Phys. Rev. B 107(18), 184314, 2023], we have derived a Floquet electronic friction model to describe nonadiabatic molecular dynamics near metal surfaces in the presence of periodic driving. In this work, we demonstrate that Floquet driving can introduce an anti-symmetric electronic friction tensor in quantum transport, resulting in circular motion of the nuclei in the long time limit. Furthermore, we show that such a Lorentz-like force strongly affects nuclear motion: at lower voltage bias, Floquet driving can increase the temperature of nuclei; at larger voltage bias, Floquet driving can decrease the temperature of nuclei. In addition, Floquet driving can affect electron transport strenuously. Finally, we show that there is an optimal frequency that maximizes electron current. We expect that the Floquet electronic friction model is a powerful tool to study nonadiabatic molecular dynamics near metal surfaces under Floquet driving in complex systems.

Authors:Keyu Su, Yi Zhong, Shanchao Zhang, Jianfeng Li, Chang-Ling Zou, Yunfei Wang, Hui Yan, Shi-Liang Zhu

Abstract: Quantum interference between identical single particles reveals the intrinsic quantum statistic nature of particles, which could not be interpreted through classical physics. Here, we demonstrate quantum interference between non-identical bosons using a generalized beam splitter based on a quantum memory. The Hong-Ou-Mandel type interference between single photons and single magnons with high visibility is demonstrated, and the crossover from the bosonic to fermionic quantum statistics is observed by tuning the beam splitter to be non-Hermitian. Moreover, multi-particle interference that simulates the behavior of three fermions by three input photons is realized. Our work extends the understanding of the quantum interference effects and demonstrates a versatile experimental platform for studying and engineering quantum statistics of particles.

Authors:Abhishek Sadhu, Meghana Ayyala Somayajula, Karol Horodecki, Siddhartha Das

Abstract: As quantum theory allows for information processing and computing tasks that otherwise are not possible with classical systems, there is a need and use of quantum Internet beyond existing network systems. At the same time, the realization of a desirably functional quantum Internet is hindered by fundamental and practical challenges such as high loss during transmission of quantum systems, decoherence due to interaction with the environment, fragility of quantum states, etc. We study the implications of these constraints by analyzing the limitations on the scaling and robustness of quantum Internet. Considering quantum networks, we present practical bottlenecks for secure communication, delegated computing, and resource distribution among end nodes. Motivated by the power of abstraction in graph theory (in association with quantum information theory), we consider graph-theoretic quantifiers to assess network robustness and provide critical values of communication lines for viable communication over quantum Internet. In particular, we begin by discussing limitations on usefulness of isotropic states as device-independent quantum key repeaters which otherwise could be useful for device-independent quantum key distribution. We consider some quantum networks of practical interest, ranging from satellite-based networks connecting far-off spatial locations to currently available quantum processor architectures within computers, and analyze their robustness to perform quantum information processing tasks. Some of these tasks form primitives for delegated quantum computing, e.g., entanglement distribution and quantum teleportation. For some examples of quantum networks, we present algorithms to perform different quantum network tasks of interest such as constructing the network structure, finding the shortest path between a pair of end nodes, and optimizing the flow of resources at a node.

Authors:Seok Hyung Lie, Nelly H. Y. Ng

Abstract: A fundamental asymmetry exists within the conventional framework of quantum theory between space and time, in terms of representing causal relations via quantum channels and acausal relations via multipartite quantum states. Such a distinction does not exist in classical probability theory. In effort to introduce this symmetry to quantum theory, a new framework has recently been proposed, such that dynamical description of a quantum system can be encapsulated by a static quantum state over time. In particular, Fullwood and Parzygnat recently proposed the state over time function based on the Jordan product as a promising candidate for such a quantum state over time function, by showing that it satisfies all the axioms required in the no-go result by Horsman et al. However, it was unclear if the axioms induce a unique state over time function. In this work, we demonstrate that the previously proposed axioms cannot yield a unique state over time function. In response, we therefore propose an alternative set of axioms that is operationally motivated, and better suited to describe quantum states over any spacetime regions beyond two points. By doing so, we establish the Fullwood-Parzygnat state over time function as the essentially unique function satisfying all these operational axioms.

Authors:Hippolyte Dourdent, Alastair A. Abbott, Ivan Šupić, Cyril Branciard

Abstract: Causal nonseparability is the property underlying quantum processes incompatible with a definite causal order. So far it has remained a central open question as to whether any process with a clear physical realisation can violate a causal inequality, so that its causal nonseparability can be certified in a device-independent way, as originally conceived. Here we present a method solely based on the observed correlations, which certifies the causal nonseparability of all the processes that can induce a causally nonseparable distributed measurement in a scenario with trusted quantum input states, as defined in [Dourdent et al., Phys. Rev. Lett. 129, 090402 (2022)]. This notably includes the celebrated quantum switch. This device-independent certification is achieved by introducing a network of untrusted operations, allowing one to self-test the quantum inputs on which the effective distributed measurement induced by the process is performed.

Authors:J. L. Montenegro Ferreira, B. de Lima Bernardo

Abstract: Quantum state tomography (QST), the process through which the density matrix of a quantum system is characterized from measurements of specific observables, is a fundamental pillar in the fields of quantum information and computation. We propose a simple QST method to reconstruct the density matrix of two qubits encoded in the polarization and path degrees of freedom of a single photon, which can be realized with a single linear-optical setup. We demonstrate that the density matrix can be fully described in terms of the one-point Stokes parameters related to the two possibles paths of the photon, together with a quantum version of the two-point Stokes parameters introduced here.

Authors:Tulja Varun Kondra, Ray Ganardi, Alexander Streltsov

Abstract: In the classical regime, thermodynamic state transformations are governed by the free energy. This is also called as the second law of thermodynamics. Previous works showed that, access to a catalytic system allows us to restore the second law in the quantum regime when we ignore coherence. However, in the quantum regime, coherence and free energy are two independent resources. Therefore, coherence places additional non-trivial restrictions on the the state transformations, that remains elusive. In order to close this gap, we isolate and study the nature of coherence, i.e. we assume access to a source of free energy. We show that allowing catalysis along with a source of free energy allows us to amplify any quantum coherence present in the quantum state arbitrarily. Additionally, any correlations between the system and the catalyst can be suppressed arbitrarily. Therefore, our results provide a key step in formulating a fully general law of quantum thermodynamics.

Authors:Yunkai Wang, Yujie Zhang, Virginia O. Lorenz

Abstract: We propose a method to build an astronomical interferometer using continuous variable quantum teleportation to overcome the transmission loss between distant telescopes. The scheme relies on two-mode squeezed states shared by distant telescopes as entanglement resources, which are distributed using continuous variable quantum repeaters. We find the optimal measurement on the teleported states, which uses beam-splitters and photon-number-resolved detection. Compared to prior proposals relying on discrete states, our scheme has the advantages of using linear optics to implement the scheme without wasting stellar photons and making use of multiphoton events, which are regarded as noise in previous discrete schemes.

Authors:Philipp A. Hoehn, Andrea Russo, Alexander R. H. Smith

Abstract: We explore the canonical description of a scalar field as a parameterized field theory on an extended phase space that includes additional embedding fields that characterize spacetime hypersurfaces $\mathsf{X}$ relative to which the scalar field is described. This theory is quantized via the Dirac prescription and physical states of the theory are used to define conditional wave functionals $|\psi_\phi[\mathsf{X}]\rangle$ interpreted as the state of the field relative to the hypersurface $\mathsf{X}$, thereby extending the Page-Wootters formalism to quantum field theory. It is shown that this conditional wave functional satisfies the Tomonaga-Schwinger equation, thus demonstrating the formal equivalence between this extended Page-Wootters formalism and standard quantum field theory. We also construct relational Dirac observables and define a quantum deparameterization of the physical Hilbert space leading to a relational Heisenberg picture, which are both shown to be unitarily equivalent to the Page-Wootters formalism. Moreover, by treating hypersurfaces as quantum reference frames, we extend recently developed quantum frame transformations to changes between classical and nonclassical hypersurfaces. This allows us to exhibit the transformation properties of a quantum field under a larger class of transformations, which leads to a frame-dependent particle creation effect.

Authors:Charikleia Troullinou, Vito Giovanni Lucivero, Morgan W. Mitchell

Abstract: We study the use of squeezed probe light and evasion of measurement back-action to enhance the sensitivity and measurement bandwidth of an optically-pumped magnetometer (OPM) at sensitivity-optimal atom number density. By experimental observation, and in agreement with quantum noise modeling, a spin-exchange-limited OPM probed with off-resonance laser light is shown to have an optimal sensitivity determined by density-dependent quantum noise contributions. Application of squeezed probe light boosts the OPM sensitivity beyond this laser-light optimum, allowing the OPM to achieve sensitivities that it cannot reach with coherent-state probing at any density. The observed quantum sensitivity enhancement at optimal number density is enabled by measurement back-action evasion.

Authors:N. L. Diaz, Paolo Braccia, Martin Larocca, J. M. Matera, R. Rossignoli, M. Cerezo

Abstract: In the past few decades, researchers have created a veritable zoo of quantum algorithm by drawing inspiration from classical computing, information theory, and even from physical phenomena. Here we present quantum algorithms for parallel-in-time simulations that are inspired by the Page and Wooters formalism. In this framework, and thus in our algorithms, the classical time-variable of quantum mechanics is promoted to the quantum realm by introducing a Hilbert space of "clock" qubits which are then entangled with the "system" qubits. We show that our algorithms can compute temporal properties over $N$ different times of many-body systems by only using $\log(N)$ clock qubits. As such, we achieve an exponential trade-off between time and spatial complexities. In addition, we rigorously prove that the entanglement created between the system qubits and the clock qubits has operational meaning, as it encodes valuable information about the system's dynamics. We also provide a circuit depth estimation of all the protocols, showing an exponential advantage in computation times over traditional sequential in time algorithms. In particular, for the case when the dynamics are determined by the Aubry-Andre model, we present a hybrid method for which our algorithms have a depth that only scales as $\mathcal{O}(\log(N)n)$. As a by product we can relate the previous schemes to the problem of equilibration of an isolated quantum system, thus indicating that our framework enable a new dimension for studying dynamical properties of many-body systems.

Authors:Bjarne Bergh, Jan Kochanowski, Robert Salzmann, Nilanjana Datta

Abstract: We study asymmetric binary channel discrimination, for qantum channels acting on separable Hilbert spaces. We establish quantum Stein's lemma for channels for both adaptive and parallel strategies, and show that under finiteness of the geometric R\'enyi divergence between the two channels for some $\alpha > 1$, adaptive strategies offer no asymptotic advantage over parallel ones. One major step in our argument is to demonstrate that the geometric R\'enyi divergence satisfies a chain rule and is additive for channels also in infinite dimensions. These results may be of independent interest. Furthermore, we not only show asymptotic equivalence of parallel and adaptive strategies, but explicitly construct a parallel strategy which approximates a given adaptive $n$-shot strategy, and give an explicit bound on the difference between the discrimination errors for these two strategies. This extends the finite dimensional result from [B. Bergh et al., arxiv:2206.08350]. Finally, this also allows us to conclude, that the chain rule for the Umegaki relative entropy in infinite dimensions, recently shown in [O. Fawzi, L. Gao, and M. Rahaman, arxiv:2212.14700v2] given finiteness of the max divergence between the two channels, also holds under the weaker condition of finiteness of the geometric R\'enyi divergence. We give explicit examples of channels which show that these two finiteness conditions are not equivalent.

Authors:S. M. Zhang, H. S. Xu, L. Jin

Abstract: The Aharonov-Bohm (AB) cage enables localized confinement with nondiffractive propagation for arbitrary excitation. In this study, we introduce an anti-parity-time (anti-$\mathcal{PT}$) symmetric imaginary coupling in a generalized Creutz ladder to construct a non-Hermitian AB cage with tunable flat-band energy. We investigate compact localized states and complete localization dynamics, and show that non-Hermiticity affects the localization probability distributions and increases the oscillation period of the AB cage dynamics. Non-Hermitian engineering of the decoupled core of the AB cage is the essential point in our proposal. Our approach is widely applicable to a more general situation and can facilitate the manipulation of localization in physics.

Authors:Jin-Lou Ma, Bobo Liu, Qing Li, Zexian Guo, Lei Tan, Lei Ying

Abstract: Disorder in one-dimensional (1D) many-body systems emerges abundant phases such as many-body localization (MBL), and thermalization. However, it remains unclear regarding their existence and behavior within hybrid quantum systems. Here, based on a simple bosonic-spin hybrid model, as known as the Jaynes-Cummings-Hubbard (JCH) array, we investigate the effect of disorder comparing to the phenomena in the clean system with the variation of atom-photon coupling strength. By using the level-spacing ratio, entanglement entropy, and the properties of observable diagonal and off-diagonal matrix elements, we find that strong disorder results in the appearance of MBL phase in the JCH model that strongly violate eigenstate thermalization hypothesis (ETH), while a conditional prethermal behavior can exist in weak disorder or weak coupling regime. The conditional prethermal dynamics is based on the choice of initial product states. This work systematically reveals abundant many-body phases in the 1D JCH model and clarifies the discrepancies in the thermalization properties of systems with and without disorder.

Authors:Sebastian Leontica, Max McGinley

Abstract: We introduce a continuous time model of many-body quantum dynamics based on infinitesimal random unitary operations, combined with projective measurements. We consider purification dynamics in this model, where the system is initialized in a mixed state, which then purifies over time as a result of the measurements. By mapping our model to a family of effective 1D quantum Hamiltonians, we are able to derive analytic expressions that capture how the entropy of the system decays in time. Our results confirm the existence of two distinct dynamical phases, where purification occurs over a timescale that is exponential vs. constant in system size. We compare our analytic expressions for this microscopic model to results derived from field theories that are expected to capture such measurement-induced phase transitions, and find quantitative agreement between the two.

Authors:Marco Parigi, Stefano Martina, Filippo Caruso

Abstract: Generative models realized with machine learning techniques are powerful tools to infer complex and unknown data distributions from a finite number of training samples in order to produce new synthetic data. Diffusion models are an emerging framework that have recently overcome the performance of the generative adversarial networks in creating synthetic text and high-quality images. Here, we propose and discuss the quantum generalization of diffusion models, i.e., three quantum-noise-driven generative diffusion models that could be experimentally tested on real quantum systems. The idea is to harness unique quantum features, in particular the non-trivial interplay among coherence, entanglement and noise that the currently available noisy quantum processors do unavoidably suffer from, in order to overcome the main computational burdens of classical diffusion models during inference. Hence, we suggest to exploit quantum noise not as an issue to be detected and solved but instead as a very remarkably beneficial key ingredient to generate much more complex probability distributions that would be difficult or even impossible to express classically, and from which a quantum processor might sample more efficiently than a classical one. Therefore, our results are expected to pave the way for new quantum-inspired or quantum-based generative diffusion algorithms addressing more powerfully classical tasks as data generation/prediction with widespread real-world applications ranging from climate forecasting to neuroscience, from traffic flow analysis to financial forecasting.

Authors:Jinao Wang, Rimika Jaiswal

Abstract: Quantum systems have historically been formidable to simulate using classical computational methods, particularly as the system size grows. The Heisenberg Model, pivotal in understanding magnetic materials, is a quintessential example where classical simulations face scalability issues. The Variational Quantum Eigensolver (VQE) algorithm is a system composed of a quantum circuit as well as a classical optimizer that can efficiently prepare the Heisenberg ground state by iteratively optimizing the variational parameters. We assess the efficacy and scalability of VQE by preparing the ground states of isotropic and anisotropic Heisenberg models. This paper also aims to provide insights into the precision and time consumption involved in classical and optimized sampling approaches in the calculation of expectation values. In preparing the ground state for the Heisenberg models, this paper paves the way for more efficient quantum algorithms and contributes to the broader field of condensed matter physics.

Authors:Shubham Kumar, Narendra N. Hegade, Enrique Solano, Francisco Albarrán-Arriagada, Gabriel Alvarado Barrios

Abstract: We propose a digital-analog quantum algorithm for simulating the Hubbard-Holstein model, describing strongly-correlated fermion-boson interactions, in a suitable architecture with superconducting circuits. It comprises a linear chain of qubits connected by resonators, emulating electron-electron (e-e) and electron-phonon (e-p) interactions, as well as fermion tunneling. Our approach is adequate for a digital-analog quantum computing (DAQC) of fermion-boson models including those described by the Hubbard-Holstein model. We show the reduction in the circuit depth of the DAQC algorithm, a sequence of digital steps and analog blocks, outperforming the purely digital approach. We exemplify the quantum simulation of a half-filling two-site Hubbard-Holstein model. In such example we obtain fidelities larger than 0.98, showing that our proposal is suitable to study the dynamical behavior of solid-state systems. Our proposal opens the door to computing complex systems for chemistry, materials, and high-energy physics.

Authors:Albert Cabot, Gianluca Giorgi, Roberta Zambrini

Abstract: We show a dissipative phase transition in a driven nonlinear quantum oscillator in which a discrete time-translation symmetry is broken either continuously or discretely. The corresponding regimes display either continuous or discrete time crystals, which we analyze numerically and analytically beyond the classical limit addressing observable dynamics, Liouvillian spectral features, and quantum fluctuations. Via an effective semiclassical description, we show that phase diffusion dominates when the symmetry is broken continuously, which manifests as a band of eigenmodes with a lifetime growing linearly with the mean-field excitation number. Instead, in the discrete symmetry broken phase, the leading fluctuation process corresponds to quantum activation with a single mode that has an exponentially growing lifetime. Interestingly, the transition between these two regimes manifests itself already in the quantum regime as a spectral singularity, namely as an exceptional point mediating between phase diffusion and quantum activation. Finally, we discuss this transition between different time-crystal orders in the context of synchronization phenomena.

Authors:Sebastian Yde Madsen, Frederik Kofoed Marqversen, Stig Elkjær Rasmussen, Nikolaj Thomas Zinner

Abstract: The task of Multiple Sequence Alignment (MSA) is a constrained combinatorial optimization problem that is generally considered a complex computational problem. In this paper, we first present a binary encoding of MSA and devise a corresponding soft-constrained cost-function that enables a Hamiltonian formulation and implementation of the MSA problem with the variational Quantum Approximate Optimization Algorithm (QAOA). Through theoretical analysis, a bound on the ratio of the number of feasible states to the size of the Hilbert space is determined. Furthermore, we consider a small instance of our QAOA-MSA algorithm in both a quantum simulator and its performance on an actual quantum computer. While the ideal solution to the instance of MSA investigated is shown to be the most probable state sampled for a shallow p<5 quantum circuit in the simulation, the level of noise in current devices is still a formidable challenge for the kind of MSA-QAOA algorithm developed here. In turn, we are not able to distinguish the feasible solutions from other states in the quantum hardware output data at this point. This indicates a need for further investigation into both the strategy utilized for compiling the quantum circuit, but also the possibility of devising a more compact ansatz, as one might achieve through constraint-preserving mixers for QAOA.

Authors:Dong Wang, Juan-Ying Zhao, Ya-Chao Wang, Liang-Jiang Zhou, Yi-Bo Zhao

Abstract: LiDAR systems that rely on classical signals are susceptible to intercept-and-recent spoofing attacks, where a target attempts to avoid detection. To address this vulnerability, we propose a quantum-secured LiDAR protocol that utilizes Gaussian modulated coherent states for both range determination and spoofing attack detection. By leveraging the Gaussian nature of the signals, the LiDAR system can accurately determine the range of the target through cross-correlation analysis. Additionally, by estimating the excess noise of the LiDAR system, the spoofing attack performed by the target can be detected, as it can introduce additional noise to the signals. We have developed a model for target detection and security check, and conducted numerical simulations to evaluate the Receiver Operating Characteristic (ROC) of the LiDAR system. The results indicate that an intercept-and-recent spoofing attack can be detected with a high probability at a low false-alarm rate. Furthermore, the proposed method can be implemented using currently available technology, highlighting its feasibility and practicality in real-world applications.

Authors:Julian Amette Estrada, Franco Mayo, Augusto J. Roncaglia, Pablo D. Mininni

Abstract: We consider a quantum Otto cycle with an interacting Bose-Einstein condensate at finite temperature. We present a procedure to evolve this system in time in three spatial dimensions, in which closed (adiabatic) strokes are described by the Gross-Pitaevskii equation, and open (isochoric) strokes are modeled using a stochastic Ginzburg-Landau equation. We analyze the effect on the thermodynamic efficiency of the strength of interactions, the frequency of the harmonic trap, and the temperatures of the reservoirs. The efficiency has little sensitivity to changes in the temperatures, but decreases as interactions increase. However, stronger interactions allow for faster cycles and for substantial increases in power.

Authors:Léo Pioge, Benoit Seron, Leonardo Novo, Nicolas J. Cerf

Abstract: In multiphon interference processes, the commonly assumed direct link between boson bunching and particle indistinguishability has recently been challenged in Seron $\textit{et al.}$ [Nat. Photon. 17, 702 (2023)]. Exploiting the connection between optical interferometry and matrix permanents, it appeared that bunching effects may surprisingly be enhanced in some interferometers by preparing specific states of partially distinguishable photons. Interestingly, all the states giving rise to such an anomalous bunching were found to be $\textit{far from}$ the state of perfectly indistinguishable particles, raising the question of whether this intriguing phenomenon might even exist for $\textit{nearly indistinguishable}$ particles. Here, we answer positively this physically motivated question by exploiting some mathematical conjecture on matrix permanents, whose physical interpretation had not yet been unveiled. Using a recently found counterexample to this conjecture, we demonstrate that there is an optical set-up involving 8~photons in 10~modes for which the probability that all photons bunch into two output modes can be enhanced by applying a suitable perturbation to the polarization states starting from photons with the same polarization. We also find out that the perturbation that decreases the bunching probability the most is not the one that takes the perfectly indistinguishable state towards a fully distinguishable state, as could naively be expected.

Authors:Tomasz Białecki, Tomasz Rybotycki, Josep Batle, Adam Bednorz

Abstract: We present a simple null test of a dimension of a quantum system, using a single repeated operation in the method of delays, assuming that each instance is identical and independent. The test is well-suited to current feasible quantum technologies, with programmed gates. We also analyze weaker versions of the test, assuming unitary or almost unitary operations and derive expressions for the statistical error.

Authors:Tim Palmer

Abstract: Superdeterminism -- where the Measurement-Independence assumption in Bell's Theorem is violated -- is typically treated with derision as it appears to imply contrived conspiratorial correlations between properties $\lambda$ of particles being measured, and nominally accurate measurement settings $x$ and $y$. Based on an analysis of Pearlean interventions needed to determine whether $x$ and $y$ are free variables, we show that whilst conspiracy implies superdeterminism, superdeterminism does not imply conspiracy. In conspiratorial superdeterminism these interventions are consistent with physical theory; in non-conspiratorial superdeterminism they are inconsistent. A non-conspiratorial locally-causal superdeterministic model is developed, based in part on the generic properties of chaotic attractors and in part on an arbitrarily fine discretisation of complex Hilbert Space. Here the required interventions are inconsistent with rational-number constraints on exact measurement settings $X$ and $Y$. In this model, hidden variables $\lambda$ are defined as the information, over and above the freely chosen determinants of $x$ and $y$, which determine $X$ and $Y$. These rationality constraints limit the freedom to vary $x$ and $y$ keeping $\lambda$ fixed. These constraints disappear with any coarse-graining of $\lambda$ and hence $X$. We show how quantum mechanics might be `gloriously explained and derived' as the singular continuum limit of a superdeterministic discretisation of Hilbert Space. We argue that the real message behind Bell's Theorem is the need to develop more holistic theories of fundamental physics -- notably gravitational physics -- some ideas for moving in this direction are discussed.

Authors:Yuxuan Du, Yibo Yang, Tongliang Liu, Zhouchen Lin, Bernard Ghanem, Dacheng Tao

Abstract: Understanding the dynamics of large quantum systems is hindered by the curse of dimensionality. Statistical learning offers new possibilities in this regime by neural-network protocols and classical shadows, while both methods have limitations: the former is plagued by the predictive uncertainty and the latter lacks the generalization ability. Here we propose a data-centric learning paradigm combining the strength of these two approaches to facilitate diverse quantum system learning (QSL) tasks. Particularly, our paradigm utilizes classical shadows along with other easily obtainable information of quantum systems to create the training dataset, which is then learnt by neural networks to unveil the underlying mapping rule of the explored QSL problem. Capitalizing on the generalization power of neural networks, this paradigm can be trained offline and excel at predicting previously unseen systems at the inference stage, even with few state copies. Besides, it inherits the characteristic of classical shadows, enabling memory-efficient storage and faithful prediction. These features underscore the immense potential of the proposed data-centric approach in discovering novel and large-scale quantum systems. For concreteness, we present the instantiation of our paradigm in quantum state tomography and direct fidelity estimation tasks and conduct numerical analysis up to 60 qubits. Our work showcases the profound prospects of data-centric artificial intelligence to advance QSL in a faithful and generalizable manner.

Authors:Irina Heinz, Adam R. Mills, Jason R. Petta, Guido Burkard

Abstract: In recent advancements of quantum computing utilizing spin qubits, it has been demonstrated that this platform possesses the potential for implementing two-qubit gates with fidelities exceeding 99.5%. However, as with other qubit platforms, it is not feasible to completely turn qubit couplings off. This study aims to investigate the impact of coherent error matrices in gate set tomography by employing a double quantum dot. We evaluate the infidelity caused by residual exchange between spins and compare various mitigation approaches, including the use of adjusted timing through simple drives, considering different parameter settings in the presence of charge noise. Furthermore, we extend our analysis to larger arrays of exchange-coupled spin qubits to provide an estimation of the expected fidelity. In particular, we demonstrate the influence of residual exchange on a single-qubit $Y$ gate and the native two-qubit SWAP gate in a linear chain. Our findings emphasize the significance of accounting for residual exchange when scaling up spin qubit devices and highlight the tradeoff between the effects of charge noise and residual exchange in mitigation techniques.

Authors:Shradhanjali Sahu, Ahmed Lawey, Mohsen Razavi

Abstract: We investigate quantum key distribution (QKD) in optical multiple-input-multiple-output (MIMO) settings. Such settings can prove useful in dealing with harsh channel conditions as in, e.g., satellite-based QKD. We study a $2\times2$ setting for continuous variable (CV) QKD with Gaussian encoding and heterodyne detection and reverse reconciliation. We present our key rate analysis for this system and compare it with single-mode and multiplexed CV QKD scenarios. We show that we can achieve multiplexing gain using multiple transmitters and receivers even if there is some crosstalk between the two channels. In certain cases, when there is nonzero correlated excess noise in the two received signals, we can even surpass the multiplexing gain.

Authors:Jose Carrasco, Marc Langer, Antoine Neven, Barbara Kraus

Abstract: We present verification protocols to gain confidence in the correct performance of the realization of an arbitrary universal quantum computation. The derivation of the protocols is based on the fact that matchgate computations, which are classically efficiently simulable, become universal if supplemented with additional resources. We combine tools from weak simulation, randomized compiling, and classical statistics to derive verification circuits. These circuits have the property that (i) they strongly resemble the original circuit and (ii) cannot only be classically efficiently simulated in the ideal, i.e. error free, scenario, but also in the realistic situation where errors are present. In fact, in one of the protocols we apply exactly the same circuit as in the original computation, however, to a slightly modified input state.

Authors:Shirin Afzal, Tyler J. Zimmerling, Mahdi Rizvandi, Majid Taghavi, Taras Hrushevskyi, Manpreet Kaur, Vien Van, Shabir Barzanjeh

Abstract: Entanglement, a fundamental concept in quantum mechanics, plays a crucial role as a valuable resource in quantum technologies. The practical implementation of entangled photon sources encounters obstacles arising from imperfections and defects inherent in physical systems and microchips, resulting in a loss or degradation of entanglement. The topological photonic insulators, however, have emerged as promising candidates, demonstrating an exceptional capability to resist defect-induced scattering, thus enabling the development of robust entangled sources. Despite their inherent advantages, building bright and programmable topologically protected entangled sources remains challenging due to intricate device designs and weak material nonlinearity. Here we present an advancement in entanglement generation achieved through a non-magnetic and tunable resonance-based anomalous Floquet insulator, utilizing an optical spontaneous four-wave mixing process. Our experiment demonstrates a substantial enhancement in entangled photon pair generation compared to devices reliant solely on topological edge states and outperforming trivial photonic devices in spectral resilience. This work marks a step forward in the pursuit of defect-robust and bright entangled sources that can open avenues for the exploration of cascaded quantum devices and the engineering of quantum states. Our result could lead to the development of resilient quantum sources with potential applications in quantum technologies.

Authors:Xuan Tang, Yunxiao Zhang, Xueshi Guo, Liang Cui, Xiaoying Li, Z. Y. Ou

Abstract: Hanbury-Brown and Twiss (HBT) effect is the foundation for stellar intensity interferometry. However, it is a phase insensitive two-photon interference effect. In this paper, we extend the HBT interferometer by mixing two phase-coherent input fields with coherent auxiliary fields before intensity correlation measurement and achieve phase sensitive two-photon interference so as to measure the complete complex second-order coherence function of the input fields. This practical scheme paves the way for synthetic aperture imaging for astronomical applications in optical regime. Pulsed input fields is also tested for potential remote sensing and ranging applications. We discuss the condition to implement recently proposed entanglement-based telescopy scheme with the more realistic cw broadband anti-bunched light fields.

Authors:Patrick Cameron, Baptiste Courme, Daniele Faccio, Hugo Defienne

Abstract: Adaptive optics (AO) has revolutionized imaging in applications ranging from astronomy to microscopy by enabling the correction of optical aberrations. In label-free microscopes, however, conventional AO methods are limited due to the absence of guidestar and the need to select an optimization metric specific to the type of sample and imaging process being used. Here, we propose a quantum-assisted AO approach that exploits correlations between entangled photons to directly access and correct the point spread function (PSF) of the imaging system. This guidestar-free method is independent of the specimen and imaging modality. We demonstrate the imaging of biological samples in the presence of aberrations using a bright-field imaging setup operating with a source of spatially-entangled photon pairs. We show that our approach performs better than conventional AO in correcting certain types of aberrations, particularly in cases involving significant defocus. Our work improves AO for label-free microscopy, and could play a major role in the development of quantum microscopes, in which optical aberrations can counteract the advantages of using entangled photons and undermine their practical use.

Authors:Andi Gu, Hong-Ye Hu, Di Luo, Taylor L. Patti, Nicholas C. Rubin, Susanne F. Yelin

Abstract: We present a comprehensive approach to quantum simulations at both zero and finite temperatures, employing a quantum information theoretic perspective and utilizing the Clifford + $k$Rz transformations. We introduce the "quantum magic ladder", a natural hierarchy formed by systematically augmenting Clifford transformations with the addition of Rz gates. These classically simulable similarity transformations allow us to reduce the quantumness of our system, conserving vital quantum resources. This reduction in quantumness is essential, as it simplifies the Hamiltonian and shortens physical circuit-depth, overcoming constraints imposed by limited error correction. We improve the performance of both digital and analog quantum computers on ground state and finite temperature molecular simulations, not only outperforming the Hartree-Fock solution, but also achieving consistent improvements as we ascend the quantum magic ladder. By facilitating more efficient quantum simulations, our approach enables near-term and early fault-tolerant quantum computers to address novel challenges in quantum chemistry.

Authors:Hrvoje Nikolic

Abstract: We develop a general formulation of quantum statistical mechanics in terms or probability currents that satisfy continuity equations in the multi-particle position space, for closed and open systems with a fixed number of particles. The continuity equation for any closed or open system suggests a natural Bohmian interpretation in terms of microscopic particle trajectories, that make the same measurable predictions as standard quantum theory. The microscopic trajectories are not directly observable, but provide a general, simple and intuitive microscopic interpretation of macroscopic phenomena in quantum statistical mechanics. In particular, we discuss how various notions of entropy, proper and improper mixtures, and thermodynamics are understood from the Bohmian perspective.

Authors:Esther Villar-Rodriguez, Aitor Gomez-Tejedor, Eneko Osaba

Abstract: The expectations arising from the latest achievements in the quantum computing field are causing that researchers coming from classical artificial intelligence to be fascinated by this new paradigm. In turn, quantum computing, on the road towards usability, needs classical procedures. Hybridization is, in these circumstances, an indispensable step but can also be seen as a promising new avenue to get the most from both computational worlds. Nonetheless, hybrid approaches have now and will have in the future many challenges to face, which, if ignored, will threaten the viability or attractiveness of quantum computing for real-world applications. To identify them and pose pertinent questions, a proper characterization of the hybrid quantum computing field, and especially hybrid solvers, is compulsory. With this motivation in mind, the main purpose of this work is to propose a preliminary taxonomy for classifying hybrid schemes, and bring to the fore some questions to stir up researchers minds about the real challenges regarding the application of quantum computing.

Authors:Fatih Ozaydin, Özgür E. Müstecaplıoğlu, Tuğrul Hakioğlu

Abstract: We consider a quantum Otto cycle with a $q$-deformed quantum oscillator working substance and classical thermal baths. We investigate the influence of the quantum statistical deformation parameter $q$ on the work and efficiency of the cycle. In usual quantum Otto cycle, a Hamiltonian parameter is varied during the quantum adiabatic stages while the quantum statistical character of the working substance remains fixed. We point out that even if the Hamiltonian parameters are not changing, work can be harvested by quantum statistical changes of the working substance. Work extraction from thermal resources using quantum statistical mutations of the working substance makes a quantum Otto cycle without any classical analog.

Authors:Amartya Bose

Abstract: Tensor networks have historically proven to be of great utility in providing compressed representations of wave functions that can be used for calculation of eigenstates. Recently, it has been shown that a variety of these networks can be leveraged to make real time non-equilibrium simulations of dynamics involving the Feynman-Vernon influence functional more efficient. In this work, tensor networks are utilized for calculating equilibrium correlation function for open quantum systems using the path integral methodology. These correlation functions are of fundamental importance in calculations of rates of reactions, simulations of response functions and susceptibilities, spectra of systems, etc. The influence of the solvent on the quantum system is incorporated through an influence functional, whose unconventional structure motivates the design of a new optimal matrix product-like operator that can be applied to the so-called path amplitude matrix product state. This complex time tensor network path integral approach provides an exceptionally efficient representation of the path integral enabling simulations for larger systems strongly interacting with baths and at lower temperatures out to longer time. The design and implementation of this method is discussed along with illustrations from rate theory, symmetrized spin correlation functions, dynamical susceptibility calculations and quantum thermodynamics.

Authors:Johannes Moerland, Nikolai Wyderka, Hermann Kampermann, Dagmar Bruß

Abstract: Bell diagonal states constitute a well-studied family of bipartite quantum states that arise naturally in various contexts in quantum information. In this paper we generalize the notion of Bell diagonal states to the case of unequal local dimensions and investigate their entanglement properties. We extend the family of entanglement criteria of Sarbicki et al. to non-Hermitian operator bases to construct entanglement witnesses for the class of generalized Bell diagonal states. We then show how to optimize the witnesses with respect to noise robustness. Finally, we use these witnesses to construct bound entangled states that are not detected by the usual computable cross norm or realignment and de Vicente criteria.

Authors:Akshat Pandey, Subir Ghosh

Abstract: In this work we introduce a new framework - Dirac's Hamiltonian formalism of constraint systems - to study different types of Superconducting Quantum Circuits (SQC) in a {\it{unified}} and unambiguous way. The Lagrangian of a SQC reveals the constraints, that are classified in a Hamiltonian framework, such that redundant variables can be removed to isolate the canonical degrees of freedom for subsequent quantization of the Dirac Brackets via a generalized Correspondence Principle. This purely algebraic approach makes the application of concepts such as graph theory, null vector, loop charge,\ etc that are in vogue, (each for a specific type of circuit), completely redundant.

Authors:Souvik Agasti

Abstract: We simulate coherent driven free dissipative Kerr nonlinear system numerically using time evolving block decimation (TEBD) algorithm and time propagation on the Heisenberg equation of motion using Eulers method to study how the numerical results are analogous to classical bistability. The system evolves through different trajectories to stabilize different branches for different external drives and initial conditions. The Wigner state reprentation confirms the system to suffer a residual effect of initial state throughout the non-classical dynamical evolution and the steady state of the system. Furthermore, we also see the numerically simulated spectral density remains significantly different from analytical counterparts when initial states do not lie to the same branch of the final state.

Authors:Dibyendu Mondal, Rahul Maitra

Abstract: Variational Quantum Eigensolver (VQE) provides a lucrative platform to determine molecular energetics in near-term quantum devices. While the VQE is traditionally tailored to determine the ground state wavefunction with the underlying Rayleigh-Ritz principle, the access to specific symmetry-adapted excited states remains elusive. This often requires high depth circuit or additional ancilla qubits along with prior knowledge of the ground state wavefunction. We propose a unified VQE framework that treats the ground and excited states in the same footings. With the knowledge of the irreducible representations of the spinorbitals, we construct a multi-determinantal reference that is adapted to a given spatial symmetry where additionally, the determinants are entangled through appropriate Clebsch-Gordan coefficients to ensure the desired spin-multiplicity. We introduce the notion of totally symmetric, spin-scalar unitary which maintains the purity of the reference at each step of the optimization. The state-selectivity safeguards the method against any variational collapse while leading to any targeted low-lying eigenroot of arbitrary symmetry. The direct access to the excited states shields our approach from the cumulative error that plagues excited state calculations in a quantum computer and with few parameter count, it is expected to be realized in near-term quantum devices.

Authors:Taisei Takabayashi, Masayuki Ohzeki

Abstract: The demand for classical-quantum hybrid algorithms to solve large-scale combinatorial optimization problems using quantum annealing (QA) has increased. One approach involves obtaining an approximate solution using classical algorithms and refining it using QA. In previous studies, such variables were determined using molecular dynamics (MD) as a continuous optimization method. We propose a method that uses the simple continuous relaxation technique called linear programming (LP) relaxation. Our method demonstrated superiority through comparative experiments with the minimum vertex cover problem versus the previous MD-based approach. Furthermore, the hybrid approach of LP relaxation and simulated annealing showed advantages in accuracy and speed compared to solving with simulated annealing alone.

Authors:István Márton, Erika Bene, Péter Diviánszky, Tamás Vértesi

Abstract: According to Bell's theorem, certain entangled states cannot be simulated classically using local hidden variables (LHV). But if can we augment LHV by classical communication, how many bits are needed to simulate them? There is a strong evidence that a single bit of communication is powerful enough to simulate projective measurements on any two-qubit entangled state. In this study, we present Bell-like scenarios where bipartite correlations resulting from projective measurements on higher dimensional states cannot be simulated with a single bit of communication. These include a three-input, a four-input, a seven-input, and a 63-input bipartite Bell-like inequality with 80089, 64, 16, and 2 outputs, respectively. Two copies of emblematic Bell expressions, such as the Magic square pseudo-telepathy game, prove to be particularly powerful, requiring a $16\times 16$ state to beat the one-bit classical bound, and look a promising candidate for implementation on an optical platform.

Authors:Aniket S. Dalvi, Jacob Whitlow, Marissa D'Onofrio, Leon Riesebos, Tianyi Chen, Samuel Phiri, Kenneth R. Brown, Jonathan M. Baker

Abstract: A large class of problems in the current era of quantum devices involve interfacing between the quantum and classical system. These include calibration procedures, characterization routines, and variational algorithms. The control in these routines iteratively switches between the classical and the quantum computer. This results in the repeated compilation of the program that runs on the quantum system, scaling directly with the number of circuits and iterations. The repeated compilation results in a significant overhead throughout the routine. In practice, the total runtime of the program (classical compilation plus quantum execution) has an additional cost proportional to the circuit count. At practical scales, this can dominate the round-trip CPU-QPU time, between 5% and 80%, depending on the proportion of quantum execution time. To avoid repeated device-level compilation, we identify that machine code can be parametrized corresponding to pulse/gate parameters which can be dynamically adjusted during execution. Therefore, we develop a device-level partial-compilation (DLPC) technique that reduces compilation overhead to nearly constant, by using cheap remote procedure calls (RPC) from the QPU control software to the CPU. We then demonstrate the performance speedup of this on optimal pulse calibration, system characterization using randomized benchmarking (RB), and variational algorithms. We execute this modified pipeline on real trapped-ion quantum computers and observe significant reductions in compilation time, as much as 2.7x speedup for small-scale VQE problems.

Authors:Wenjie Liu, Qingshan Wu

Abstract: It is essential to select efficient topology of parameterized quantum circuits (PQCs) in variational quantum algorithms (VQAs). However, there are problems in current circuits, i.e. optimization difficulties caused by too many parameters or performance is hard to guarantee. How to reduce the number of parameters (number of single-qubit rotation gates and 2-qubit gates) in PQCs without reducing the performance has become a new challenge. To solve this problem, we propose a novel topology, called Block-Ring (BR) topology, to construct the PQCs. This topology allocate all qubits to several blocks, all-to-all mode is adopt inside each block and ring mode is applied to connect different blocks. Compared with the pure all-to-all topology circuits which own the best power, BR topology have similar performance and the number of parameters and 2-qubit gate reduced from 0(n^2) to 0(mn) , m is a hyperparameter set by ourselves. Besides, we compared BR topology with other topology circuits in terms of expressibility and entangling capability. Considering the effects of different 2-qubit gates on circuits, we also make a distinction between controlled X-rotation gates and controlled Z-rotation gates. Finally, the 1- and 2-layer configurations of PQCs are taken into consideration as well, which shows the BR's performance improvement in the condition of multilayer circuits.

Authors:Yilun Yang, Arthur Christianen, Mari Carmen Bañuls, Dominik S. Wild, J. Ignacio Cirac

Abstract: Many quantum algorithms rely on the measurement of complex quantum amplitudes. Standard approaches to obtain the phase information, such as the Hadamard test, give rise to large overheads due to the need for global controlled-unitary operations. We introduce a quantum algorithm based on complex analysis that overcomes this problem for amplitudes that are a continuous function of time. Our method only requires the implementation of real-time evolution and a shallow circuit that approximates a short imaginary-time evolution. We show that the method outperforms the Hadamard test in terms of circuit depth and that it is suitable for current noisy quantum computers when combined with a simple error-mitigation strategy.

Authors:Choong Pak Shen, Davide Girolami

Abstract: We address the problem of evaluating the difference between quantum states before and after being affected by errors encoded in unitary transformations. Standard distance functions, e.g., the Bures length, are not fully adequate for such a task. Weighted distances are instead appropriate information measures to quantify distinguishability of multipartite states. Here, we employ the previously introduced weighted Bures length and the newly defined weighted Hilbert-Schmidt distance to quantify how much single-qubit Pauli errors alter cluster states. We find that different errors of the same dimension change cluster states in a different way, i.e., their detectability is in general different. Indeed, they transform an ideal cluster state into a state whose weighted distance from the input depends on the specific chosen Pauli rotation, as well as the position of the affected qubit in the graph related to the state. As these features are undetected by using standard distances, the study proves the usefulness of weighted distances to monitor key but elusive properties of many-body quantum systems.

Authors:Graham R. Enos, Matthew J. Reagor, Eric Hulburd

Abstract: A limited set of tools exist for assessing whether the behavior of quantum machine learning models diverges from conventional models, outside of abstract or theoretical settings. We present a systematic application of explainable artificial intelligence techniques to analyze synthetic data generated from a hybrid quantum-classical neural network adapted from twenty different real-world data sets, including solar flares, cardiac arrhythmia, and speech data. Each of these data sets exhibits varying degrees of complexity and class imbalance. We benchmark the quantum-generated data relative to state-of-the-art methods for mitigating class imbalance for associated classification tasks. We leverage this approach to elucidate the qualities of a problem that make it more or less likely to be amenable to a hybrid quantum-classical generative model.

Authors:Ziqi Niu, Yue Jiang, Jianming Wen, Chuanwei Zhang, Shengwang Du, Irina Novikova

Abstract: We report an experimental demonstration of anti-parity-time (anti-PT) symmetric optical four-wave mixing in thermal Rubidium vapor, where the propagation of two conjugate optical fields in a double-$\Lambda$ scheme is governed by a non-Hermitian Hamiltonian. We are particularly interested in studying quantum intensity correlations between the two conjugate fields near the exceptional point, taking into account loss and accompanied Langevin noise. Our experimental measurements of classical four-wave mixing gain and the associated two-mode relative-intensity squeezing are in reasonable agreement with the theoretical predictions.

Authors:Zewen Sun, Yi Hong Teoh, Fereshteh Rajabi, Rajibul Islam

Abstract: We numerically investigate a hybrid trapping architecture for 2D ion crystals using static electrode voltages and optical cavity fields for in-plane and out-of-plane confinements, respectively. By studying the stability of 2D crystals against 2D-3D structural phase transitions, we identify the necessary trapping parameters for ytterbium ions. Multiple equilibrium configurations for 2D crystals are possible, and we analyze their stability by estimating potential barriers between them. We find that scattering to anti-trapping states limits the trapping lifetime, which is consistent with recent experiments employing other optical trapping architectures. These 2D ion crystals offer an excellent platform for quantum simulation of frustrated spin systems, benefiting from their 2D triangular lattice structure and phonon-mediated spin-spin interactions. Quantum information processing with tens of ions is feasible in this scheme with current technologies.

Authors:Mark Field, Angela Q. Chen, Ben Scharmann, Eyob A. Sete, Feyza Oruc, Kim Vu, Valentin Kosenko, Joshua Y. Mutus, Stefano Poletto, Andrew Bestwick

Abstract: We use a floating tunable coupler to mediate interactions between qubits on separate chips to build a modular architecture. We demonstrate three different designs of multi-chip tunable couplers using vacuum gap capacitors or superconducting indium bump bonds to connect the coupler to a microwave line on a common substrate and then connect to the qubit on the next chip. We show that the zero-coupling condition between qubits on separate chips can be achieved in each design and that the relaxation rates for the coupler and qubits are not noticeably affected by the extra circuit elements. Finally, we demonstrate two-qubit gate operations with fidelity at the same level as qubits with a tunable coupler on a single chip. Using one or more indium bonds does not degrade qubit coherence or impact the performance of two-qubit gates.

Authors:Kento Sasaki, Eisuke Abe

Abstract: We study a mechanism by which nuclear hyperpolarization due to the polarization transfer from a microwave-pulse-controlled electron spin is suppressed. From analytical and numerical calculations of the unitary dynamics of multiple nuclear spins, we uncover that, combined with the formation of the dark state within a cluster of nuclei, coherent higher-order nuclear spin dynamics impose limits on the efficiency of the polarization transfer even in the absence of mundane depolarization processes such as nuclear spin diffusion and relaxation. Furthermore, we show that the influence of the dark state can be partly mitigated by introducing a disentangling operation. Our analysis is applied to the nuclear polarizations observed in $^{13}$C nuclei coupled with a single nitrogen-vacancy center in diamond [Science 374, 1474 (2021) by J. Randall et al.]. Our work sheds light on collective engineering of nuclear spins as well as future designs of pulsed dynamic nuclear polarization protocols.

Authors:Pooja Lamba, Akshat Rana, Sougata Halder, Siddharth Dhomkar, Dieter Suter, Rama K. Kamineni

Abstract: Nitrogen-Vacancy (NV) centers in diamond have useful properties for detecting both AC and DC magnetic fields with high sensitivity at nano-scale resolution. Vector detection of AC magnetic fields can be achieved by using NV centers having three different orientations. Here, we propose a method to achieve this by using NV centers of single orientation. In this method, a static magnetic field is applied perpendicular to the NV axis, leading to strong mixing of the $m_{s}=-1$ and $1$ electron spin states. As a result, all three electron spin transitions of the triplet ground state have non-zero dipole moments, with each transition coupling to a single component of the magnetic field. This can be used to measure both strength and orientation of the applied AC field. To validate the technique, we perform a proof of principle experiment using a subset of ensemble NV centers in diamond, all having the same orientation.

Authors:Ioannis Chremmos

Abstract: A way is presented to design quantum wave functions that exhibit backflow, namely negative probability current despite having a strictly positive spectrum of momentum. These wave functions are derived from rational complex functions which are analytic in the upper half-plane and have zeros in the lower half-plane through which the backflowing behavior is controlled. In analogy, backflowing periodic wave functions are derived from rational complex functions which are analytic in the interior and have appropriately placed zeros or poles in the exterior of the unit circle. The concept is combined with a Pad\'e-type procedure to design wave functions of this type that approximate a desired profile along the interval of backflow.

Authors:Soronzonbold Otgonbaatar, Dieter Kranzlmüller

Abstract: We first review the current state of the art of quantum computing for Earth observation and satellite images. There are the persisting challenges of profiting from quantum advantage, and finding the optimal sharing between high-performance computing (HPC) and quantum computing (QC), i.e. the HPC+QC paradigm, for computational EO problems and Artificial Intelligence (AI) approaches. Secondly, we assess some quantum models transpiled into a Clifford+T universal gate set, where the Clifford+T quantum gate set sheds light on the quantum resources required for deploying quantum models either on an HPC system or several QCs. If the Clifford+T quantum gate set cannot be simulated efficiently on an HPC system then we can apply a quantum computer and its computational power over conventional computers. Our resulting quantum resource estimation demonstrates that Quantum Machine Learning (QML) models, which do not comprise a large number of T-gates, can be deployed on an HPC system during the training and validation process; otherwise, we can execute them on several QCs. Namely, QML models having a sufficient number of T-gates provide the quantum advantage if and only if they generalize on unseen data points better than their classical counterparts deployed on the HPC system, and they break the symmetry in their weights at each learning iteration like in conventional deep neural networks. As an initial innovation, we estimate the quantum resources required for some QML models. Secondly, we define the optimal sharing between an HPC+QC system for executing QML models for hyperspectral images (HSIs); HSIs are a specific dataset compared to multispectral images to be deployed on quantum computers due to the limited number of their input qubits, and the commonly used small number of labeled benchmark HSIs.

Authors:Irene Ada Picatoste, Alessandra Colla, Heinz-Peter Breuer

Abstract: Employing a recently developed approach to dynamically emergent quantum thermodynamics, we revisit the thermodynamic behavior of the quantum Otto cycle with a focus on memory effects and strong system-bath couplings. Our investigation is based on an exact treatment of non-Markovianity by means of an exact quantum master equation, modelling the dynamics through the Fano-Anderson model featuring a peaked environmental spectral density. By comparing the results to the standard Markovian case, we find that non-Markovian baths can induce work transfer to the system, and identify specific parameter regions which lead to enhanced work output and efficiency of the cycle. In particular, we demonstrate that these improvements arise when the cycle operates in a frequency interval which contains the peak of the spectral density. This can be understood from an analysis of the renormalized frequencies emerging through the system-baths couplings.

Authors:Antonio Jiménez-Pastor, Kim G. Larsen, Mirco Tribastone, Max Tschaikowski

Abstract: Efficient methods for the simulation of quantum circuits on classic computers are crucial for their improvement and better understanding. Unfortunately, classic array-based simulation of quantum circuits suffers from the curse of dimensionality because the size of the arrays is exponential in the number of qubits. Starting from the observation that results of quantum circuits are often evaluated by means of quantum measurements that capture only a subpart of the entire quantum state, we introduce measurement-preserving reductions. The proposed technique complements existing approaches and can be closely aligned to model reduction approaches from systems biology and control engineering. By providing a publicly available prototype implementation, we demonstrate the applicability of the approach by obtaining substantial reductions of common quantum computing benchmarks.

Authors:Mario G. Silveirinha, Hugo Terças, Mauro Antezza

Abstract: We present a theoretical study of the interaction between an atom characterized by a degenerate ground state and a reciprocal environment, such as a semiconductor nanoparticle, without the presence of external bias. Our analysis reveals that the combined influence of the electron's intrinsic spin magnetic moment on the environment and the chiral atomic dipolar transitions may lead to either the spontaneous breaking of time-reversal symmetry or the emergence of time-crystal-like states with remarkably long relaxation times. The different behavior is ruled by the handedness of the precession motion of the atom's spin vector, which is induced by virtual chiral-dipolar transitions. Specifically, when the relative orientation of the precession angular velocity and the electron spin vector is as in a spinning top, the system manifests time-crystal-like states. Conversely, with the opposite relative orientation, the system experiences spontaneous symmetry breaking of time-reversal symmetry. Our findings introduce a novel mechanism for the spontaneous breaking of time-reversal symmetry in atomic systems, and unveil an exciting opportunity to engineer a nonreciprocal response at the nanoscale, exclusively driven by the quantum vacuum fluctuations.

Authors:Sebastian Brandhofer, Ilia Polian, Kevin Krsulich

Abstract: A limited number of qubits, high error rates, and limited qubit connectivity are major challenges for effective near-term quantum computations. Quantum circuit partitioning divides a quantum computation into a set of computations that include smaller-scale quantum (sub)circuits and classical postprocessing steps. These quantum subcircuits require fewer qubits, incur a smaller effort for satisfying qubit connectivity requirements, and typically incur less error. Thus, quantum circuit partitioning has the potential to enable quantum computations that would otherwise only be available on more matured hardware. However, partitioning quantum circuits generally incurs an exponential increase in quantum computing runtime by repeatedly executing quantum subcircuits. Previous work results in non-optimal subcircuit executions hereby limiting the scope of quantum circuit partitioning. In this work, we develop an optimal partitioning method based on recent advances in quantum circuit knitting. By considering wire cuts and gate cuts in conjunction with ancilla qubit insertions and classical communication, the developed method can determine a minimal cost quantum circuit partitioning. Compared to previous work, we demonstrate the developed method to reduce the overhead in quantum computing time by 73% on average for 56% of evaluated quantum circuits. Given a one hour runtime budget on a typical near-term quantum computer, the developed method could reduce the qubit requirement of the evaluated quantum circuits by 40% on average. These results highlight the ability of the developed method to extend the computational reach of near-term quantum computers by reducing the qubit requirement at a lower increase in quantum circuit executions.

Authors:Katharina Senkalla, Genko Genov, Mathias H. Metsch, Petr Siyushev, Fedor Jelezko

Abstract: Negatively charged group IV defects in diamond show great potential as quantum network nodes due to their efficient spin-photon interface. However, reaching sufficiently long coherence times remains a challenge. In this work, we demonstrate coherent control of germanium-vacancy center (GeV) at millikelvin temperatures and extend its coherence time by several orders of magnitude to more than 20 ms. We model the magnetic and amplitude noise as an Ornstein-Uhlenbeck process, reproducing the experimental results well. The utilized method paves the way to optimized coherence times of group IV defects in various experimental conditions and their successful applications in quantum technologies.

Authors:Rebecca Erbanni, Antonios Varvitsiotis, Dario Poletti

Abstract: We consider a class of games between two competing players that take turns acting on the same many-body quantum register. Each player can perform unitary operations on the register, and after each one of them acts on the register the energy is measured. Player A aims to maximize the energy while player B to minimize it. This class of zero-sum games has a clear second mover advantage if both players can entangle the same portion of the register. We show, however, that if the first player can entangle a larger number of qubits than the second player (which we refer to as having quantum advantage), then the second mover advantage can be significantly reduced. We study the game for different types of quantum advantage of player A versus player B and for different sizes of the register, in particular, scenarios in which absolutely maximally entangled states cannot be achieved. In this case, we also study the effectiveness of using random unitaries. Last, we consider mixed initial preparations of the register, in which case the player with a quantum advantage can rely on strategies stemming from the theory of ergotropy of quantum batteries.

Authors:João F. Doriguello, Alessandro Luongo, Ewin Tang

Abstract: Clustering is one of the most important tools for analysis of large datasets, and perhaps the most popular clustering algorithm is Lloyd's iteration for $k$-means. This iteration takes $N$ vectors $v_1,\dots,v_N\in\mathbb{R}^d$ and outputs $k$ centroids $c_1,\dots,c_k\in\mathbb{R}^d$; these partition the vectors into clusters based on which centroid is closest to a particular vector. We present an overall improved version of the "$q$-means" algorithm, the quantum algorithm originally proposed by Kerenidis, Landman, Luongo, and Prakash (2019) which performs $\varepsilon$-$k$-means, an approximate version of $k$-means clustering. This algorithm does not rely on the quantum linear algebra primitives of prior work, instead only using its QRAM to prepare and measure simple states based on the current iteration's clusters. The time complexity is $O\big(\frac{k^{2}}{\varepsilon^2}(\sqrt{k}d + \log(Nd))\big)$ and maintains the polylogarithmic dependence on $N$ while improving the dependence on most of the other parameters. We also present a "dequantized" algorithm for $\varepsilon$-$k$-means which runs in $O\big(\frac{k^{2}}{\varepsilon^2}(kd + \log(Nd))\big)$ time. Notably, this classical algorithm matches the polylogarithmic dependence on $N$ attained by the quantum algorithms.

Authors:Salvatore Mandrà, Gianni Mossi, Eleanor G. Rieffel

Abstract: In this paper we present a novel method to generate hard instances with planted solutions based on the public-private McEliece post-quantum cryptographic protocol. Unlike other planting methods rooted in the infinite-size statistical analysis, our cryptographic protocol generates instances which are all hard (in cryptographic terms), with the hardness tuned by the size of the private key, and with a guaranteed unique ground state. More importantly, because of the private-public key protocol, planted solutions cannot be easily recovered by a direct inspection of the planted instances without the knowledge of the private key used to generate them, therefore making our protocol suitable to test and evaluate quantum devices without the risk of "backdoors" being exploited.

Authors:Karl Svozil

Abstract: Space-time in quantum mechanics is about bridging Hilbert and configuration space. Thereby, an entirely new perspective is obtained by replacing the Newtonian space-time theater with the image of a presumably high-dimensional Hilbert space, through which space-time becomes an epiphenomenon construed by internal observers.

Authors:Subrata Das, Avimita Chatterjee, Swaroop Ghosh

Abstract: Quantum computing, with its transformative computational potential, is gaining prominence in the technological landscape. As a new and exotic technology, quantum computers involve innumerable Intellectual Property (IP) in the form of fabrication recipe, control electronics and software techniques, to name a few. Furthermore, complexity of quantum systems necessitates extensive involvement of third party tools, equipment and services which could risk the IPs and the Quality of Service and enable other attack surfaces. This paper is a first attempt to explore the quantum computing ecosystem, from the fabrication of quantum processors to the development of specialized software tools and hardware components, from a security perspective. By investigating the publicly disclosed information from industry front runners like IBM, Google, Honeywell and more, we piece together various components of quantum computing supply chain. We also uncover some potential vulnerabilities and attack models and suggest defenses. We highlight the need to scrutinize the quantum computing supply chain further through the lens of security.

Authors:Peyman Azodi, Herschel A Rabitz

Abstract: A new framework is formulated to study entanglement dynamics in many-body quantum systems along with an associated geometric description. In this formulation, called the Quantum Correlation Transfer Function (QCTF), the system's wave function or density matrix is transformed into a new space of complex functions with isolated singularities. Accordingly, entanglement dynamics is encoded in specific residues of the QCTF, and importantly, the explicit evaluation of the system's time dependence is avoided. Notably, the QCTF formulation allows for various algebraic simplifications and approximations to address the normally encountered complications due to the exponential growth of the many-body Hilbert space with the number of bodies. These simplifications are facilitated through considering the patterns, in lieu of the elements, lying within the system's state. Consequently, a main finding of this paper is the exterior (Grassmannian) algebraic expression of many-body entanglement as the collective areas of regions in the Hilbert space spanned by pairs of projections of the wave function onto an arbitrary basis. This latter geometric measure is shown to be equivalent to the second-order Renyi entropy. Additionally, the geometric description of the QCTF shows that characterizing features of the reduced density matrix can be related to experimentally observable quantities. The QCTF-based geometric description offers the prospect of theoretically revealing aspects of many-body entanglement, by drawing on the vast scope of methods from geometry.

Authors:Stan Gudder

Abstract: Measurement models (MMs) stand at the highest structural level of quantum measurement theory. MMs can be employed to construct instruments which stand at the next level. An instrument is thought of as an apparatus that is used to measure observables and update states. Observables, which are still at the next level, are used to determine probabilities of quantum events. The main ingredient of an MM is an interaction channel $\nu$ between the system being measured and a probe system. For a general $\nu$, the measured observable $A$ need not have an explicit useful form. In this work we introduce a condition for $\nu$ called separability and in this case $A$ has an explicit form. Under the assumption that $\nu$ is separable, we study product MMs and conditioned MMs. We also consider the statistics of MMs and their uncertainty principle. Various concepts are illustrated using examples of L\"uders and Holevo instruments.

Authors:Hanan Saidi, Hanane El Hadfi, Abdallah Slaoui, Rachid Ahl Laamara

Abstract: In quantum phase estimation, the Heisenberg limit provides the ultimate accuracy over quasi-classical estimation procedures. However, realizing this limit hinges upon both the detection strategy employed for output measurements and the characteristics of the input states. This study delves into quantum phase estimation using $s$-spin coherent states superposition. Initially, we delve into the explicit formulation of spin coherent states for a spin $s=3/2$. Both the quantum Fisher information and the quantum Cramer-Rao bound are meticulously examined. We analytically show that the ultimate measurement precision of spin cat states approaches the Heisenberg limit, where uncertainty decreases inversely with the total particle number. Moreover, we investigate the phase sensitivity introduced through operators $e^{i\zeta{S}_{z}}$, $e^{i\zeta{S}_{x}}$ and $e^{i\zeta{S}_{y}}$, subsequently comparing the resultants findings. In closing, we provide a general analytical expression for the quantum Cramer-Rao boundary applied to these three parameter-generating operators, utilizing general $s$-spin coherent states. We remarked that attaining Heisenberg-limit precision requires the careful adjustment of insightful information about the geometry of $s$-spin cat states on the Bloch sphere. Additionally, as the number of $s$-spin increases, the Heisenberg limit decreases, and this reduction is inversely proportional to the $s$-spin number.

Authors:Ningyi Xie, Xinwei Lee, Dongsheng Cai, Yoshiyuki Saito, Nobuyoshi Asai, Hoong Chuin Lau

Abstract: The Capacitated Vehicle Routing Problem (CVRP) is an NP-optimization problem (NPO) that arises in various fields including transportation and logistics. The CVRP extends from the Vehicle Routing Problem (VRP), aiming to determine the most efficient plan for a fleet of vehicles to deliver goods to a set of customers, subject to the limited carrying capacity of each vehicle. As the number of possible solutions skyrockets when the number of customers increases, finding the optimal solution remains a significant challenge. Recently, a quantum-classical hybrid algorithm known as Quantum Approximate Optimization Algorithm (QAOA) can provide better solutions in some cases of combinatorial optimization problems, compared to classical heuristics. However, the QAOA exhibits a diminished ability to produce high-quality solutions for some constrained optimization problems including the CVRP. One potential approach for improvement involves a variation of the QAOA known as the Grover-Mixer Quantum Alternating Operator Ansatz (GM-QAOA). In this work, we attempt to use GM-QAOA to solve the CVRP. We present a new binary encoding for the CVRP, with an alternative objective function of minimizing the shortest path that bypasses the vehicle capacity constraint of the CVRP. The search space is further restricted by the Grover-Mixer. We examine and discuss the effectiveness of the proposed solver through its application to several illustrative examples.

Authors:Yan Zhu, Ya-Dong Wu, Qiushi Liu, Yuexuan Wang, Giulio Chiribella

Abstract: Complete characterization of an unknown quantum process can be achieved by process tomography, or, for continuous time processes, by Hamiltonian learning. However, such a characterization becomes unfeasible for high dimensional quantum systems. In this paper, we develop the first neural network algorithm for predicting the behavior of an unknown quantum process when applied on a given ensemble of input states. The network is trained with classical data obtained from measurements on a few pairs of input/output quantum states. After training, it can be used to predict the measurement statistics of a set of measurements of interest performed on the output state corresponding to any input in the state ensemble. Besides learning a quantum gate or quantum circuit, our model can also be applied to the task of learning a noisy quantum evolution and predicting the measurement statistics on a time-evolving quantum state. We show numerical results using our neural network model for various relevant processes in quantum computing, quantum many-body physics, and quantum optics.

Authors:Xiao-Yu Cao, Bing-Hong Li, Yang Wang, Yao Fu, Hua-Lei Yin, Zeng-Bing Chen

Abstract: E-commerce, a type of trading that occurs at a high frequency on the Internet, requires guaranteeing the integrity, authentication and non-repudiation of messages through long distance. As current e-commerce schemes are vulnerable to computational attacks, quantum cryptography, ensuring information-theoretic security against adversary's repudiation and forgery, provides a solution to this problem. However, quantum solutions generally have much lower performance compared to classical ones. Besides, when considering imperfect devices, the performance of quantum schemes exhibits a significant decline. Here, for the first time, we demonstrate the whole e-commerce process of involving the signing of a contract and payment among three parties by proposing a quantum e-commerce scheme, which shows resistance of attacks from imperfect devices. Results show that with a maximum attenuation of 25 dB among participants, our scheme can achieve a signature rate of 0.82 times per second for an agreement size of approximately 0.428 megabit. This proposed scheme presents a promising solution for providing information-theoretic security for e-commerce.

Authors:V. Krutyanskiy, M. Canteri, M. Meraner, V. Krcmarsky, B. P. Lanyon

Abstract: A three-qubit quantum network node based on trapped atomic ions is presented. The ability to establish entanglement between each of the qubits in the node and a separate photon that has travelled over a 101km-long optical fiber is demonstrated. By sending those photons through the fiber in close succession, a remote entanglement rate is achieved that is greater than when using only a single qubit in the node. Once extended to more qubits, this multimode approach can be a useful technique to boost entanglement distribution rates in future long-distance quantum networks of light and matter.

Authors:Yiru Zhou, Pooja Malik, Florian Fertig, Matthias Bock, Tobias Bauer, Tim van Leent, Wei Zhang, Christoph Becher, Harald Weinfurter

Abstract: Long-distance entanglement distribution is the key task for quantum networks, enabling applications such as secure communication and distributed quantum computing. Here we report on novel developments extending the reach for sharing entanglement between a single $^{87}$Rb atom and a single photon over long optical fibers. To maintain a high fidelity during the long flight times through such fibers, the coherence time of the single atom is prolonged to 7 ms by applying a long-lived qubit encoding. In addition, the attenuation in the fibers is minimized by converting the photon's wavelength to the telecom S-Band via polarization-preserving quantum frequency conversion. This enables to observe entanglement between the atomic quantum memory and the emitted photon after passing 101 km of optical fiber with a fidelity better than 70.8$\pm$2.4%. The fidelity, however, is no longer reduced due to loss of coherence of the atom or photon but in the current setup rather due to detector dark counts, showing the suitability of our platform to realize city-to-city scale quantum network links.

Authors:Mats O. Tholén, Riccardo Borgani, Christian Križan, Jonas Bylander, David B. Haviland

Abstract: We implement an iSWAP gate with two transmon qubits using a flux-tunable coupler. Precise control of the relative phase of the qubit-control pulses and the parametric-coupler drive is achieved with a multi-channel instrument called Presto using direct digital synthesis (DDS), a promising technique for scaling up quantum systems. We describe the process of tuning and benchmarking the iSWAP gate, where the relative phase of the pulses is controlled via software. We perform the iSWAP gate in 290 ns, validate it with quantum-state tomography, and measure 2\% error with interleaved randomized benchmarking.

Authors:Milan Liepelt, Tommaso Peduzzi, James R. Wootton

Abstract: Achieving fault-tolerance will require a strong relationship between the hardware and the protocols used. Different approaches will therefore naturally have tailored proof-of-principle experiments to benchmark progress. Nevertheless, repetition codes have become a commonly used basis of experiments that allow cross-platform comparisons. Here we propose methods by which repetition code experiments can be expanded and improved, while retaining cross-platform compatibility. We also consider novel methods of analyzing the results, which offer more detailed insights than simple calculation of the logical error rate.

Authors:Xueying Liang, Xiangfu Zou, Xin Wang, Shenggen Zheng, Zhenbang Rong, Zhiming Huang, Jianfeng Liu, Ying Chen, Jianxiong Wu

Abstract: From the perspective of resource theory, it is interesting to achieve the same quantum task using as few quantum resources as possible. Semiquantum key distribution (SQKD), which allows a quantum user to share a confidential key with a classical user who prepares and operates qubits in only one basis, is an important example for studying this issue. To further limit the quantum resources used by users, in this paper, we constructed the first SQKD protocol which restricts the quantum user to prepare quantum states in only one basis and removes the classical user's measurement capability. Furthermore, we prove that the constructed protocol is unconditionally secure by deriving a key rate expression of the error rate in the asymptotic scenario. The work of this paper provides inspiration for achieving quantum superiority with minimal quantum resources.

Authors:Babatunde M. Ayeni

Abstract: In the age of noisy quantum processors, the exploitation of quantum symmetries can be quite beneficial in the efficient preparation of trial states, an important part of the variational quantum eigensolver algorithm. The benefits include building quantum circuits which are more compact, with lesser number of paramaters, and more robust to noise, than their non-symmetric counterparts. Leveraging on ideas from representation theory we show how to construct symmetric quantum circuits. Similar ideas have been previously used in the field of tensor networks to construct symmetric tensor networks. We focus on the specific case of particle number conservation, that is systems with U(1) symmetry. Based on the representation theory of U(1), we show how to derive the particle-conserving exchange gates, which are commonly used in constructing hardware-efficient quantum circuits for fermionic systems, like in quantum chemistry, material science, and condensed-matter physics. We tested the effectiveness of our circuits with the Heisenberg XXZ model.

Authors:Zurab K. Silagadze

Abstract: Tsang and Caves suggested the idea of a quantum-mechanics-free subsystem in 2012. We contend that Sudarshan's viewpoint on Koopman-von Neumann mechanics is realized in the quantum-mechanics-free subsystem. Since quantum-mechanics-free subsystems are being experimentally realized, Koopman-von Neumann mechanics is essentially transformed into an engineering science.

Authors:Calum J. Robson

Abstract: This paper discusses several issues around Relational Quantum Mechanics. First, I discuss possible ontologies underlying the interpretation, before settling on the hypothesis that RQM follows from contextuality of measurements, due to quantum measurements changing the system measured. I then examine how the consistent histories formalism can be used to clarify which infomation about a system can be shared between different observers. Finally I discuss the similarities and differences between special relativity and RQM.

Authors:Lambert Lin, Steven R White

Abstract: We introduce a novel extrapolation algorithm inspired by quantum mechanics and evaluate its performance against linear prediction. Our method involves mapping function values onto a quantum state and estimating future function values by minimizing entanglement entropy. We demonstrate the effectiveness of our approach on various simple functions, both with and without noise, comparing it to linear prediction. Our results show that the proposed algorithm produces extrapolations comparable to linear prediction, while exhibiting improved performance for functions with sharp features.

Authors:Pablo Tieben, Andreas W. Schell

Abstract: Single photon emitters in hexagonal boron nitride have gathered a lot of attention due to their favourable emission properties and the manifold of possible applications. Despite extensive scientific effort, the exact atomic origin of these emitters has remained unkown thus far. Recently, several studies have tied the emission in the yellow spectral region to carbon-related defects, but the exact atomic structure of the defects remains elusive. In this study, photoluminescence emission and excitation spectroscopy is performed on a large number of emitters within this region. By comparison of the experimental data with theoretical predictions, the origin of yellow single photon emission in hexagonal boron nitride is determined. Knowledge of this atomic structure and its optical properties is crucial for the reliable implementation of these emitters in quantum technologies.

Authors:Chenxu Liu, Rafail Frantzeskakis, Sophia E. Economou, Edwin Barnes

Abstract: Photonic parity projection plays a significant role in photonic quantum information processing. Non-destructive parity projections normally require high-fidelity Controlled-Z gates between photonic and matter qubits, which can be experimentally demanding. In this paper, we propose a nearly deterministic parity projection protocol on two photonic qubits which only requires stable matter-photon Controlled-Phase gates. The fact that our protocol does not require perfect Controlled-Z gates makes it more amenable to experimental implementation.

Authors:Jonathan C. Marcks, Mykyta Onizhuk, Nazar Delegan, Yu-Xin Wang, Masaya Fukami, Maya Watts, Aashish A. Clerk, F. Joseph Heremans, Giulia Galli, David D. Awschalom

Abstract: The nitrogen vacancy (NV) center in diamond, a well-studied, optically active spin defect, is the prototypical system in many state of the art quantum sensing and communication applications. In addition to the enticing properties intrinsic to the NV center, its diamond host's nuclear and electronic spin baths can be leveraged as resources for quantum information, rather than considered solely as sources of decoherence. However, current synthesis approaches result in stochastic defect spin positions, reducing the technology's potential for deterministic control and yield of NV-spin bath systems, as well as scalability and integration with other technologies. Here, we demonstrate the use of theoretical calculations of electronic central spin decoherence as an integral part of an NV-spin bath synthesis workflow, providing a path forward for the quantitative design of NV center-based quantum sensing systems. We use computationally generated coherence data to characterize the properties of single NV center qubits across relevant growth parameters to find general trends in coherence time distributions dependent on spin bath dimensionality and density. We then build a maximum likelihood estimator with our theoretical model, enabling the characterization of a test sample through NV T2* measurements. Finally, we explore the impact of dimensionality on the yield of strongly coupled electron spin systems. The methods presented herein are general and applicable to other qubit platforms that can be appropriately simulated.

Authors:Manuel S. Rudolph, Enrico Fontana, Zoë Holmes, Lukasz Cincio

Abstract: We introduce LOWESA as a classical algorithm for faithfully simulating quantum systems via a classically constructed surrogate expectation landscape. After an initial overhead to build the surrogate landscape, one can rapidly study entire families of Hamiltonians, initial states and target observables. As a case study, we simulate the 127-qubit transverse-field Ising quantum system on a heavy-hexagon lattice with up to 20 Trotter steps which was recently presented in Nature 618, 500-505 (2023). Specifically, we approximately reconstruct (in minutes to hours on a laptop) the entire expectation landscape spanned by the heavy-hex Ising model. The expectation of a given observable can then be evaluated at different parameter values, i.e. with different onsite magnetic fields and coupling strengths, in fractions of a second on a laptop. This highlights that LOWESA can attain state-of-the-art performance in quantum simulation tasks, with the potential to become the algorithm of choice for scanning a wide range of systems quickly.

Authors:Pooja Siwach, Kaytlin Harrison, A. Baha Balantekin

Abstract: We simulate the time evolution of collective neutrino oscillations in two-flavor settings on a quantum computer. We explore the generalization of Trotter-Suzuki approximation to time-dependent Hamiltonian dynamics. The trotterization steps are further optimized using the Cartan decomposition of two-qubit unitary gates U $\in$ SU (4) in the minimum number of controlled-NOT (CNOT) gates making the algorithm more resilient to the hardware noise. A more efficient hybrid quantum-classical algorithm is also explored to solve the problem on noisy intermediate-scale quantum (NISQ) devices.

Authors:Philipp A. Hoehn, Isha Kotecha, Fabio M. Mele

Abstract: It was recently noted that different internal quantum reference frames (QRFs) partition a system in different ways into subsystems, much like different inertial observers in special relativity decompose spacetime in different ways into space and time. Here we expand on this QRF relativity of subsystems and elucidate that it is the source of all novel QRF dependent effects, just like the relativity of simultaneity is the origin of all characteristic special relativistic phenomena. We show that subsystem relativity, in fact, also arises in special relativity with internal frames and, by implying the relativity of simultaneity, constitutes a generalisation of it. Physical consequences of the QRF relativity of subsystems, which we explore here systematically, and the relativity of simultaneity may thus be seen in similar light. We focus on investigating when and how subsystem correlations and entropies, interactions and types of dynamics (open vs. closed), as well as quantum thermodynamical processes change under QRF transformations. We show that thermal equilibrium is generically QRF relative and find that, remarkably, $\textit{QRF transformations not only can change a subsystem temperature, but even map positive into negative temperature states}$. We further examine how non-equilibrium notions of heat and work exchange, as well as entropy production and flow depend on the QRF. Along the way, we develop the first study of how reduced subsystem states transform under QRF changes. Focusing on physical insights, we restrict to ideal QRFs associated with finite abelian groups. Besides being conducive to rigour, the ensuing finite-dimensional setting is where quantum information-theoretic quantities and quantum thermodynamics are best developed. We anticipate, however, that our results extend qualitatively to more general groups and frames, and even to subsystems in gauge theory and gravity.

Authors:Bo Xing, Xhek Turkeshi, Marco Schiró, Rosario Fazio, Dario Poletti

Abstract: Interspersing unitary dynamics with local measurements results in measurement-induced phases and transitions in many-body quantum systems. When the evolution is driven by a local Hamiltonian, two types of transitions have been observed, characterized by an abrupt change in the system size scaling of entanglement entropy. The critical point separates the strongly monitored area-law phase from a volume law or a sub-extensive, typically logarithmic-like one at low measurement rates. Identifying the key ingredients responsible for the entanglement scaling in the weakly monitored phase is the key purpose of this work. For this purpose, we consider prototypical one-dimensional spin chains with local monitoring featuring the presence/absence of U(1) symmetry, integrability, and interactions. Using exact numerical methods, the system sizes studied reveal that the presence of interaction is always correlated to a volume-law weakly monitored phase. In contrast, non-interacting systems present sub-extensive scaling of entanglement. Other characteristics, namely integrability or U(1) symmetry, do not play a role in the character of the entanglement phase.

Authors:E. P. Ruddy, Y. Jiang, N. E. Frattini, K. O. Quinlan, K. W. Lehnert

Abstract: Noiseless measurement of a single quadrature in systems of parametrically coupled oscillators is theoretically possible by pumping at the sum and difference frequencies of the two oscillators, realizing a backaction-evading (BAE) scheme. Although this would hold true in the simplest scenario for a system with pure three-wave mixing, implementations of this scheme are hindered by unwanted higher-order parametric processes that destabilize the system and add noise. We show analytically that detuning the two pumps from the sum and difference frequencies can stabilize the system and fully recover the BAE performance, enabling operation at otherwise inaccessible cooperativities. We also show that the acceleration demonstrated in a weak signal detection experiment [PRX QUANTUM 4, 020302 (2023)] was only achievable because of this detuning technique.

Authors:Hyung S. Choi, Ye Jin Han, Collin Kessinger, Qiaoren Wang

Abstract: We propose a new quantum key distribution scheme that is based on the optimum expectation values of maximally entangled Greenberger-Horne-Zeilinger states. Our protocol makes use of the degrees of freedom in continuously variable angles, thereby increasing the security of the key distribution. Outlined are two protocols that distribute a key from Alice to Bob using the above idea, followed by an extension that allows for the same key to be shared with Charlie. We show how this scheme provides for certain detection of any eavesdropper through absolute violation rather than the probabilistic violation used in many protocols.

Authors:Ragesh Jaiswal

Abstract: We give a quantum approximation scheme (i.e., $(1 + \varepsilon)$-approximation for every $\varepsilon > 0$) for the classical $k$-means clustering problem in the QRAM model with a running time that has only polylogarithmic dependence on the number of data points. More specifically, given a dataset $V$ with $N$ points in $\mathbb{R}^d$ stored in QRAM data structure, our quantum algorithm runs in time $\tilde{O} \left( 2^{\tilde{O}(\frac{k}{\varepsilon})} \eta^2 d\right)$ and with high probability outputs a set $C$ of $k$ centers such that $cost(V, C) \leq (1+\varepsilon) \cdot cost(V, C_{OPT})$. Here $C_{OPT}$ denotes the optimal $k$-centers, $cost(.)$ denotes the standard $k$-means cost function (i.e., the sum of the squared distance of points to the closest center), and $\eta$ is the aspect ratio (i.e., the ratio of maximum distance to minimum distance). This is the first quantum algorithm with a polylogarithmic running time that gives a provable approximation guarantee of $(1+\varepsilon)$ for the $k$-means problem. Also, unlike previous works on unsupervised learning, our quantum algorithm does not require quantum linear algebra subroutines and has a running time independent of parameters (e.g., condition number) that appear in such procedures.

Authors:Dibakar Das, Shiva Kumar Malapaka, Jyotsna Bapat, Debabrata Das

Abstract: Quantum networks are of great interest of late which apply quantum mechanics to transfer information securely. One of the key properties which are exploited is entanglement to transfer information from one network node to another. Applications like quantum teleportation rely on the entanglement between the concerned nodes. Thus, efficient entanglement distribution among network nodes is of utmost importance. Several entanglement distribution methods have been proposed in the literature which primarily rely on attributes, such as, fidelities, link layer network topologies, proactive distribution, etc. This paper studies the centralities of the network when the link layer topology of entanglements (referred to as entangled graph) is driven by usage patterns of peer-to-peer connections between remote nodes (referred to as connection graph) with different characteristics. Three different distributions (uniform, gaussian, and power law) are considered for the connection graph where the two nodes are selected from the same distribution. For the entangled graph, both reactive and proactive entanglements are employed to form a random graph. Results show that the edge centralities (measured as usage frequencies of individual edges during entanglement distribution) of the entangled graph follow power law distributions whereas the growth in entanglements with connections and node centralities (degrees of nodes) are monomolecularly distributed for most of the scenarios. These findings will help in quantum resource management, e.g., quantum technology with high reliability and lower decoherence time may be allocated to edges with high centralities.

Authors:Mohammad Umar, Vibhav Narayan Singh, Mohammad Hasan, Bhabani Prasad Mandal

Abstract: We introduce a new type of potential system that combines the families of general Cantor (fractal system) and general Smith-Volterra-Cantor (non-fractal system) potentials. We call this system as Unified Cantor Potential (UCP) system. The UCP system of total span $L$ is characterized by scaling parameter $\rho >1$, stage $G$ and two real numbers $\alpha$ and $\beta$. For $\alpha=1$, $\beta=0$, the UCP system represents general Cantor potential while for $\alpha=0$, $\beta=1$, this system represent general Smith-Volterra-Cantor (SVC) potential. We provide close-form expression of transmission probability from UCP system for arbitrary $\alpha$ and $\beta$ by using $q$-Pochhammer symbol. Several new features of scattering are reported for this system. The transmission probability $T_{G}(k)$ shows a scaling behavior with $k$ which is derived analytically for this potential. The proposed system also opens up the possibility for further generalization of new potential systems that encompass a large class of fractal and non-fractal systems. The analytical formulation of tunneling from this system would help to study the transmission feature at breaking threshold when a system transit from fractal to non-fractal domain.

Authors:Hidetaka Manabe, Yasunari Suzuki, Andrew S. Darmawan

Abstract: Leakage errors, in which a qubit is excited to a level outside the qubit subspace, represent a significant obstacle in the development of robust quantum computers. We present a computationally efficient simulation methodology for studying leakage errors in quantum error correcting codes (QECCs) using tensor network methods, specifically Matrix Product States (MPS). Our approach enables the simulation of various leakage processes, including thermal noise and coherent errors, without approximations (such as the Pauli twirling approximation) that can lead to errors in the estimation of the logical error rate. We apply our method to two QECCs: the one-dimensional (1D) repetition code and a thin $3\times d$ surface code. By leveraging the small amount of entanglement generated during the error correction process, we are able to study large systems, up to a few hundred qudits, over many code cycles. We consider a realistic noise model of leakage relevant to superconducting qubits to evaluate code performance and a variety of leakage removal strategies. Our numerical results suggest that appropriate leakage removal is crucial, especially when the code distance is large.

Authors:Dongni Chen, Zhengyang Peng, Jiahui Li, Stefano Chesi, Yingdan Wang

Abstract: The spontaneous breaking of time translation symmetry in periodically driven Floquet systems can lead to a discrete time crystal. Here we study the occurrence of such dynamical phase in a driven-dissipative optomechanical system with two membranes in the middle. We find that, under certian conditions, the system can be mapped to an open Dicke model and realizes a superradianttype phase transition. Furthermore, applying a suitable periodically modulated drive, the system dynamics exhibits a robust subharmonic oscillation persistent in the thermodynamic limit.

Authors:Sisi Miao, Alexander Schnerring, Haizheng Li, Laurent Schmalen

Abstract: Quantum low-density parity-check (QLDPC) codes are promising candidates for error correction in quantum computers. One of the major challenges in implementing QLDPC codes in quantum computers is the lack of a universal decoder. In this work, we first propose to decode QLDPC codes with a belief propagation (BP) decoder operating on overcomplete check matrices. Then, we extend the neural BP (NBP) decoder, which was originally studied for suboptimal binary BP decoding of QLPDC codes, to quaternary BP decoders. Numerical simulation results demonstrate that both approaches as well as their combination yield a low-latency, high-performance decoder for several short to moderate length QLDPC codes.

Authors:Anja Seegebrecht, Tanja Schilling

Abstract: We compare definitions of the internal energy of an open quantum system and strategies to split the internal energy into work and heat contributions as given by four different approaches from autonomous system framework. Our discussion focuses on methods that allow for arbitrary environments (not just heat baths) and driving by a quantum mechanical system. As a simple application we consider an atom as the system of interest and an oscillator field mode as the environment. Three different types of coupling are analyzed. We discuss ambiguities in the definitions and highlight differences that appear if one aims at constructing environments that act as pure heat or work reservoirs. Further, we identify different sources of work (e.g. coherence, correlations, or frequency offset), depending on the underlying framework. Finally, we give arguments to favour the approach based on minimal dissipation.

Authors:Shao-Hen Chiew, Kilian Poirier, Rajesh Mishra, Ulrike Bornheimer, Ewan Munro, Si Han Foon, Christopher Wanru Chen, Wei Sheng Lim, Chee Wei Nga

Abstract: Multi-objective optimization is a ubiquitous problem that arises naturally in many scientific and industrial areas. Network routing optimization with multi-objective performance demands falls into this problem class, and finding good quality solutions at large scales is generally challenging. In this work, we develop a scheme with which near-term quantum computers can be applied to solve multi-objective combinatorial optimization problems. We study the application of this scheme to the network routing problem in detail, by first mapping it to the multi-objective shortest path problem. Focusing on an implementation based on the quantum approximate optimization algorithm (QAOA) -- the go-to approach for tackling optimization problems on near-term quantum computers -- we examine the Pareto plot that results from the scheme, and qualitatively analyze its ability to produce Pareto-optimal solutions. We further provide theoretical and numerical scaling analyses of the resource requirements and performance of QAOA, and identify key challenges associated with this approach. Finally, through Amazon Braket we execute small-scale implementations of our scheme on the IonQ Harmony 11-qubit quantum computer.

Authors:Veronika Baumann

Abstract: The famous Wigner's friend experiment considers an observer -- the friend -- and a superobserver -- Wigner -- who treats the friend as a quantum system and her interaction with other quantum systems as unitary dynamics. This is at odds with the friend describing this interaction via collapse dynamics, if she interacts with the quantum system in a way that she would consider a measurement. These different descriptions constitute the Wigner's friend paradox. Extended Wigner's friend experiments combine the original thought experiment with non-locality setups. This allows for deriving local friendliness inequalities, similar to Bell's theorem, which can be violated for certain extended Wigner's friend scenarios. A Wigner's friend paradox and the violation of local friendliness inequalities require that no classical record exists, which reveals the result the friend observed during her measurement. Otherwise Wigner agrees with his friend's description and no local friendliness inequality can be violated. In this article, I introduce classical communication between Wigner and his friend and discuss its effects on the simple as well as extended Wigner's friend experiments. By controlling the properties of a (quasi) classical communication channel between Wigner and the friend one can regulate how much outcome information about the friend's measurement is revealed. This gives a smooth transition between the paradoxical description and the possibility of violating local friendliness inequalities, on the one hand, and the effectively collapsed case, on the other hand.

Authors:Sai Li, Zhongchu Ni, Libo Zhang, Yanyan Cai, Jiasheng Mai, Shengcheng Wen, Pan Zheng, Xiaowei Deng, Song Liu, Yuan Xu, Dapeng Yu

Abstract: Fock states with a well-defined number of photons in an oscillator have shown a wide range of applications in quantum information science. Nonetheless, their usefulness has been marred by single and multiple photon losses due to unavoidable environment-induced dissipation. Though several dissipation engineering methods have been developed to counteract the leading single-photon loss error, averting multiple photon losses remains elusive. Here, we experimentally demonstrate a dissipation engineering method that autonomously stabilizes multi-photon Fock states against losses of multiple photons using a cascaded selective photon-addition operation in a superconducting quantum circuit. Through measuring the photon-number populations and Wigner tomography of the oscillator states, we observe a prolonged preservation of quantum coherence properties for the stabilized Fock states $\vert N\rangle$ with $N=1,2,3$ for a duration of about $10$~ms, far surpassing their intrinsic lifetimes of less than $50~\mu$s. Furthermore, the dissipation engineering method demonstrated here also facilitates the implementation of a non-unitary operation for resetting a binomially-encoded logical qubit. These results highlight the potential application in error-correctable quantum information processing against multi-photon-loss errors.

Authors:Egor E. Nuzhin, Dmitry Yudin

Abstract: Advances in the experimental demonstration of quantum processors have provoked a surge of interest to the idea of practical implementation of quantum computing over last years. It is expected that the use of quantum algorithms will significantly speed up the solution to certain problems in numerical optimization and machine learning. In this paper, we propose a quantum-enhanced policy iteration (QEPI) algorithm as widely used in the domain of reinforcement learning and validate it with the focus on the mountain car problem. In practice, we elaborate on the soft version of the value iteration algorithm, which is beneficial for policy interpretation, and discuss the stochastic discretization technique in the context of continuous state reinforcement learning problems for the purposes of QEPI. The complexity of the algorithm is analyzed for dense and (typical) sparse cases. Numerical results on the example of a mountain car with the use of a quantum emulator verify the developed procedures and benchmark the QEPI performance.

Authors:David Raveh, Rafael I. Nepomechie

Abstract: Dicke states are completely symmetric states of multiple qubits (2-level systems), and qudit Dicke states are their $d$-level generalization. We define here $q$-deformed qudit Dicke states using the quantum algebra $su_q(d)$. We show that these states can be compactly expressed as a weighted sum over permutations with $q$-factors involving the so-called inversion number, an important permutation statistic in Combinatorics. We use this result to compute the bipartite entanglement entropy of these states. We also discuss the preparation of these states on a quantum computer, and show that introducing a $q$-dependence does not change the circuit gate count.

Authors:Shi Jin, Nana Liu, Chuwen Ma

Abstract: We present quantum algorithms for electromagnetic fields governed by Maxwell's equations. The algorithms are based on the Schr\"odingersation approach, which transforms any linear PDEs and ODEs with non-unitary dynamics into a system evolving under unitary dynamics, via a warped phase transformation that maps the equation into one higher dimension. In this paper, our quantum algorithms are based on either a direct approximation of Maxwell's equations combined with Yee's algorithm, or a matrix representation in terms of Riemann-Silberstein vectors combined with a spectral approach and an upwind scheme. We implement these algorithms with physical boundary conditions, including perfect conductor and impedance boundaries. We also solve Maxwell's equations for a linear inhomogeneous medium, specifically the interface problem. Several numerical experiments are performed to demonstrate the validity of this approach. In addition, instead of qubits, the quantum algorithms can also be formulated in the continuous variable quantum framework, which allows the quantum simulation of Maxwell's equations in analog quantum simulation.

Authors:Ben Avnit, Doron Cohen

Abstract: We consider a quantized version of the Sinai-Derrida model for "random walk in random environment". The model is defined in terms of a Lindblad master equation. For a ring geometry (a chain with periodic boundary condition) it features a delocalization-transition as the bias in increased beyond a critical value, indicating that the relaxation becomes under-damped. Counter intuitively, the effective disorder is enhanced due to coherent hopping. We analyze in detail this enhancement and its dependence on the model parameters. The non-monotonic dependence of the Lindbladian spectrum on the rate of the coherent transitions is highlighted.

Authors:S. I. Pavlik

Abstract: The exact representation of the atomic inversion in the Jaynes-Cummings model as an integral over the Hankel contour is used. For a field in a binomial state, the integral is evaluated using the saddle point method. Simple approximate analytical expressions for collapse and revivals are obtained.

Authors:Lars Simon, Manuel Radons

Abstract: In \cite{simon2023algorithms} we introduced four algorithms for the training of neural support vector machines (NSVMs) and demonstrated their feasibility. In this note we introduce neural quantum support vector machines, that is, NSVMs with a quantum kernel, and extend our results to this setting.

Authors:Mar Ferri-Cortés, Jose A. Almanza-Marrero, Rosa López, Roberta Zambrini, Gonzalo Manzano

Abstract: The thermodynamic behavior of Markovian open quantum systems can be described at the level of fluctuations by using continuous monitoring approaches. However, practical applications require assessing imperfect detection schemes, where the definition of main thermodynamic quantities becomes subtle and universal fluctuation relations are unknown. Here we fill this gap by deriving a universal fluctuation relation that links entropy production in ideal and in inefficient monitoring setups. This provides a suitable estimator of dissipation using imperfect detection records that lower bounds the underlying entropy production at the level of single trajectories. We illustrate our findings with a driven-dissipative two-level system following quantum jump trajectories.

Authors:John Harding, Andre Kornell

Abstract: An irreducible complete atomic OML of infinite height cannot both be algebraic and have the covering property. However, Kalmbach's construction provides an example of such an OML that is algebraic and has the 2-covering property, and Keller's construction provides an example of such an OML that has the covering property and is completely hereditarily atomic. Completely hereditarily atomic OMLs generalize algebraic OMLs suitably to quantum predicate logic.

Authors:Abinash Sahu, Naga Dileep Varikuti, Bishal Kumar Das, Vaibhav Madhok

Abstract: We study operator spreading in many-body quantum systems by its potential to generate an informationally complete measurement record in quantum tomography. We adopt continuous weak measurement tomography for this purpose. We generate the measurement record as a series of expectation values of an observable evolving under the desired dynamics, which can show a transition from integrability to full chaos. We find that the amount of operator spreading, as quantified by the fidelity in quantum tomography, increases with the degree of chaos in the system. We also observe a remarkable increase in information gain when the dynamics transitions from integrable to non-integrable. We find our approach in quantifying operator spreading is a more consistent indicator of quantum chaos than Krylov complexity as the latter may correlate/anti-correlate or show no clear behavior with the level of chaos in the dynamics. We support our argument through various metrics of information gain for two models; the Ising spin chain with a tilted magnetic field and the Heisenberg XXZ spin chain with an integrability breaking field. Our study gives an operational interpretation for operator spreading in quantum chaos.

Authors:Naghi Behzadi

Abstract: We introduce two types of thermodynamic refrigeration cycles obtained through modification of the Otto cycle refrigerator by a generalized measurement channel. These refrigerators are corresponding to the activation of the measurement-based stroke before (first type) and after (second type) the full thermalization of the cooling medium by the cold reservoir in the related familiar Otto cycle. We show that the coefficient of performance for the first type modified refrigerator increases linearly in terms of measurement strength parameter, beyond the classical cooling of the known Otto cycle refrigerator. The second type interestingly introduces another autonomous refrigerator whose supplying work is provided by a quantum engine induced by the measurement channel along the modified cycle. By the considered measurement channel, we also establish such modifications on the swap refrigerator. It is observed that the thermodynamic properties of the obtained modified swap refrigerators are the same as of the modified Otto cycle ones respectively.

Authors:Jonathan Allcock, Jinge Bao, João F. Doriguello, Alessandro Luongo, Miklos Santha

Abstract: We explore the power of the unbounded Fan-Out gate and the Global Tunable gates generated by Ising-type Hamiltonians in constructing constant-depth quantum circuits, with particular attention to quantum memory devices. We propose two types of constant-depth constructions for implementing Uniformly Controlled Gates. These gates include the Fan-In gates defined by $|x\rangle|b\rangle\mapsto |x\rangle|b\oplus f(x)\rangle$ for $x\in\{0,1\}^n$ and $b\in\{0,1\}$, where $f$ is a Boolean function. The first of our constructions is based on computing the one-hot encoding of the control register $|x\rangle$, while the second is based on Boolean analysis and exploits different representations of $f$ such as its Fourier expansion. Via these constructions, we obtain constant-depth circuits for the quantum counterparts of read-only and read-write memory devices -- Quantum Random Access Memory (QRAM) and Quantum Random Access Gate (QRAG) -- of memory size $n$. The implementation based on one-hot encoding requires either $O(n\log{n}\log\log{n})$ ancillae and $O(n\log{n})$ Fan-Out gates or $O(n\log{n})$ ancillae and $6$ Global Tunable gates. On the other hand, the implementation based on Boolean analysis requires only $2$ Global Tunable gates at the expense of $O(n^2)$ ancillae.

Authors:Fei Hua, Meng Wang, Gushu Li, Bo Peng, Chenxu Liu, Muqing Zheng, Samuel Stein, Yufei Ding, Eddy Z. Zhang, Travis S. Humble, Ang Li

Abstract: The success of a quantum algorithm hinges on the ability to orchestrate a successful application induction. Detrimental overheads in mapping general quantum circuits to physically implementable routines can be the deciding factor between a successful and erroneous circuit induction. In QASMTrans, we focus on the problem of rapid circuit transpilation. Transpilation plays a crucial role in converting high-level, machine-agnostic circuits into machine-specific circuits constrained by physical topology and supported gate sets. The efficiency of transpilation continues to be a substantial bottleneck, especially when dealing with larger circuits requiring high degrees of inter-qubit interaction. QASMTrans is a high-performance C++ quantum transpiler framework that demonstrates up to 369X speedups compared to the commonly used Qiskit transpiler. We observe speedups on large dense circuits such as uccsd_n24 and qft_n320 which require O(10^6) gates. QASMTrans successfully transpiles the aforementioned circuits in 69s and 31s, whilst Qiskit exceeded an hour of transpilation time. With QASMTrans providing transpiled circuits in a fraction of the time of prior transpilers, potential design space exploration, and heuristic-based transpiler design becomes substantially more tractable. QASMTrans is released at http://github.com/pnnl/qasmtrans.

Authors:Shikun Zhang, Guofeng Zhang

Abstract: This article concerns the attraction domain analysis for steady states in Markovian open quantum systems. The central question is proposed as: given a steady state, which part of the state space of density operators does it attract and which part does it not attract? We answer this question by presenting necessary and sufficient conditions that determine, for any steady state and initial state, whether the latter belongs to the attraction domain of the former. Moreover, we show that steady states without uniqueness in the set of density operators have attraction domains with measure zero under some translation invariant and locally finite measures. Finally, an example regarding an open Heisenberg XXZ spin chain is presented.

Authors:Miloslav Znojil

Abstract: A $(K+1)-$plet of non-Hermitian and time-dependent operators (say, $\Lambda_j(t)$, $j=0,1,\ldots,K$) can be interpreted as the set of observables characterizing a unitary quantum system. What is required is the existence of a self-adjoint and, in general, time-dependent operator (say, $\Theta(t)$ called inner product metric) making the operators quasi-Hermitian, $\Lambda_j^\dagger(t)\Theta(t)=\Theta(t)\Lambda_j(t)$. The theory (called non-Hermitian interaction-picture, NIP) requires a separate description of the evolution of the states $\psi(t)$ (realized, via Schr\"{o}dinger-type equation, by a generator, say, $G(t)$) and of the observables themselves (a different generator (say, $\Sigma(t)(t)$) occurs in the related non-Hermitian Heisenberg-type equation). Every $\Lambda_j(t)$ (and, in particular, Hamiltonian $H(t)=\Lambda_0(t)$) appears isospectral to its hypothetical isospectral and self-adjoint (but, by assumption, prohibitively user-unfriendly) avatar $\lambda_j(t)=\Omega(t)\Lambda_j(t)\Omega^{-1}(t)$ with $\Omega^\dagger(t)\Omega(t)=\Theta(t)$. In our paper the key role played by identity $H(t)=G(t)+\Sigma(t)$ is shown to imply that there exist just three alternative meaningful implementations of the NIP approach, viz., ``number one'' (a ``dynamical'' strategy based on the knowledge of $H(t)$), ``number two'' (a ``kinematical'' one, based on the Coriolis force $\Sigma(t)$) and ``number three'' (in the literature, such a construction based on $G(t)$ is most popular but, paradoxically, it is also most complicated).

Authors:Yao Wang

Abstract: The uncertainty principle lies at the heart of quantum mechanics, as it describes the fundamental trade-off between the precision of position and momentum measurements. In this work, we study the quantum particle in the Boltzmann states and derive a refined lower bound on the product of {\Delta}x and {\Delta}p. Our new bound is expressed in terms of the ratio between {\Delta}x and the thermal de Broglie wavelength, and provides a valuable tool for characterizing thermodynamic precision. We apply our results to the Brownian oscillator system, where we compare our new bound with the well-known Heisenberg uncertainty principle. Our analysis shows that our new bound offers a more precise measure of the thermodynamic limits of precision.

Authors:Ege Özgün

Abstract: We calculated the eigenvalues for a general N-channel coupled system with parity-time symmetry due to equal loss/gain. We found that the eigenspectrum displays a mixing of parity-time symmetric and broken phases, with N-2 of the eigenvalues being parity-time broken whereas the remaining two being either parity-time symmetric or broken depending on the loss/gain and coupling parameters. Our results also show that mixing of parity-time symmetric and parity-time broken phases can only be obtained for at least four-channels if other degrees of freedom like polarization is not taken into account.

Authors:Lukas Wolff, Jörg Evers

Abstract: Up to now, experiments involving M\"ossbauer nuclei have been restricted to the low-excitation regime. The reason for this is the narrow spectral line width of the nuclei. This defining feature enables M\"ossbauer spectroscopy with remarkable resolution and convenient control and measurements in the time domain, but at the same time implies that only a tiny part of the photons delivered by accelerator-based x-ray sources with orders-of-magnitude larger pulse bandwidth are resonant with the nuclei. X-ray free-electron lasers promise a substantial enhancement of the number of nuclear-resonant photons per pulse, such that excitations beyond the low-excitation (LER) regime come within reach. This raises the question, how the onset of non-linear excitations could be experimentally verified. Here, we develop and explore a method to detect an excitation of nuclear ensembles beyond the LER for ensembles of nuclei embedded in x-ray waveguides. It relies on the comparison of the x-rays coherently and incoherently scattered off of the nuclei. As a key result, we show that the ratio of the two observables is constant within the LER, essentially independent of the details of the nuclear system and the characteristics of the exciting x-rays. Conversely, deviations from this equivalence serve as a direct indication of excitations beyond the LER. Building upon this observation, we develop a variety of experimental signatures both, for near-instantaneous impulsive and for temporally-extended non-impulsive x-ray excitation. Correlating coherently and incoherently scattered intensities further allows one to compare theoretical models of nonlinear excitations more rigorously to corresponding experiments.

Authors:Cristian A. Galvis-Florez, Daniel Reitzner, Simo Särkkä

Abstract: Current quantum computers have the potential to overcome classical computational methods, however, the capability of the algorithms that can be executed on noisy intermediate-scale quantum devices is limited due to hardware imperfections. Estimating the state of a qubit is often needed in different quantum protocols, due to the lack of direct measurements. In this paper, we consider the problem of estimating the quantum state of a qubit in a quantum processing unit without conducting direct measurements of it. We consider a parameterized measurement model to estimate the quantum state, represented as a quantum circuit, which is optimized using the quantum tomographic transfer function. We implement and test the circuit using the quantum computer of the Technical Research Centre of Finland as well as an IBM quantum computer. We demonstrate that the set of positive operator-valued measurements used for the estimation is symmetric and informationally complete. Moreover, the resources needed for qubit estimation are reduced when direct measurements are allowed, keeping the symmetric property of the measurements.

Authors:Yannic Rath

Abstract: Capturing the correlation emerging between constituents of many-body systems accurately is one of the key challenges for the appropriate description of various systems whose properties are underpinned by quantum mechanical fundamentals. This thesis discusses novel tools and techniques for the (classical) numerical modelling of quantum many-body wavefunctions exhibiting non-trivial correlations with the ultimate goal to introduce a universal framework for finding efficient quantum state representations. It is outlined how synergies with standard machine learning frameworks can be exploited to enable an automated inference of the relevant intrinsic characteristics, essentially without restricting the approximated state to specific (physically expected) correlation characteristics of the target. It is presented how rigorous Bayesian regression techniques, e.g. formalized via Gaussian Processes, can be utilized to introduce compact forms for various many-body states. Based on the probabilistic regression techniques forming the foundation of the resulting ansatz, coined the Gaussian Process State, different compression techniques are explored to efficiently extract a numerically feasible representation from which physical properties can be extracted. By following intuitively motivated modelling principles, the model carries a high degree of interpretability and offers an easily applicable tool for the study of different quantum systems, including ones inherently hard to simulate due to their strong correlation. This thesis outlines different perspectives on Gaussian Process States, and demonstrates the practical applicability of the numerical framework based on several benchmark applications, in particular, ground state approximations for prototypical quantum lattice models, Fermi-Hubbard models and $J_1-J_2$ models, as well as simple ab-initio quantum chemical systems.

Authors:Shreyans Jain, Tobias Sägesser, Pavel Hrmo, Celeste Torkzaban, Martin Stadler, Robin Oswald, Chris Axline, Amado Bautista-Salvador, Christian Ospelkaus, Daniel Kienzler, Jonathan Home

Abstract: Trapped ions in radio-frequency traps are among the leading approaches for realizing quantum computers, due to high-fidelity quantum gates and long coherence times. However, the use of radio-frequencies presents a number of challenges to scaling, including requiring compatibility of chips with high voltages, managing power dissipation and restricting transport and placement of ions. By replacing the radio-frequency field with a 3 T magnetic field, we here realize a micro-fabricated Penning ion trap which removes these restrictions. We demonstrate full quantum control of an ion in this setting, as well as the ability to transport the ion arbitrarily in the trapping plane above the chip. This unique feature of the Penning micro-trap approach opens up a modification of the Quantum CCD architecture with improved connectivity and flexibility, facilitating the realization of large-scale trapped-ion quantum computing, quantum simulation and quantum sensing.

Authors:Arthur Vesperini, Roberto Franzosi

Abstract: In this work, we present a comprehensive exploration of the entanglement and graph connectivity properties of graph states. We quantify the entanglement in pseudo graph states using the entanglement distance, a recently introduced measure of entanglement. Additionally, we propose a novel approach to probe the underlying graph connectivity of genuine graph states, using quantum correlators of Pauli matrices. Our findings also reveal interesting implications for measurement processes, demonstrating the equivalence of certain projective measurements. Finally, we emphasize the simplicity of data analysis within this framework. This work contributes to a deeper understanding of the entanglement and connectivity properties of graph states, offering valuable insights for quantum information processing and quantum computing applications. In this work, we do not resort to the celebrated stabilizer formalism, which is the framework typically preferred for the study of this type of state; on the contrary, our approach is solely based on the concepts of expectation values, quantum correlations and projective measurement, which have the advantage of being very intuitive and fundamental tools of quantum theory.

Authors:Liam Fuderer, Joseph J Hope, Simon A Haine

Abstract: We introduce a new spin-squeezing technique that is a hybrid of two well established spin-squeezing techniques, quantum nondemolition measurement (QND) and one-axis twisting (OAT). This hybrid method aims to improve spin-squeezing over what is currently achievable using QND and OAT. In practical situations, the strength of both the QND and OAT interactions is limited. We found that in these situations, the hybrid scheme performed considerably better than either OAT or QND used in isolation. As QND and OAT have both been realised experimentally, this technique could be implemented in current atom interferometry setups with only minor modifications to the experiment.

Authors:Varun Narasimhachar

Abstract: Quantum coherence is an indispensable resource for quantum technological applications. It is known to be distillable from a noisy form using operations that cannot create coherence. However, distillation exacts a hidden coherent measurement cost, whose extent has not previously been estimated. Here we show that this cost (quantified by an equivalent number of Hadamard measurements) is related to what we call the irretrievable coherence: the difference between the coherence of formation and the distillable coherence. We conjecture (and make partial progress towards proving) that when distilling from many copies of a given noisy coherent state, the coherent measurement cost scales extensively in the number of copies, at an asymptotic rate exactly equalling the input's irretrievable coherence. This cost applies to any application whereof coherence distillation is an incidental outcome (e.g. incoherent randomness extraction), but the implications are more dramatic if pure coherence is the only desired outcome: the measurement cost may often be higher than the distilled yield, in which case coherence should rather be prepared afresh than distilled from a noisy input.

Authors:Teiko Heinosaari, Oskari Kerppo, Leevi Leppäjärvi, Martin Plávala

Abstract: Communication scenarios between two parties can be implemented by first encoding messages into some states of a physical system which acts as the physical medium of the communication and then decoding the messages by measuring the state of the system. We show that already in the simplest possible scenarios it is possible to detect a definite, unbounded advantage of quantum systems over classical systems. We do this by constructing a family of operationally meaningful communication tasks each of which on one hand can be implemented by using just a single qubit but which on the other hand require unboundedly larger classical system for classical implementation. Furthemore, we show that even though with the additional resource of shared randomness the proposed communication tasks can be implemented by both quantum and classical systems of the same size, the number of coordinated actions needed for the classical implementation also grows unboundedly. In particular, no finite storage can be used to store all the coordinated actions needed to implement all the possible quantum communication tasks with classical systems. As a consequence, shared randomness cannot be viewed as a free resource.

Authors:Paulo J. Cavalcanti, Giovanni Scala, Antonio Mandarino, Cosmo Lupo

Abstract: We frame entanglement detection as a problem of random variable inference to introduce a quantitative method to measure and understand whether entanglement witnesses lead to an efficient procedure for that task. Hence we quantify how many bits of information a family of entanglement witnesses can infer about the entanglement of a given quantum state sample. The bits are computed in terms of the mutual information and we unveil there exists hidden information not \emph{efficiently} processed. We show that there is more information in the expected value of the entanglement witnesses, i.e. $\mathbb{E}[W]=\langle W \rangle_\rho$ than in the sign of $\mathbb{E}[W]$. This suggests that an entanglement witness can provide more information about the entanglement if for our decision boundary we compute a different functional of its expectation value, rather than $\mathrm{sign}\left(\mathbb{E}\right [ W ])$.

Authors:Jonathan Nemirovsky, Rafi Weill, Ilan Meltzer, Yoav Sagi

Abstract: Atomic interferometers measure forces and acceleration with exceptional precision. The conventional approach to atomic interferometry is to launch an atomic cloud into a ballistic trajectory and perform the wave-packet splitting in momentum space by Raman transitions. This places severe constraints on the possible atomic trajectory, positioning accuracy and probing duration. Here, we propose and analyze a novel atomic interferometer that uses micro-optical traps (optical tweezers) to manipulate and control the motion of atoms. The new interferometer allows long probing time, sub micrometer positioning accuracy, and utmost flexibility in shaping of the atomic trajectory. The cornerstone of the tweezer interferometer are the coherent atomic splitting and combining schemes. We present two adiabatic schemes with two or three tweezers that are robust to experimental imperfections and work simultaneously with many vibrational states. The latter property allows for multi-atom interferometry in a single run. We also highlight the advantage of using fermionic atoms to obtain single-atom occupation of vibrational states and to eliminate mean-field shifts. We examine the impact of tweezer intensity noise and demonstrate that, when constrained by shot noise, the interferometer can achieve a relative accuracy better than $10^{-12}$ in measuring Earth's gravitational acceleration. The sub-micrometer resolution and extended measurement duration offer promising opportunities for exploring fundamental physical laws in new regimes. We discuss two applications well-suited for the unique capabilities of the tweezer interferometer: the measurement of gravitational forces and the study of Casimir-Polder forces between atoms and surfaces. Crucially, our proposed tweezer interferometer is within the reach of current technological capabilities.

Authors:Kapil Goswami, Rick Mukherjee, Herwig Ott, Peter Schmelcher

Abstract: We provide a non-unit disk framework to solve combinatorial optimization problems such as Maximum Cut (Max-Cut) and Maximum Independent Set (MIS) on a Rydberg quantum annealer. Our setup consists of a many-body interacting Rydberg system where locally controllable light shifts are applied to individual qubits in order to map the graph problem onto the Ising spin model. Exploiting the flexibility that optical tweezers offer in terms of spatial arrangement, our numerical simulations implement the local-detuning protocol while globally driving the Rydberg annealer to the desired many-body ground state, which is also the solution to the optimization problem. Using optimal control methods, these solutions are obtained for prototype graphs with varying sizes at time scales well within the system lifetime and with approximation ratios close to one. The non-blockade approach facilitates the encoding of graph problems with specific topologies that can be realized in two-dimensional Rydberg configurations and is applicable to both unweighted as well as weighted graphs. A comparative analysis with fast simulated annealing is provided which highlights the advantages of our scheme in terms of system size, hardness of the graph, and the number of iterations required to converge to the solution.

Authors:Willie Aboumrad, Dominic Widdows, Ananth Kaushik

Abstract: The availability of working quantum computers has led to several proposals and claims of quantum advantage. In 2023, this has included claims that quantum computers can successfully factor large integers, by optimizing the search for nearby integers whose prime factors are all small. This paper demonstrates that the hope of factoring numbers of commercial significance using these methods is unfounded. Mathematically, this is because the density of smooth numbers (numbers all of whose prime factors are small) decays exponentially as n grows. Our experimental reproductions and analysis show that lattice-based factoring does not scale successfully to larger numbers, that the proposed quantum enhancements do not alter this conclusion, and that other simpler classical optimization heuristics perform much better for lattice-based factoring. However, many topics in this area have interesting applications and mathematical challenges, independently of factoring itself. We consider particular cases of the CVP, and opportunities for applying quantum techniques to other parts of the factorization pipeline, including the solution of linear equations modulo 2. Though the goal of factoring 1000-bit numbers is still out-of-reach, the combinatoric landscape is promising, and warrants further research with more circumspect objectives.

Authors:Michael J. Kewming, Anthony Kiely, Steve Campbell, Gabriel T. Landi

Abstract: The First Passage Time (FPT) is the time taken for a stochastic process to reach a desired threshold. It finds wide application in various fields and has recently become particularly important in stochastic thermodynamics, due to its relation to kinetic uncertainty relations (KURs). In this letter we address the FPT of the stochastic measurement current in the case of continuously measured quantum systems. Our approach is based on a charge-resolved master equation, which is related to the Full-Counting statistics of charge detection. In the quantum jump unravelling we show that this takes the form of a coupled system of master equations, while for quantum diffusion it becomes a type of quantum Fokker-Planck equation. In both cases, we show that the FPT can be obtained by introducing absorbing boundary conditions, making their computation extremely efficient. The versatility of our framework is demonstrated with two relevant examples. First, we show how our method can be used to study the tightness of recently proposed KURs for quantum jumps. Second, we study the homodyne detection of a single two-level atom, and show how our approach can unveil various non-trivial features in the FPT distribution.

Authors:Hector Bombin, Chris Dawson, Terry Farrelly, Yehua Liu, Naomi Nickerson, Mihir Pant, Fernando Pastawski, Sam Roberts

Abstract: Fault-tolerant complexes describe surface-code fault-tolerant protocols from a single geometric object. We first introduce fusion complexes that define a general family of fusion-based quantum computing (FBQC) fault-tolerant quantum protocols based on surface codes. We show that any 3-dimensional cell complex where each edge has four incident faces gives a valid fusion complex. This construction enables an automated search for fault tolerance schemes, allowing us to identify 627 examples within a moderate search time. We implement this using the open-source software tool Gavrog and present threshold results for a variety of schemes, finding fusion networks with higher erasure and Pauli thresholds than those existing in the literature. We then define more general structures we call fault-tolerant complexes that provide a homological description of fault tolerance from a large family of low-level error models, which include circuit-based computation, floquet-based computation, and FBQC with multi-qubit measurements. This extends the applicability of homological descriptions of fault tolerance, and enables the generation of many new schemes which have not been previously identified. We also define families of fault-tolerant complexes for color codes and 3d single-shot subsystem codes, which enables similar constructive methods, and we present several new examples of each.

Authors:Sahel Ashhab, Ziqiao Ao, Fumiki Yoshihara, Kouichi Semba

Abstract: We perform theoretical calculations to investigate the naturally occurring high-frequency cutoff in a circuit comprising a flux qubit coupled inductively to a transmission line resonator (TLR). Our results agree with those of past studies that considered somewhat similar circuit designs. In particular, a decoupling occurs between the qubit and the high-frequency modes. As a result, the coupling strength between the qubit and resonator modes increases with mode frequency $\omega$ as $\sqrt{\omega}$ at low frequencies and decreases as $1/\sqrt{\omega}$ at high frequencies. We derive expressions for the multimode-resonator-induced Lamb shift in the qubit's characteristic frequency. Because of the natural decoupling between the qubit and high-frequency modes, the Lamb-shift-renormalized qubit frequency remains finite.

Authors:Carys Harvey, Richie Yeung, Konstantinos Meichanetzidis

Abstract: We introduce complex-valued tensor network models for sequence processing motivated by correspondence to probabilistic graphical models, interpretability and resource compression. Inductive bias is introduced to our models via network architecture, and is motivated by the correlation structure inherent in the data, as well as any relevant compositional structure, resulting in tree-like connectivity. Our models are specifically constructed using parameterised quantum circuits, widely used in quantum machine learning, effectively using Hilbert space as a feature space. Furthermore, they are efficiently trainable due to their tree-like structure. We demonstrate experimental results for the task of binary classification of sequences from real-world datasets relevant to natural language and bioinformatics, characterised by long-range correlations and often equipped with syntactic information. Since our models have a valid operational interpretation as quantum processes, we also demonstrate their implementation on Quantinuum's H2-1 trapped-ion quantum processor, demonstrating the possibility of efficient sequence processing on near-term quantum devices. This work constitutes the first scalable implementation of near-term quantum language processing, providing the tools for large-scale experimentation on the role of tensor structure and syntactic priors. Finally, this work lays the groundwork for generative sequence modelling in a hybrid pipeline where the training may be conducted efficiently in simulation, while sampling from learned probability distributions may be done with polynomial speed-up on quantum devices.

Authors:Ernest Y. -Z. Tan

Abstract: In device-independent cryptography, it is known that reuse of devices across multiple protocol instances can introduce a vulnerability against memory attacks. This is an introductory note to highlight that even if we restrict ourselves to device-dependent QKD and only consider a single protocol instance, memory effects across rounds are enough to cause substantial difficulties in applying many existing non-IID proof techniques, such as de Finetti reductions and complementarity-based arguments (e.g. analysis of phase errors). We present a quick discussion of these issues, including some tailored scenarios where protocols admitting security proofs via those techniques become insecure when memory effects are allowed, and we highlight connections to recently discussed attacks on DIQKD protocols that have public announcements based on the measurement outcomes. This discussion indicates the challenges that would need to be addressed in order to apply those techniques in the presence of memory effects (for either the device-dependent or device-independent case), even for a single protocol instance.

Authors:Cisco Gooding, Allison Sachs, Robert B. Mann, Silke Weinfurtner

Abstract: This study explores the transfer of nonclassical correlations from an ultra-cold atom system to a pair of pulsed laser beams. Through nondestructive local probe measurements, we introduce an alternative to destructive techniques for mapping BEC entanglement. Operating at ultralow temperatures, the setup emulates a relativistic vacuum field, using lasers as Unruh-DeWitt detectors for phonons. The vacuum holds intrinsic entanglement, transferable to distant probes briefly interacting with it - a phenomenon termed ``entanglement harvesting''. Our study accomplishes two primary objectives: first, establishing a mathematical equivalence between a pair of pulsed laser probes interacting with an effective relativistic field and the entanglement harvesting protocol; and second, to closely examine the potential and persisting obstacles for realising this protocol in an ultra-cold atom experiment.

Authors:Xinwei Li, Yijia Zhou, Hao Zhang

Abstract: We investigate a ring cavity comprising two degenerate counter-propagating modes coupled to a one-dimensional atomic chain, leading to bidirectional light scattering. The spatial configuration of the atomic chain, described by a structure factor, plays a crucial role in manipulation of the atom-cavity interactions and formation of the collective excitation modes. Remarkably, we observe that a cavity dark mode is induced when the atomic spacing is an integer multiple of half-wavelength. The nodes of this standing-wave dark mode align precisely with the atomic positions, enabling intracavity field conversion without free space scattering. By adjusting the configuration of the atomic chain, we realize optical mode conversion with almost no photon loss and a broad tuning range, making it suitable for various practical applications in quantum technologies.

Authors:Adam L. Shaw, Zhuo Chen, Joonhee Choi, Daniel K. Mark, Pascal Scholl, Ran Finkelstein, Andreas Elben, Soonwon Choi, Manuel Endres

Abstract: Quantum systems have entered a competitive regime where classical computers must make approximations to represent highly entangled quantum states. However, in this beyond-classically-exact regime, fidelity comparisons between quantum and classical systems have so far been limited to digital quantum devices, and it remains unsolved how to estimate the actual entanglement content of experiments. Here we perform fidelity benchmarking and mixed-state entanglement estimation with a 60-atom analog Rydberg quantum simulator, reaching a high entanglement entropy regime where exact classical simulation becomes impractical. Our benchmarking protocol involves extrapolation from comparisons against many approximate classical algorithms with varying entanglement limits. We then develop and demonstrate an estimator of the experimental mixed-state entanglement, finding our experiment is competitive with state-of-the-art digital quantum devices performing random circuit evolution. Finally, we compare the experimental fidelity against that achieved by various approximate classical algorithms, and find that only one, which we introduce here, is able to keep pace with the experiment on the classical hardware we employ. Our results enable a new paradigm for evaluating the performance of both analog and digital quantum devices in the beyond-classically-exact regime, and highlight the evolving divide between quantum and classical systems.

Authors:Sergey Bravyi, Andrew W. Cross, Jay M. Gambetta, Dmitri Maslov, Patrick Rall, Theodore J. Yoder

Abstract: Quantum error correction becomes a practical possibility only if the physical error rate is below a threshold value that depends on a particular quantum code, syndrome measurement circuit, and a decoding algorithm. Here we present an end-to-end quantum error correction protocol that implements fault-tolerant memory based on a family of LDPC codes with a high encoding rate that achieves an error threshold of $0.8\%$ for the standard circuit-based noise model. This is on par with the surface code which has remained an uncontested leader in terms of its high error threshold for nearly 20 years. The full syndrome measurement cycle for a length-$n$ code in our family requires $n$ ancillary qubits and a depth-7 circuit composed of nearest-neighbor CNOT gates. The required qubit connectivity is a degree-6 graph that consists of two edge-disjoint planar subgraphs. As a concrete example, we show that 12 logical qubits can be preserved for ten million syndrome cycles using 288 physical qubits in total, assuming the physical error rate of $0.1\%$. We argue that achieving the same level of error suppression on 12 logical qubits with the surface code would require more than 4000 physical qubits. Our findings bring demonstrations of a low-overhead fault-tolerant quantum memory within the reach of near-term quantum processors.

Authors:Lukas Johannes Splitthoff, Jaap Joachim Wesdorp, Marta Pita-Vidal, Arno Bargerbos, Christian Kraglund Andersen

Abstract: Superconducting parametric amplifiers play a crucial role in the preparation and readout of quantum states at microwave frequencies, enabling high-fidelity measurements of superconducting qubits. Most existing implementations of these amplifiers rely on the nonlinearity from Josephson junctions, superconducting quantum interference devices or disordered superconductors. Additionally, frequency tunability arises typically from either flux or current biasing. In contrast, semiconductor-based parametric amplifiers are tunable by local electric fields, which impose a smaller thermal load on the cryogenic setup than current and flux biasing and lead to vanishing crosstalk to other on-chip quantum systems. In this work, we present a gate-tunable parametric amplifier that operates without Josephson junctions, utilizing a proximitized semiconducting nanowire. This design achieves near-quantum-limited performance, featuring more than 20 dB gain and a 30 MHz gain-bandwidth product. The absence of Josephson junctions allows for advantages, including substantial saturation powers of -120dBm, magnetic field compatibility up to 500 mT and frequency tunability over a range of 15 MHz. Our realization of a parametric amplifier supplements efforts towards gate-controlled superconducting electronics, further advancing the abilities for high-performing quantum measurements of semiconductor-based and superconducting quantum devices.

Authors:Mikolaj Lasota, Olena Kovalenko, Vladyslav C. Usenko

Abstract: Discrete-variable (DV) and continuous-variable (CV) schemes constitute the two major families of quantum key distribution (QKD) protocols. Unfortunately, since the setup elements required by these schemes are quite different, making a fair comparison of their potential performance in particular applications is often troublesome, limiting the experimenters' capability to choose an optimal solution. In this work we perform a general comparison of the major entanglement-based DV and CV QKD protocols in terms of their resistance to the channel noise, with the otherwise perfect setup, showing the definite superiority of the DV family. We analytically derive fundamental bounds on the tolerable channel noise and attenuation for entanglement-based CV QKD protocols. We also investigate the influence of DV QKD setup imperfections on the obtained results in order to determine benchmarks for the parameters of realistic photon sources and detectors, allowing the realistic DV protocols to outperform even the ideal CV QKD analogs. Our results indicate the realistic advantage of DV EPR-based schemes over their CV counterparts and suggests the practical efforts for maximizing this advantage.

Authors:Nai-Hui Chial, Ching-Yi Lai, Han-Hsuan Lin

Abstract: One of the primary objectives in the field of quantum state learning is to develop algorithms that are time-efficient for learning states generated from quantum circuits. Earlier investigations have demonstrated time-efficient algorithms for states generated from Clifford circuits with at most $\log(n)$ non-Clifford gates. However, these algorithms necessitate multi-copy measurements, posing implementation challenges in the near term due to the requisite quantum memory. On the contrary, using solely single-qubit measurements in the computational basis is insufficient in learning even the output distribution of a Clifford circuit with one additional $T$ gate under reasonable post-quantum cryptographic assumptions. In this work, we introduce an efficient quantum algorithm that employs only nonadaptive single-copy measurement to learn states produced by Clifford circuits with a maximum of $O(\log n)$ non-Clifford gates, filling a gap between the previous positive and negative results.

Authors:Vahid Jannesary, Vahid Karimipour

Abstract: We present a simple approach for generation of a diverse set of positive maps between spaces of different dimensions. The proposed method enables the construction of Entanglement Witnesses tailored for systems characterized by $d_1 \times d_2$ dimensions. We also present an alternative argument for directly generating a wide range of Entanglement Witnesses in these dimensions. With this method, it is possible to construct Entanglement Witnesses that consist solely of a chosen set of desired measurements. We demonstrate the effectiveness and generality of our approach using concrete examples.

Authors:Lewis Wooltorton, Peter Brown, Roger Colbeck

Abstract: Nonlocal tests on multipartite quantum correlations form the basis of protocols that certify randomness in a device-independent (DI) way. Such correlations admit a rich structure, making the task of choosing an appropriate test difficult. For example, extremal Bell inequalities are tight witnesses of nonlocality, however achieving their maximum violation places constraints on the underlying quantum system, which can reduce the rate of randomness generation. As a result there is often a trade-off between maximum randomness and the amount of violation of a given Bell inequality. Here, we explore this trade-off for more than two parties. More precisely, we study the maximum amount of randomness that can be certified by correlations exhibiting a violation of the Mermin-Ardehali-Belinskii-Klyshko (MABK) inequality. We find that maximum quantum violation and maximum randomness are incompatible for any even number of parties, with incompatibility diminishing as the number of parties grow, and conjecture the precise trade-off. We also show that maximum MABK violation is not necessary for maximum randomness for odd numbers of parties. To obtain our results, we derive new families of Bell inequalities certifying maximum randomness from a technique for randomness certification, which we call "expanding Bell inequalities". Our technique allows one to take a bipartite Bell expression, known as the seed, and transform it into a multipartite Bell inequality tailored for randomness certification, showing how intuition learned in the bipartite case can find use in more complex scenarios.

Authors:Balázs Pozsgay, Ian M. Wanless

Abstract: Absolutely maximally entangled (AME) states of $k$ qudits (also known as perfect tensors) are quantum states that have maximal entanglement for all possible bipartitions of the sites/parties. We consider the problem of whether such states can be decomposed into a tensor network with a small number of tensors, such that all physical and all auxiliary spaces have the same dimension $D$. We find that certain AME states with $k=6$ can be decomposed into a network with only three 4-leg tensors; we provide concrete solutions for local dimension $D=5$ and higher. Our result implies that certain AME states with six parties can be created with only three two-site unitaries from a product state of three Bell pairs, or equivalently, with six two-site unitaries acting on a product state on six qudits. We also consider the problem for $k=8$, where we find similar tensor network decompositions with six 4-leg tensors.

Authors:J. Neuser, T. Thiemann

Abstract: In a recent contribution we showed that there exists a smooth, dense domain for singular potential Schr\"odinger operators on the real line which is invariant under taking derivatives of arbitrary order and under multiplication by positive and negative integer powers of the coordinate. Moreover, inner products between basis elements of that domain were shown to be easily computable analytically. A task left open was to construct an orthonormal basis from elements of that domain by using Gram-Schmidt orthonormalisation. We perform that step in the present manuscript. We also consider the application of these methods to the positive real line for which one can no longer perform the integrals analytically but for which one can give tight analytical estimates.

Authors:Kui An, Zilei Liu, Ting Zhang, Siqi Li, You Zhou, Xiao Yuan, Leiran Wang, Wenfu Zhang, Guoxi Wang, He Lu

Abstract: Metasurface enables the generation and manipulation of multiphoton entanglement with flat optics, providing a more efficient platform for large-scale photonic quantum information processing. Here, we show that a single metasurface optical chip would allow more efficient characterizations of multiphoton entangled states, such as shadow tomography, which generally requires fast and complicated control of optical setups to perform projective measurements in different bases, a demanding task using conventional optics. The compact and stable device here allows implementations of general positive observable value measures with a reduced sample complexity and significantly alleviates the experimental complexity to implement shadow tomography. Integrating self-learning and calibration algorithms, we observe notable advantages in the reconstruction of multiphoton entanglement, including using fewer measurements, having higher accuracy, and being robust against optical loss. Our work unveils the feasibility of metasurface as a favorable integrated optical device for efficient characterization of multiphoton entanglement, and sheds light on scalable photonic quantum technologies with ultra-thin integrated optics.

Authors:Ting Zhang, Graeme Smith, John A. Smolin, Lu Liu, Xu-Jie Peng, Qi Zhao, Davide Girolami, Xiongfeng Ma, Xiao Yuan, He Lu

Abstract: Entanglement and coherence are fundamental properties of quantum systems, promising to power the near future quantum technologies. Yet, their quantification, rather than mere detection, generally requires reconstructing the spectrum of quantum states, i.e., experimentally challenging measurement sets that increase exponentially with the system size. Here, we demonstrate quantitative bounds to operationally useful entanglement and coherence that are universally valid, analytically computable, and experimentally friendly. Specifically, our main theoretical results are lower and upper bounds to the coherent information and the relative entropy of coherence in terms of local and global purities of quantum states. To validate our proposal, we experimentally implement two purity detection methods in an optical system: shadow estimation with random measurements and collective measurements on pairs of state copies. The experiment shows that both the coherent information and the relative entropy of coherence of pure and mixed unknown quantum states can be bounded by purity functions. Our research offers an efficient means of verifying large-scale quantum information processing without spectrum reconstruction.

Authors:D. Sych, A. Kodukhov, V. Pastushenko, N. Kirsanov, D. Kronberg, M. Pflitsch

Abstract: Quantum communication offers unique features that have no classical analog, in particular, it enables provably secure quantum key distribution (QKD). Despite the benefits of quantum communication are well understood within the scientific community, the practical implementations sometimes meet with scepticism or even resistance. In a recent publication [1], NSA claims that QKD is inferior to "quantum-resistant" cryptography and does not recommend it for use. Here we show that such a sceptical approach to evaluation of quantum security is not well justified. We hope that our arguments will be helpful to clarify the issue.

Authors:Kirill Petrovnin, Jiaming Wang, Michael Perelshtein, Pertti Hakonen, Gheorghe Sorin Paraoanu

Abstract: The detection of microwave fields at single-photon power levels is a much sought-after technology, with practical applications in nanoelectronics and quantum information science. Here we demonstrate a simple yet powerful criticality-enhanced method of microwave photon detection by operating a magnetic-field tunable Kerr Josephson parametric amplifier near a first-order quantum phase transition. We obtain a 73% efficiency and a dark-count rate of 167 kHz, corresponding to a responsivity of $1.3 \times 10^{17}~\mathrm{W}^{-1}$ and noise-equivalent power of 3.28 zW/$\sqrt{\rm Hz}$. We verify the single-photon operation by extracting the Poissonian statistics of a coherent probe signal.

Authors:Oliver Neyenhuys, Mikhail V. Fistul, Ilya M. Eremin

Abstract: Geometrical frustration in correlated systems can give rise to a plethora of novel ordered states and intriguing phases. Here, we analyze theoretically vertex-sharing frustrated Kagome lattice of Josephson junctions and identify various classical and quantum phases. The frustration is provided by periodically arranged $0$- and $\pi$- Josephson junctions. In the frustrated regime the macroscopic phases are composed of different patterns of vortex/antivortex penetrating each basic element of the Kagome lattice, i.e., a superconducting triangle interrupted by three Josephson junctions. We obtain that numerous topological constraints, related to the flux quantization in any hexagon loop, lead to highly anisotropic and long-range interaction between well separated vortices (antivortices). Taking into account this interaction and a possibility of macroscopic "tunneling" between vortex and antivortex in single superconducting triangles we derive an effective Ising-type spin Hamiltonian with strongly anisotropic long-range interaction. In the classically frustrated regime we calculate numerically the temperature-dependent spatially averaged spins polarization, $\overline{m}(T)$, characterizing the crossover between the ordered and disordered vortex/antivortex states. In the coherent quantum regime we analyze the lifting of the degeneracy of the ground state and the appearance of the highly entangled states.

Authors:Wei Zhong, Wen-Yi Zhu, Yang Li, Lan Zhou, Ming-Ming Du, Yu-Bo Sheng

Abstract: Quantum illumination (QI) leverages entangled lights to detect the potential presence of low-reflective objects in a region surrounded by a thermal bath. Homologously, quantum parameter estimation utilizes non-classical probes to accurately estimate the value of the unknown parameter(s) of interest in a system. There appears to be a certain connection between these two areas. However, they are commonly studied using different figures of merit: signal-to-noise ratio and quantum Fisher information. In this study, we prove that the two measures are equivalent to QI in the limit of zero object reflectivity. We further demonstrate this equivalence by investigating QI protocols employing non-Gaussian states, which are obtained by de-Gaussifying the two-mode squeezed vacuum state with photon addition and photon subtraction. However, our analysis leads to a no-go result which demonstrates that de-Gaussification operations do not offer an advantage compared to the null case.

Authors:Michael J. Bremner, Bin Cheng, Zhengfeng Ji

Abstract: Sampling problems demonstrating beyond classical computing power with noisy intermediate-scale quantum (NISQ) devices have been experimentally realized. In those realizations, however, our trust that the quantum devices faithfully solve the claimed sampling problems is usually limited to simulations of smaller-scale instances and is, therefore, indirect. The problem of verifiable quantum advantage aims to resolve this critical issue and provides us with greater confidence in a claimed advantage. Instantaneous quantum polynomial-time (IQP) sampling has been proposed to achieve beyond classical capabilities with a verifiable scheme based on quadratic-residue codes (QRC). Unfortunately, this verification scheme was recently broken by an attack proposed by Kahanamoku-Meyer. In this work, we revive IQP-based verifiable quantum advantage by making two major contributions. Firstly, we introduce a family of IQP sampling protocols called the \emph{stabilizer scheme}, which builds on results linking IQP circuits, the stabilizer formalism, coding theory, and an efficient characterization of IQP circuit correlation functions. This construction extends the scope of existing IQP-based schemes while maintaining their simplicity and verifiability. Secondly, we introduce the \emph{Hidden Structured Code} (HSC) problem as a well-defined mathematical challenge that underlies the stabilizer scheme. To assess classical security, we explore a class of attacks based on secret extraction, including the Kahanamoku-Meyer's attack as a special case. We provide evidence of the security of the stabilizer scheme, assuming the hardness of the HSC problem. We also point out that the vulnerability observed in the original QRC scheme is primarily attributed to inappropriate parameter choices, which can be naturally rectified with proper parameter settings.

Authors:Genevieve Clark, Hamza Raniwala, Matthew Koppa, Kevin Chen, Andrew Leenheer, Matthew Zimmermann, Mark Dong, Linsen Li, Y. Henry Wen, Daniel Dominguez, Matthew Trusheim, Gerald Gilbert, Matt Eichenfield, Dirk Englund

Abstract: Atom-like defects or color centers (CC's) in nanostructured diamond are a leading platform for optically linked quantum technologies, with recent advances including memory-enhanced quantum communication, multi-node quantum networks, and spin-mediated generation of photonic cluster states. Scaling to practically useful applications motivates architectures meeting the following criteria: C1 individual optical addressing of spin qubits; C2 frequency tuning of CC spin-dependent optical transitions; C3 coherent spin control in CC ground states; C4 active photon routing; C5 scalable manufacturability; and C6 low on-chip power dissipation for cryogenic operations. However, no architecture meeting C1-C6 has thus far been demonstrated. Here, we introduce a hybrid quantum system-on-chip (HQ-SoC) architecture that simultaneously achieves C1-C6. Key to this advance is the realization of piezoelectric strain control of diamond waveguide-coupled tin vacancy centers to meet C2 and C3, with ultra-low power dissipation necessary for C6. The DC response of our device allows emitter transition tuning by over 20 GHz, while the large frequency range (exceeding 2 GHz) enables low-power AC control. We show acoustic manipulation of integrated tin vacancy spins and estimate single-phonon coupling rates over 1 kHz in the resolved sideband regime. Combined with high-speed optical routing with negligible static hold power, this HQ-SoC platform opens the path to scalable single-qubit control with optically mediated entangling gates.

Authors:Zong-Xing Xiong, Yongli Zhang

Abstract: A set of orthogonal multipartite quantum states is said to be distinguishability-based genuinely nonlocal (also genuinely nonlocal, for abbreviation) if the states are locally indistinguishable across any bipartition of the subsystems. In this work, we study the (distinguishability-based) genuine nonlocality of the generalized GHZ states, primarily for the case when a large number of partites are considered. For the N-qubit case, we show that genuinely nonlocal subsets of the GHZ basis with cardianlity {\Theta}(2^(N/2)) exist. We also generalize this result to the cases when d > 2 is an even number.

Authors:Jinyong Ma, Y. S. S. Patil, Jiaxin Yu, Yiqi Wang, J. G. E. Harris

Abstract: The electron bubble in superfluid helium has two degrees of freedom that may offer exceptionally low dissipation: the electron's spin and the bubble's motion. If these degrees of freedom can be read out and controlled with sufficient sensitivity, they would provide a novel platform for realizing a range of quantum technologies and for exploring open questions in the physics of superfluid helium. Here we propose a practical scheme for accomplishing this by trapping an electron bubble inside a superfluid-filled opto-acoustic cavity.

Authors:Sabee Grewal, Vishnu Iyer, William Kretschmer, Daniel Liang

Abstract: Recent work has shown that $n$-qubit quantum states output by circuits with at most $t$ single-qubit non-Clifford gates can be learned to trace distance $\epsilon$ using $\mathsf{poly}(n,2^t,1/\epsilon)$ time and samples. All prior algorithms achieving this runtime use entangled measurements across two copies of the input state. In this work, we give a similarly efficient algorithm that learns the same class of states using only single-copy measurements.

Authors:Henrique Santos Lima, Matheus M. A. Paixão, Constantino Tsallis

Abstract: Within the de Broglie-Bohm theory, we numerically study a generic two-dimensional anharmonic oscillator including cubic and quartic interactions. Our analysis of the quantum velocity fields and trajectories reveals the emergence of dynamical vortices. In their vicinity, fingerprints of chaotic behavior such as unpredictability and sensitivity to initial conditions are detected. The simultaneous presence of off-diagonal and nonlinear terms leads to robust quantum chaos very analogous to its classical version.

Authors:Hakon Volkmann AG Moderne Optik, Institut für Physik, Humboldt-Universität zu Berlin, Germany, Raamamurthy Sathyanarayanan AG Moderne Optik, Institut für Physik, Humboldt-Universität zu Berlin, Germany, Alejandro Saenz AG Moderne Optik, Institut für Physik, Humboldt-Universität zu Berlin, Germany, Karl Jansen CQTA, DESY Zeuthen, Germany, and Computation-Based Science and Technology Research Center, The Cyprus Institute, Nicosia, Cyprus, Stefan Kühn CQTA, DESY Zeuthen, Germany, and Computation-Based Science and Technology Research Center, The Cyprus Institute, Nicosia, Cyprus

Abstract: With the recent advances in the development of devices capable of performing quantum computations, a growing interest in finding near-term applications has emerged in many areas of science. In the era of non-fault tolerant quantum devices, algorithms that only require comparably short circuits accompanied by high repetition rates are considered to be a promising approach for assisting classical machines with finding solution on computationally hard problems. The ADAPT approach previously introduced in Nat. Commun. 10, 3007 (2019) extends the class of variational quantum eigensolver (VQE) algorithms with dynamically growing ans\"atze in order to find approximations to ground and excited state energies of molecules. In this work, the ADAPT algorithm has been combined with a first-quantized formulation for the hydrogen molecule in the Born-Oppenheimer approximation, employing the explicitly correlated basis functions introduced in J. Chem. Phys. 43, 2429 (1965). By the virtue of their explicit electronic correlation properties, it is shown in classically performed simulations that relatively short circuits yield chemical accuracy ($< 1.6$ mHa) for ground and excited state potential curves that can compete with second quantized approaches such as Unitary Coupled Cluster.

Authors:Michael R Grace, Saikat Guha, Zachary Dutton

Abstract: Detecting if and when objects change is difficult in passive sub-diffraction imaging of dynamic scenes. We consider the best possible tradeoff between responsivity and accuracy for detecting a change from one arbitrary object model to another in the context of sub-diffraction incoherent imaging. We analytically evaluate the best possible average latency, for a fixed false alarm rate, optimizing over all physically allowed measurements of the optical field collected by a finite 2D aperture. We find that direct focal-plane detection of the incident optical intensity achieves sub-optimal detection latencies compared to the best possible average latency, but that a three-mode spatial-mode demultiplexing measurement in concert with on-line statistical processing using the well-known CUSUM algorithm achieves this quantum limit for sub-diffraction objects. We verify these results via Monte Carlo simulation of the change detection procedure and quantify a growing gap between the conventional and quantum-optimal receivers as the objects are more and more diffraction-limited.

Authors:Alejandro Hnilo

Abstract: The Bell's basis is composed of four maximally entangled states of two qubits, named Bell states. They are usual tools in many theoretical studies and experiments. The aim of this paper is to find out the symmetries that determine a Bell state. For this purpose, starting from a general density matrix, physical constraints and symmetry conditions are added until the elements of the Bell's basis are univocally determined. It is found that the usual physical constraints and symmetry conditions do not suffice to determine a Bell state. The additional restriction needed is named here atomic symmetry. It is a sort of global symmetry of the system, not derived from the action = reaction law. It is also found that the imperfection in fulfilling the atomic symmetry is linearly proportional to the deviation of the Concurrence from its maximum value. The atomic symmetry allows a different insight on the nature of entanglement, and might be useful as a criterion to define the condition of maximal entanglement for states with more than two qubits.

Authors:J. Furtado

Abstract: In this paper, we investigate the influence of the geometry in the electronic states of a quantum Beltrami surface. We have considered an electron governed by the spinless stationary Schr\"{o}dinger equation constrained to move on the Beltrami surface due to a confining potential from which the Da Costa potential emerges. We investigate the role played by the geometry and orbital angular momentum on the electronic states of the system.

Authors:Kiyoto Nakamura, Joachim Ankerhold

Abstract: With increasing experimental performance of qubit devices, highly accurate theoretical predictions are needed to describe the open system dynamics. Here, we make use of three equations of motion for the reduced density matrix, the conventional Lindblad equation (LE), the Universal Lindblad Equation (ULE), and the Hierarchical Equations of Motion (HEOM). While the HEOM provides numerically exact benchmark data, the LE is based on the Born-Markov approximation in combination with the rotating wave approximation (RWA) which is not imposed in the ULE. This allows us to analyze the distinction between the Born-Markov approximation and the RWA, which may be sometimes confused. As a demonstration, predictions for relaxation and decoherence of a two-level system in presence of reservoirs with Ohmic and sub-Ohmic spectral densities are explored. With the aid of a recently proposed protocol based on Ramsey experiments, the role of the Born-Markov approximation and the RWA is revealed.

Authors:Renzo Testa, Alex Rodriguez, Alberto d'Onofrio, Andrea Trombettoni, Fabio Benatti, Fabio Anselmi

Abstract: Given the key role that quantum tunneling plays in a wide range of applications, a crucial objective is to maximize the probability of tunneling from one quantum state/level to another, while keeping the resources of the underlying physical system fixed. In this work, we demonstrate that an effective solution to this challenge can be achieved by coupling the tunneling system with an ancillary system of the same kind. By utilizing machine learning techniques, the parameters of both the ancillary system and the coupling can be optimized, leading to the maximization of the tunneling probability. We provide illustrative examples for the paradigmatic scenario involving a two-mode system and a two-mode ancilla with arbitrary couplings and in the presence of several interacting particles. Importantly, the enhancement of the tunneling probability appears to be minimally affected by noise and decoherence in both the system and the ancilla.

Authors:Ismail Yunus Akhalwaya, Adam Connolly, Roland Guichard, Steven Herbert, Cahit Kargi, Alexandre Krajenbrink, Michael Lubasch, Conor Mc Keever, Julien Sorci, Michael Spranger, Ifan Williams

Abstract: We present the Quantum Monte Carlo Integration (QMCI) engine developed by Quantinuum. It is a quantum computational tool for evaluating multi-dimensional integrals that arise in various fields of science and engineering such as finance. This white paper presents a detailed description of the architecture of the QMCI engine, including a variety of distribution-loading methods, a novel quantum amplitude estimation method that improves the statistical robustness of QMCI calculations, and a library of statistical quantities that can be estimated. The QMCI engine is designed with modularity in mind, allowing for the continuous development of new quantum algorithms tailored in particular to financial applications. Additionally, the engine features a resource mode, which provides a precise resource quantification for the quantum circuits generated. The paper also includes extensive benchmarks that showcase the engine's performance, with a focus on the evaluation of various financial instruments.

Authors:A. C. Quillen, Rayleigh Parker, Nathan Skerrett

Abstract: Using the recent ability of quantum computers to initialize quantum states rapidly with high fidelity, we use a function operating on a discrete set to create a simple class of quantum channels. Fixed points and periodic orbits, that are present in the function, generate fixed points and periodic orbits in the associated quantum channel. Phenomenology such as periodic doubling is visible in a 6 qubit dephasing channel constructed from a truncated version of the logistic map. Using disjoint subsets, discrete function-generated channels can be constructed that preserve coherence within subspaces. Error correction procedures can be in this class as syndrome detection uses an initialized quantum register. A possible application for function-generated channels is in hybrid classical/quantum algorithms. We illustrate how these channels can aid in carrying out classical computations involving iteration of non-invertible functions on a quantum computer with the Euclidean algorithm for finding the greatest common divisor of two integers.

Authors:Oleg Morzhin, Alexander Pechen

Abstract: This work considers two-qubit open quantum systems driven by coherent and incoherent controls. Incoherent control induces time-dependent decoherence rates via time-dependent spectral density of the environment which is used as a resource for controlling the system. The system evolves according to the Gorini-Kossakowski-Sudarshan-Lindblad master equation with time-dependent coefficients. For two types of interaction with coherent control, three types of objectives are considered: 1) maximizing the Hilbert-Schmidt overlap between the final and target density matrices; 2) minimizing the Hilbert-Schmidt distance between these matrices; 3) steering the overlap to a given value. For the first problem, we develop the Krotov type methods directly in terms of density matrices with or without regularization for piecewise continuous constrained controls and find the cases where the methods produce (either exactly or with some precision) zero controls which satisfy the Pontryagin maximum principle and produce the overlap's values close to their upper estimates. For the problems 2) and 3), we find cases when the dual annealing method steers the objectives close to zero and produces a non-zero control.

Authors:Gang Zhang, Zhi Song

Abstract: We investigate the influence of the external fields on the stability of spin helix states in a XXZ Heisenberg model. Exact diagonalization on finite system shows that random transverse fields in x and y directions drive the transition from integrability to nonintegrability. It results in the fast decay of a static helix state, which is the eigenstate of an unperturbed XXZ Heisenberg model. However, in the presence of uniform z field, the static helix state becomes a dynamic helix state with a relatively long life as a quantum scar state.

Authors:Andreas Fring, Marta Reboiro

Abstract: We propose a new type of quantum thermodynamic cycle whose efficiency is greater than the one of the classical Carnot cycle for the same conditions. In our model this type of cycle only exists in the low temperature regime in the spontaneously broken parity-time-reversal (PT) symmetry regime of a non-Hermitian quantum theory and does not manifest in the PT-symmetric regime. We discuss this effect for an ensemble based on a model of a single boson coupled in a non Hermitian way to a bath of different types of bosons with and without a time-dependent boundary.

Authors:Michele Fava, Jorge Kurchan, Silvia Pappalardi

Abstract: Unitary Designs have become a vital tool for investigating pseudorandomness since they approximate the statistics of the uniform Haar ensemble. Despite their central role in quantum information, their relation to quantum chaotic evolution and in particular to the Eigenstate Thermalization Hypothesis (ETH) are still largely debated issues. This work provides a bridge between the latter and $k$-designs through Free Probability theory. First, by introducing the more general notion of $k$-freeness, we show that it can be used as an alternative probe of designs. In turn, free probability theory comes with several tools, useful for instance for the calculation of mixed moments or for quantum channels. Our second result is the connection to quantum dynamics. Quantum ergodicity, and correspondingly ETH, apply to a restricted class of physical observables, as already discussed in the literature. In this spirit, we show that unitary evolution with generic Hamiltonians always leads to freeness at sufficiently long times, but only when the operators considered are restricted within the ETH class. Our results provide a direct link between unitary designs, quantum chaos and the Eigenstate Thermalization Hypothesis, and shed new light on the universality of late-time quantum dynamics.

Authors:Hugo Molinares, Fernanda Pinilla, Enrique Muñoz, Francisco Muñoz, Vitalie Eremeev

Abstract: Hexagonal boron nitride hosts one dimensional topologically-protected phonons at certain grain boundaries. Here we show that it is possible to use these phonons for the transmission of information. Particularly, \textit{(i)} a color center (a single photon emitter) can be used to induce single-, two- and qubit-phonon states in the one dimensional channel, and \textit{(ii)} two distant color centers can be coupled by the topological phonons transmitted along a line of defects that acts as a waveguide, thus exhibiting strong quantum correlations.

Authors:Palash Pandya, Marcin Wieśniak

Abstract: In this article we provide a three step algorithm to obtain the Closest Separable State to the given state in Hilbert space dimensions $2\times 2$ and $2\times 3$, or in the higher dimensional Hilbert spaces, 'Closest Positive Partial Transpose (PPT) state' for the chosen bipartition. In the process, a tight lower bound to the minimum Hilbert-Schmidt distance is brought forth together with the relation between the minimum Hilbert-Schmidt distance and Negativity. This also leads us to discuss the validity of the said distance from the set of separable quantum states as an entanglement measure. Any Entanglement measure defined as the minimum of a distance measure to the set of separable states needs to follow certain widely accepted rules. Most significantly, contractiveness of the distance (also, CP non-expansive property) under LOCC maps. While the Hilbert-Schmidt distance does not have this property, it is still an open question if the measure constructed using it is non-increasing under LOCC operations. While we outline some of the difficulties in such a proof, we also provide numerical evidence that brings one step closer to closing the question.

Authors:Annan Fan, Shi-Dong Liang

Abstract: Non-Hermitian systems as theoretical models of open or dissipative systems exhibit rich novel physical properties and fundamental issues in condensed matter physics.We propose a generalized local-global correspondence between the pseudo-boundary states in the complex energy plane and topological invariants of quantum states. We find that the patterns of the pseudo-boundary states in the complex energy plane mapped to the Brillouin zone are topological invariants against the parameter deformation. We demonstrate this approach by the non-Hermitian Chern insulator model. We give the consistent topological phases obtained from the Chern number and vorticity. We also find some novel topological invariants embedded in the topological phases of the Chern insulator model, which enrich the phase diagram of the non-Hermitian Chern insulators model beyond that predicted by the Chern number and vorticity. We also propose a generalized vorticity and its flipping index to understand physics behind this novel local-global correspondence and discuss the relationships between the local-global correspondence and the Chern number as well as the transformation between the Brillouin zone and the complex energy plane. These novel approaches provide insights to how topological invariants may be obtained from local information as well as the global property of quantum states, which is expected to be applicable in more generic non-Hermitian systems.

Authors:Zhen-Qiang Ren, Xian-Liang Lu, Ze-Liang Xiang

Abstract: In this paper, we investigate spin squeezing in a hybrid quantum system consisting of a Silicon-Vacancy (SiV) center ensemble coupled to a diamond acoustic waveguide via the strain interaction. Two sets of non-overlapping driving fields, each contains two time-dependent microwave fields, are applied to this hybrid system. By modulating these fields, the one-axis twist (OAT) interaction and two-axis two-spin (TATS) interaction can be independently realized. In the latter case the squeezing parameter scales to spin number as $\xi_R^2\sim1.61N^{-0.64}$ with the consideration of dissipation, which is very close to the Heisenberg limit. Furthermore, this hybrid system allows for the study of spin squeezing generated by the simultaneous presence of OAT and TATS interactions, which reveals sensitivity to the parity of the number of spins $N_{tot}$, whether it is even or odd. Our scheme enriches the approach for generating Heisenberg-limited spin squeezing in spin-phonon hybrid systems and offers the possibility for future applications in quantum information processing.

Authors:Qing Zhou, Xueming Tang, Songfeng Lu, Hao Yang

Abstract: In this paper, we develop a generic controlled alternate quantum walk model (called CQWM-P) by combining parity-dependent quantum walks with distinct arbitrary memory lengths and then construct a quantum-inspired hash function (called QHFM-P) based on this model. Numerical simulation shows that QHFM-P has near-ideal statistical performance and is on a par with the state-of-the-art hash functions based on discrete quantum walks in terms of sensitivity of hash value to message, diffusion and confusion properties, uniform distribution property, and collision resistance property. Stability test illustrates that the statistical properties of the proposed hash function are robust with respect to the coin parameters, and theoretical analysis indicates that QHFM-P has the same computational complexity as that of its peers.

Authors:Alvin Gonzales, Anjala M Babu, Ji Liu, Zain Saleem, Mark Byrd

Abstract: Typically, fault-tolerant operations and code concatenation are reserved for quantum error correction due to their resource overhead. Here, we show that fault tolerant operations have a large impact on the performance of symmetry based error mitigation techniques. We also demonstrate that similar to results in fault tolerant quantum computing, code concatenation in fault-tolerant quantum error mitigation (FTQEM) can exponentially suppress the errors to arbitrary levels. We also provide analytical error thresholds for FTQEM with the repetition code. The post-selection rate in FTQEM can also be increased by correcting some of the outcomes. The benefits of FTQEM are demonstrated with numerical simulations and hardware demonstrations.

Authors:Shi-Xin Zhang, Shuai Yin

Abstract: Quantum computers promise a highly efficient approach to investigate quantum phase transitions, which describe abrupt changes between different ground states of many-body systems. At quantum critical points, the divergent correlation length and entanglement entropy render the ground state preparation difficult. In this work, we explore the imaginary-time evolution for probing the universal critical behavior as the universal information of the ground state can be extracted in the early-time relaxation process. We propose a systematic and scalable scheme to probe the universal behaviors via imaginary-time critical dynamics on quantum computers and demonstrate the validness of our approach by both numerical simulation and quantum hardware experiments. With the full form of the universal scaling function in terms of imaginary time, system size, and circuit depth, we successfully probe the universality by scaling analysis of the critical dynamics at an early time and with shallower quantum circuit depth. Equipped with quantum error mitigation, we also confirm the expected scaling behavior from experimental results on a superconducting quantum processor which stands as the first experimental demonstration on universal imaginary-time quantum critical dynamics.

Authors:Yue Ban, Xi Chen

Abstract: We investigate the behavior of relativistic electrons encountering a potential step through analogies with optical phenomena. By accounting for the conservation of Dirac current, we elucidate that the Goos-H\"anchen shift can be understood as a combination of two components: one arising from the current entering the transmission region and the other originating from the interference between the incident and reflected beams. This result has been proven to be consistent with findings obtained utilizing the stationary phase method. Moreover, we explore the transverse Imbert-Fedorov shift, by applying both current conservation and total angular momentum conservation, revealing intriguing parallel to the spin Hall effect. Beyond enriching our comprehension of fundamental quantum phenomena, our findings have potential applications for designing and characterizing devices using Dirac and topological materials.

Authors:Sebastian Schulz, Dennis Willsch, Kristel Michielsen

Abstract: We utilize the theory of local amplitude transfers (LAT) to gain insights into quantum walks (QWs) and quantum annealing (QA) beyond the adiabatic theorem. By representing the eigenspace of the problem Hamiltonian as a hypercube graph, we demonstrate that probability amplitude traverses the search space through a series of local Rabi oscillations. We argue that the amplitude movement can be systematically guided towards the ground state using a time-dependent hopping rate based solely on the problem's energy spectrum. Building upon these insights, we extend the concept of multi-stage QW by introducing the guided quantum walk (GQW) as a bridge between QW-like and QA-like procedures. We assess the performance of the GQW on exact cover, traveling salesperson and garden optimization problems with 9 to 30 qubits. Our results provide evidence for the existence of optimal annealing schedules, beyond the requirement of adiabatic time evolutions. These schedules might be capable of solving large-scale combinatorial optimization problems within evolution times that scale linearly in the problem size.

Authors:Arindam Dutta, Anirban Pathak

Abstract: Here we present a new protocol for controlled quantum key agreement and another protocol for key agreement with a specific focus on the security analysis. Specifically, detailed security proof is provided against impersonated fraudulent attack and collective attacks and it is established that the proposed protocols are not only secure, but they also satisfy other desired properties of such schemes (i.e., fairness and correctness). Further, the proposed schemes are critically compared with a set of schemes for quantum key agreement and an existing scheme for controlled quantum key agreement (Tang et al.'s protocol) in terms of efficiency and the required quantum resources. Especially, it is observed that in contrast to the existing schemes, the present scheme does not require quantum memory. In addition, the protocol for controlled quantum key agreement proposed here is found to require quantum resources (Bell state and single photon state) that are easier to produce and maintain compared to the quantum resources (GHZ states) required by the only known existing protocol for the same purpose, i.e., Tang et al.'s protocol.

Authors:Vlatko Vedral

Abstract: We show how imaginary numbers in quantum physics can be eliminated by enlarging the Hilbert Space followed by an imposition of - what effectively amounts to - a superselection rule. We illustrate this procedure with a qubit and apply it to the Mach-Zehnder interferometer. The procedure is somewhat reminiscent of the constrained quantization of the electromagnetic field, where, in order to manifestly comply with relativity, one enlargers the Hilbert Space by quantizing the longitudinal and scalar modes, only to subsequently introduce a constraint to make sure that they are actually not directly observable.

Authors:Yilun Xu, Daoquan Zhu, Feng-Xiao Sun, Qiongyi He, Wei Zhang

Abstract: Continuous U(1) gauge symmetry, which guarantees the conservation of the total excitations in linear bosonic systems, will be broken when it comes to the strong-coupling regime where the rotation wave approximation (RWA) fails. Here we develop analytic solutions for multi-mode bosonic systems with XX-type couplings beyond RWA, and proposed a novel scheme to implement high-fidelity quantum state transfer (QST) and entanglement preparation (EP) with high speed. The scheme can be realized with designated coupling strength and pulse duration with which the excitation number keeps unchanged regardless of the breakdown of the global U(1) symmetry. In the QST tasks, we consider several typical quantum states and demonstrate that this method is robust against thermal noise and imperfections of experimental sequence. In the EP tasks, the scheme is successfully implemented for the preparation of Bell states and W-type states, within a shortest preparation time.

Authors:Juan M. Cruz-Martinez, Matteo Robbiati, Stefano Carrazza

Abstract: In this work we present a novel strategy to evaluate multi-variable integrals with quantum circuits. The procedure first encodes the integration variables into a parametric circuit. The obtained circuit is then derived with respect to the integration variables using the parameter shift rule technique. The observable representing the derivative is then used as the predictor of the target integrand function following a quantum machine learning approach. The integral is then estimated using the fundamental theorem of integral calculus by evaluating the original circuit. Embedding data according to a reuploading strategy, multi-dimensional variables can be easily encoded into the circuit's gates and then individually taken as targets while deriving the circuit. These techniques can be exploited to partially integrate a function or to quickly compute parametric integrands within the training hyperspace.

Authors:Ao-Xiang Liu, Ma-Cheng Yang, Cong-Feng Qiao

Abstract: We derive by lattice theory a universal quantum certainty relation for arbitrary $M$ observables in $N$-dimensional system, which provides a state-independent maximum lower bound on the direct-sum of the probability distribution vectors (PDVs) in terms of majorization relation. While the utmost lower bound coincides with $(1/N,...,1/N)$ for any two orthogonal bases, the majorization certainty relation for $M\geqslant3$ is shown to be nontrivial. The universal majorization bounds for three mutually complementary observables and a more general set of observables in dimension-2 are achieved. It is found that one cannot prepare a quantum state with PDVs of incompatible observables spreading out arbitrarily. Moreover, we obtain a complementary relation for the quantum coherence as well, which characterizes a trade-off relation of quantum coherence with different bases.

Authors:Aaron Z. Goldberg, Khabat Heshami

Abstract: A recent theoretical proposal for teleamplification requires preparation of Fock states, programmable interferometers, and photon-number resolving detectors to herald the teleamplification of an input state. These enable teleportation and heralded noiseless linear amplification of a photonic state up to an arbitrarily large energy cutoff. We report on adapting this proposal for Borealis and demonstrating teleamplification of squeezed-vacuum states with variable amplification factors. The results match the theoretical predictions and exhibit features of amplification in the teleported mode. This demonstration motivates the continued development of photonic quantum computing hardware for noiseless linear amplification's applications across quantum communication, sensing, and error correction.

Authors:H. Cao, L. M. Hansen, F. Giorgino, L. Carosini, P. Zahalka, F. Zilk, J. C. Loredo, P. Walther

Abstract: Generating large multiphoton entangled states is of main interest due to enabling universal photonic quantum computing and all-optical quantum repeater nodes. These applications exploit measurement-based quantum computation using cluster states. Remarkably, it was shown that photonic cluster states of arbitrary size can be generated by using feasible heralded linear optics fusion gates that act on heralded three-photon Greenberger-Horne-Zeilinger (GHZ) states as the initial resource state. Thus, the capability of generating heralded GHZ states is of great importance for scaling up photonic quantum computing. Here, we experimentally demonstrate this required building block by reporting a polarisation-encoded heralded GHZ state of three photons, for which we build a high-rate six-photon source ($547{\pm}2$ Hz) from a solid-state quantum emitter and a stable polarisation-based interferometer. The detection of three ancillary photons heralds the generation of three-photon GHZ states among the remaining particles with fidelities up to $\mathcal{F}=0.7278{\pm}0.0106$. Our results initiate a path for scalable entangling operations using heralded linear-optics implementations.

Authors:Gabriel M. Lando, Olivier Giraud, Denis Ullmo

Abstract: We study the time evolution of mean values of quantum operators in a regime plagued by two difficulties: The smallness of $\hbar$ and the presence of strong and ubiquitous classical chaos. While numerics become too computationally expensive for purely quantum calculations as $\hbar \to 0$, methods that take advantage of the smallness of $\hbar$ -- that is, semiclassical methods -- suffer from both conceptual and practical difficulties in the deep chaotic regime. We implement an approach which addresses these conceptual problems, leading to a deeper understanding of the origin of the interference contributions to the operator's mean value. We show that in the deep chaotic regime our approach is capable of unprecedented accuracy, while a typical semiclassical method (the Herman-Kluk propagator) produces only numerical noise. Our work paves the way to the development and employment of more efficient and accurate methods for quantum simulations of systems with strongly chaotic classical limits.

Authors:Zheng-Rong Zhu, Bin Shao, Jian Zou, Lian-Ao Wu

Abstract: We investigate the orthogonality catastrophe and quantum speed limit in the Creutz model for dynamical quantum phase transitions. We demonstrate that exact zeros of the Loschmidt echo can exist in finite-size systems for specific discrete values. We highlight the role of the zero-energy mode when analyzing quench dynamics near the critical point. We also examine the behavior of the time for the first exact zeros of the Loschmidt echo and the corresponding quantum speed limit time as the system size increases. While the bound is not tight, it can be attributed to the scaling properties of the band gap and energy variance with respect to system size. As such, we establish a relation between the orthogonality catastrophe and quantum speed limit by referencing the full form of the Loschmidt echo. Significantly, we find the possibility of using the quantum speed limit to detect the critical point of a static quantum phase transition, along with a decrease in the amplitude of noise induced quantum speed limit.

Authors:Ken Mochizuki, Ryusuke Hamazaki

Abstract: We show that noisy non-unitary dynamics of bosons drives arbitrary initial states into a novel fluctuating bunching state, where all bosons occupy one time-dependent mode. We propose a concept of the noisy spectral gap, a generalization of the spectral gap in noiseless systems, and demonstrate that exponentially fast absorption to the fluctuating bunching state takes place asymptotically. The fluctuating bunching state is unique to noisy non-unitary dynamics with no counterpart in any unitary dynamics and non-unitary dynamics described by a time-independent generator. We also argue that the times of relaxation to the fluctuating bunching state obey a universal power law as functions of the noise parameter in generic noisy non-unitary dynamics.

Authors:Yadong Wu, Juan Yao, Pengfei Zhang, Xiaopeng Li

Abstract: As a hybrid of artificial intelligence and quantum computing, quantum neural networks (QNNs) have gained significant attention as a promising application on near-term, noisy intermediate-scale quantum (NISQ) devices. Conventional QNNs are described by parametrized quantum circuits, which perform unitary operations and measurements on quantum states. In this work, we propose a novel approach to enhance the expressivity of QNNs by incorporating randomness into quantum circuits. Specifically, we introduce a random layer, which contains single-qubit gates sampled from an trainable ensemble pooling. The prediction of QNN is then represented by an ensemble average over a classical function of measurement outcomes. We prove that our approach can accurately approximate arbitrary target operators using Uhlmann's theorem for majorization, which enables observable learning. Our proposal is demonstrated with extensive numerical experiments, including observable learning, R\'enyi entropy measurement, and image recognition. We find the expressivity of QNNS is enhanced by introducing randomness for multiple learning tasks, which could have broad application in quantum machine learning.

Authors:Chen Chen, Jun-Yong Yan, Hans-Georg Babin, Xing Lin, Wei Fang, Run-Ze Liu, Yong-Heng Huo, Wei E. I. Sha, Jiaxiang Zhang, Christian Heyn, Andreas D. Wieck, Arne Ludwig, Da-Wei Wang, Chao-Yuan Jin, Feng Liu

Abstract: The construction of a large-scale quantum internet requires quantum repeaters containing multiple entangled photon sources with identical wavelengths. Semiconductor quantum dots can generate entangled photon pairs deterministically with high fidelity. However, realizing quantum dot-based quantum repeaters faces two difficulties: the non-uniformity of emission wavelength and exciton fine-structure splitting induced fidelity reduction. Typically, these two factors are not independently tunable, making it challenging to achieve simultaneous improvement. In this work, we demonstrate wavelength-tunable entangled photon sources based on droplet-etched GaAs quantum dots through the combined use of the AC and quantum-confined Stark effects. The emission wavelength can be tuned by ~1 meV while preserving entanglement fidelity above 0.955(1) across the entire tuning range. Our work paves a way towards robust and scalable on-demand entangled photon sources for large-scale quantum internet and integrated quantum optical circuits.

Authors:Qian Wang, Marko Robnik

Abstract: The properties of mixed eigenstates in a generic quantum system with classical counterpart that has mixed-type phase space, although important to understand several fundamental questions that arise in both theoretical and experimental studies, are still not clear. Here, following a recent work [\v{C}.~Lozej {\it et al}. Phys. Rev. E {\bf 106}, 054203 (2022)], we perform an analysis of the features of mixed eigenstates in a time-dependent Hamiltonian system, the celebrated kicked top model. As a paradigmatic model for studying quantum chaos, kicked top model is known to exhibit both classical and quantum chaos. The types of eigenstates are identified by means of the phase space overlap index, which is defined as the overlap of the Husimi function with regular and chaotic regions in classical phase space. We show that the mixed eigenstates appear due to various tunneling precesses between different phase space structures, while the regular and chaotic eigenstates are, respectively, associated with invariant tori and chaotic component in phase space. We examine how the probability distribution of the phase space overlap index evolves with increasing system size for different kicking strengths. In particular, we find that the relative fraction of mixed states exhibits a power-law decay as the system size increases, indicating that only purely regular and chaotic eigenstates are left in the strict semiclassical limit. We thus provide further verification of the principle of uniform semiclassical condensation of Husimi functions and confirm the correctness of the Berry-Robnik picture.

Authors:Nishikanta Mohanty, Bikash K. Behera, Christopher Ferrie

Abstract: The Vehicle Routing Problem (VRP) is an example of a combinatorial optimization problem that has attracted academic attention due to its potential use in various contexts. VRP aims to arrange vehicle deliveries to several sites in the most efficient and economical manner possible. Quantum machine learning offers a new way to obtain solutions by harnessing the natural speedups of quantum effects, although many solutions and methodologies are modified using classical tools to provide excellent approximations of the VRP. In this paper, we implement and test hybrid quantum machine learning methods for solving VRP of 3 and 4-city scenarios, which use 6 and 12 qubit circuits, respectively. The proposed method is based on quantum support vector machines (QSVMs) with a variational quantum eigensolver on a fixed or variable ansatz. Different encoding strategies are used in the experiment to transform the VRP formulation into a QSVM and solve it. Multiple optimizers from the IBM Qiskit framework are also evaluated and compared.

Authors:Galina Weinstein

Abstract: This paper serves as an addendum to my previously published work, which delves into the experimentation with the Google Sycamore quantum processor under the title "Debating the Reliability and Robustness of the Learned Hamiltonian in the Traversable Wormhole Experiment." In the preceding publication, I extensively discussed the quantum system functioning as a dual to a traversable wormhole and the ongoing efforts to discover a sparse model that accurately depicts the dynamics of this intriguing phenomenon. In this paper, I bring to light an important insight regarding applying Nancy Cartwright's ideas about reliability and reproducibility, which are deeply rooted in classical scientific practices and experiments. I show that when applied to the realm of quantum devices, such as Google's Sycamore quantum processor and other Noisy Intermediate-Scale Quantum (NISQ) devices, these well-established notions demand careful adaptation and consideration. These systems' inherent noise and quantum nature introduce complexities that necessitate rethinking traditional perspectives on reliability and reproducibility. In light of these complexities, I propose the term "noisy reliability" as a means to effectively capture the nuanced nature of assessing the reliability of quantum devices, particularly in the presence of inherent quantum noise. This addendum seeks to enrich the discussion by highlighting the challenges and implications of assessing quantum device reliability, thereby contributing to a deeper understanding of quantum experimentation and its potential applications in various domains.

Authors:Dennis Willsch, Madita Willsch, Fengping Jin, Hans De Raedt, Kristel Michielsen

Abstract: Shor's factoring algorithm is one of the most anticipated applications of quantum computing. However, the limited capabilities of today's quantum computers only permit a study of Shor's algorithm for very small numbers. Here we show how large GPU-based supercomputers can be used to assess the performance of Shor's algorithm for numbers that are out of reach for current and near-term quantum hardware. First, we study Shor's original factoring algorithm. While theoretical bounds suggest success probabilities of only 3-4 %, we find average success probabilities above 50 %, due to a high frequency of "lucky" cases, defined as successful factorizations despite unmet sufficient conditions. Second, we investigate a powerful post-processing procedure, by which the success probability can be brought arbitrarily close to one, with only a single run of Shor's quantum algorithm. Finally, we study the effectiveness of this post-processing procedure in the presence of typical errors in quantum processing hardware. We find that the quantum factoring algorithm exhibits a particular form of universality and resilience against the different types of errors. The largest semiprime that we have factored by executing Shor's algorithm on a GPU-based supercomputer, without exploiting prior knowledge of the solution, is 549755813701 = 712321 * 771781. We put forward the challenge of factoring, without oversimplification, a non-trivial semiprime larger than this number on any quantum computing device.

Authors:Jwo-Sy Chen, Erik Nielsen, Matthew Ebert, Volkan Inlek, Kenneth Wright, Vandiver Chaplin, Andrii Maksymov, Eduardo Páez, Amrit Poudel, Peter Maunz, John Gamble

Abstract: Quantum computers are rapidly becoming more capable, with dramatic increases in both qubit count and quality. Among different hardware approaches, trapped-ion quantum processors are a leading technology for quantum computing, with established high-fidelity operations and architectures with promising scaling. Here, we demonstrate and thoroughly benchmark the IonQ Forte system: configured here as a single-chain 30-qubit trapped-ion quantum computer with all-to-all operations. We assess the performance of our quantum computer operation at the component level via direct randomized benchmarking (DRB) across all 30 choose 2 = 435 gate pairs. We then show the results of application-oriented benchmarks, indicating that the system passes the suite of algorithmic qubit (AQ) benchmarks up to #AQ 29. Finally, we use our component-level benchmarking to build a system-level model to predict the application benchmarking data through direct simulation, including error mitigation. We find that the system-level model correlates well with the observations in many cases, though in some cases out-of-model errors lead to higher predicted performance than is observed. This highlights that as quantum computers move toward larger and higher-quality devices, characterization becomes more challenging, suggesting future work required to push performance further.

Authors:Tomislav Begušić, Johnnie Gray, Garnet Kin-Lic Chan

Abstract: A recent quantum simulation of observables of the kicked Ising model on 127 qubits [Nature 618, 500 (2023)] implemented circuits that exceed the capabilities of exact classical simulation. We show that several approximate classical methods, based on sparse Pauli dynamics and tensor network algorithms, can simulate these observables orders of magnitude faster than the quantum experiment, and can also be systematically converged beyond the experimental accuracy. Our most accurate technique combines a mixed Schr\"odinger and Heisenberg tensor network representation with the free entropy relation of belief propagation to compute expectation values with an effective wavefunction-operator sandwich bond dimension ${>}16,000,000$, achieving an absolute accuracy, without extrapolation, in the observables of ${<}0.01$, which is converged for many practical purposes. We thereby identify inaccuracies in the experimental extrapolations and suggest how future experiments can be implemented to increase the classical hardness.

Authors:François Le Gall, Yupan Liu, Qisheng Wang

Abstract: Driven by exploring the power of quantum computation with a limited number of qubits, we present a novel complete characterization for space-bounded quantum computation, which encompasses settings with one-sided error (unitary coRQL) and two-sided error (BQL), approached from a quantum state testing perspective: - The first family of natural complete problems for unitary coRQL, i.e., space-bounded quantum state certification for trace distance and Hilbert-Schmidt distance; - A new family of natural complete problems for BQL, i.e., space-bounded quantum state testing for trace distance, Hilbert-Schmidt distance, and quantum entropy difference. In the space-bounded quantum state testing problem, we consider two logarithmic-qubit quantum circuits (devices) denoted as $Q_0$ and $Q_1$, which prepare quantum states $\rho_0$ and $\rho_1$, respectively, with access to their ``source code''. Our goal is to decide whether $\rho_0$ is $\epsilon_1$-close to or $\epsilon_2$-far from $\rho_1$ with respect to a specified distance-like measure. Interestingly, unlike time-bounded state testing problems, which exhibit computational hardness depending on the chosen distance-like measure (either QSZK-complete or BQP-complete), our results reveal that the space-bounded state testing problems, considering all three measures, are computationally as easy as preparing quantum states. Our results primarily build upon a space-efficient variant of the quantum singular value transformation (QSVT) introduced by Gily\'en, Su, Low, and Wiebe (STOC 2019), which is of independent interest. Our technique provides a unified approach for designing space-bounded quantum algorithms. Specifically, we show that implementing QSVT for any bounded polynomial that approximates a piecewise-smooth function incurs only a constant overhead in terms of the space required for special forms of the projected unitary encoding.

Authors:ChunJun Cao, Michael J. Gullans, Brad Lackey, Zitao Wang

Abstract: We provide the first tensor network method for computing quantum weight enumerator polynomials in the most general form. As a corollary, if a quantum code has a known tensor network construction of its encoding map, our method produces an algorithm that computes its distance. For non-(Pauli)-stabilizer codes, this constitutes the current best algorithm for computing the code distance. For degenerate stabilizer codes, it can provide up to an exponential speed up compared to the current methods. We also introduce a few novel applications of different weight enumerators. In particular, for any code built from the quantum lego method, we use enumerators to construct its (optimal) decoders under any i.i.d. single qubit or qudit error channels and discuss their applications for computing logical error rates. As a proof of principle, we perform exact analyses of the deformed surface codes, the holographic pentagon code, and the 2d Bacon-Shor code under (biased) Pauli noise and limited instances of coherent error at sizes that are inaccessible by brute force.

Authors:Pan-Yu Hou, Jenny J. Wu, Stephen D. Erickson, Giorgio Zarantonello, Adam D. Brandt, Daniel C. Cole, Andrew C. Wilson, Daniel H. Slichter, Dietrich Leibfried

Abstract: Cooling the motion of trapped ions to near the quantum ground state is crucial for many applications in quantum information processing and quantum metrology. However, certain motional modes of trapped-ion crystals can be difficult to cool due to weak or zero interaction between the modes and the cooling radiation, typically laser beams. We overcome this challenge by coupling a mode with weak cooling radiation interaction to one with strong cooling radiation interaction using parametric modulation of the trapping potential, thereby enabling indirect cooling of the former. In this way, we demonstrate near-ground-state cooling of motional modes with weak or zero cooling radiation interaction in multi-ion crystals of the same and mixed ion species, specifically $^9$Be$^+$-$^9$Be$^+$, $^9$Be$^+$-$^{25}$Mg$^+$, and $^9$Be$^+$-$^{25}$Mg$^+$-$^9$Be$^+$ crystals. This approach can be generally applied to any Coulomb crystal where certain motional modes cannot be directly cooled efficiently, including crystals containing molecular ions, highly-charged ions, charged fundamental particles, or charged macroscopic objects.

Authors:Luka Medic, Anton Ramšak, Tomaž Prosen

Abstract: We consider the spectral and initial value problem for the Lindblad-Gorini-Kossakowski-Sudarshan master equation describing an open quantum system of bosons and spins, where the bosonic parts of the Hamiltonian and Lindblad jump operators are quadratic and linear respectively, while the spins couple to bosons via mutually commuting spin operators. Needless to say, the spin degrees of freedom can be replaced by any set of finite-level quantum systems. A simple, yet non-trivial example of a single open spin-boson model is worked out in some detail.

Authors:Zhiyuan Wang, Kaden R. A. Hazzard

Abstract: It is commonly believed that there are only two types of particle exchange statistics in quantum mechanics, fermions and bosons, with the exception of anyons in two dimension. In principle, a second exception known as parastatistics, which extends outside of two dimensions, has been considered but was believed to be physically equivalent to fermions and bosons. In this paper we show that nontrivial parastatistics inequivalent to either fermions or bosons can exist in physical systems. These new types of identical particles obey generalized exclusion principles, leading to exotic free-particle thermodynamics distinct from any system of free fermions and bosons. We formulate our theory by developing a second quantization of paraparticles, which naturally includes exactly solvable non-interacting theories, and incorporates physical constraints such as locality. We then construct a family of one-dimensional quantum spin models where free parastatistical particles emerge as quasiparticle excitations. This demonstrates the possibility of a new type of quasiparticle in condensed matter systems, and, more speculatively, the potential for previously unconsidered types of elementary particles.

Authors:Mads Bjerregaard Kristensen, Nenad Kralj, Eric Langman, Albert Schliesser

Abstract: We demonstrate a memory for light based on optomechanically induced transparency. We achieve a long storage time by leveraging the ultra-low dissipation of a soft-clamped mechanical membrane resonator, which oscillates at MHz frequencies. At room temperature, we demonstrate a lifetime $T_1 \approx 23\,\mathrm{ms}$ and a retrieval efficiency $\eta \approx 40\%$ for classical coherent pulses. We anticipate storage of quantum light to be possible at moderate cryogenic conditions ($T\approx 10\,\mathrm{K}$). Such systems could find applications in emerging quantum networks, where they can serve as long-lived optical quantum memories by storing optical information in a phononic mode.

Authors:Mahesh N. Jayakody, Eliahu Cohen

Abstract: Theoretical and applied studies of quantum walks are abundant in quantum science and technology thanks to their relative simplicity and versatility. Here we derive closed-form expressions for the probability distribution of quantum walks on a line. The most general two-state coin operator and the most general (pure) initial state are considered in the derivation. The general coin operator includes the common choices of Hadamard, Grover, and Fourier coins. The method of Fibonacci-Horner basis for the power decomposition of a matrix is employed in the analysis. Moreover, we also consider mixed initial states and derive closed-form expression for the probability distribution of the Quantum walk on a line. To prove the accuracy of our derivations, we retrieve the simulated probability distribution of Hadamard walk on a line using our closed-form expressions. With a broader perspective in mind, we argue that our approach has the potential to serve as a helpful mathematical tool in obtaining precise analytical expressions for the time evolution of qubit-based systems in a general context.

Authors:Shrobona Bagchi

Abstract: Quantum correlations are one of the most important aspects of the modern day quantum information and computation theory. However, the majority of understanding of the quantum correlations is in the field of non-relativistic quantum mechanics. To develop the quantum information and computation tasks fully, one must inevitably take into account the relativistic effects. In this regard, the spin is one of the central tools to implement these qubit operations in almost all quantum information processing tasks. For this purpose, it is of paramount importance to understand and characterize fully the theory of spin in relativistic quantum mechanics and relativistic quantum information theory where the spin states act as qubit. This area is still far from being resolved as a current state of art. As a result, this article will explore the recent studies of the concepts of the spin and spin quantum correlations in inertial frames and some apparent paradoxes regarding this concept. We will mainly focus on the problem of characterizing the concept of spin, reduced spin density matrices and consequently spin quantum correlations in inertial reference frames and the apparent paradoxes involved therein, yet to be verified experimentally. Another important aspect is the use of tools of quantum field theory to extend concepts in non-relativistic domain to relativistic one. In this regard, we will analyze the development of the theory of relativistic secret sharing and a correlation measure namely the entanglement of purification. We will also explore how these developments may be mapped to quantum information processing task and discuss the future promises.

Authors:Nouhaila Innan, Muhammad Al-Zafar Khan, Mohamed Bennai

Abstract: In this research, a comparative study of four Quantum Machine Learning (QML) models was conducted for fraud detection in finance. We proved that the Quantum Support Vector Classifier model achieved the highest performance, with F1 scores of 0.98 for fraud and non-fraud classes. Other models like the Variational Quantum Classifier, Estimator Quantum Neural Network (QNN), and Sampler QNN demonstrate promising results, propelling the potential of QML classification for financial applications. While they exhibit certain limitations, the insights attained pave the way for future enhancements and optimisation strategies. However, challenges exist, including the need for more efficient Quantum algorithms and larger and more complex datasets. The article provides solutions to overcome current limitations and contributes new insights to the field of Quantum Machine Learning in fraud detection, with important implications for its future development.

Authors:Ricardo R. Ancheyta

Abstract: The nonadiabatic dynamic of the electromagnetic field triggers photons generation from the quantum vacuum. Shortcuts to adiabaticity, instead, are protocols that mimic the field's adiabatic dynamic in a finite time. Here, we show how the counterdiabatic term of the transitionless tracking algorithm cancels out, exactly, the term responsible for the photon production in the dynamical Casimir effect. This result suggests that the energy of producing photons out of the vacuum is related to the energetic cost of the shortcut. Furthermore, if the system operates under a quantum thermodynamic cycle, we confirm the equivalence between the adiabatic and nonadiabatic work outputs. Finally, our study reveals that identifying these unreported observations can only be possible using the so-called effective Hamiltonian approach.

Authors:Jyotsna Gidugu, Daniel P. Arovas

Abstract: We generalize the recent work of Shibata and Katsura, who considered a S=1/2 chain with alternating XX and YY couplings in the presence of dephasing, the dynamics of which are described by the GKLS master equation. Their model is equivalent to a non-Hermitian system described by the Kitaev formulation in terms of a single Majorana species hopping on a two-leg ladder in the presence of a nondynamical Z_2 gauge field. Our generalization involves Dirac gamma matrix `spin' operators on the square lattice, and maps onto a non-Hermitian square lattice bilayer which is also Kitaev-solvable. We describe the exponentially many non-equilibrium steady states in this model. We identify how the spin degrees of freedom can be accounted for in the 2d model in terms of the gauge-invariant quantities and then proceed to study the Liouvillian spectrum. We use a genetic algorithm to estimate the Liouvillian gap and the first decay modes for large system sizes. We observe a transition in the first decay modes, similar to that found by Shibata and Katsura. The results we obtain are consistent with a perturbative analysis for small and large values of the dissipation strength.

Authors:Oleg Morzhin, Alexander Pechen

Abstract: The work considers an open qutrit system whose density matrix $\rho(t)$ evolution is governed by the Gorini-Kossakowski-Sudarshan-Lindblad master equation with simultaneous coherent (in the Hamiltonian) and incoherent (in the superoperator of dissipation) controls. Incoherent control makes the decoherence rates depending on time in a specific controlled manner and within clear physical mechanics. We consider the problem of maximizing the Hilbert-Schmidt overlap between the system's final state $\rho(T)$ and a given target state $\rho_{\rm target}$ and the problem of minimizing the squared Hilbert-Schmidt distance between these states. For the both problems, we perform their realifications, derive the corresponding Pontryagin function, adjount system (with the two cases of transversality conditions in view of the two terminal objectives), and gradients of the objectives, adapt the one-, two-, three-step gradient projection methods. For the problem of maximizing the overlap, we also adapt the regularized first-order Krotov method. In the numerical experiments, we analyze, first, the methods' operation and, second, the obtained control processes, in respect to considering the environment as a resource via incoherent control.

Authors:Michael Maroun

Abstract: Analyzing the point spectrum, i.e. bound state energy eigenvalue, of the Dirac delta function in two and three dimensions is notoriously difficult without recourse to regularization or renormalization, typically both. The reason for this in two dimensions is two fold; 1) the coupling constant, together with the mass and Planck's constant form an unitless quantity. This causes there to be a missing anomalous length scale. 2) The immediately obvious L$^2$ solution is divergent at the origin, where the Dirac Delta potential has its important point of support as a measure. Due to the uniqueness of the solution presented here, it is immediate that the linear operator (the two dimensional Laplace operator on all of $\mathbb{R}^2$), with the specialized domain constructed here, ensures that the point spectrum has exactly one element. This element is determined precisely, and a natural mathematically rigorous resolution to the anomalous length scale arises. In this work, there is no recourse to renormalization or regularization of any kind.

Authors:Pradeep Kumar, Tanoy Kanti Konar, Leela Ganesh Chandra Lakkaraju, Aditi Sen De

Abstract: Quantum algorithms have the ability to reduce runtime for executing tasks beyond the capabilities of classical algorithms. Therefore, identifying the resources responsible for quantum advantages is an interesting endeavour. We prove that nonvanishing quantum correlations, both bipartite and genuine multipartite entanglement, are required for solving nontrivial linear systems of equations in the Harrow-Hassidim-Lloyd (HHL) algorithm. Moreover, we find a nonvanishing l1-norm quantum coherence of the entire system and the register qubit which turns out to be related to the success probability of the algorithm. Quantitative analysis of the quantum resources reveals that while a significant amount of bipartite entanglement is generated in each step and required for this algorithm, multipartite entanglement content is inversely proportional to the performance indicator. In addition, we report that when imperfections chosen from Gaussian distribution are incorporated in controlled rotations, multipartite entanglement increases with the strength of the disorder, albeit error also increases while bipartite entanglement and coherence decreases, confirming the beneficial role of bipartite entanglement and coherence in this algorithm.

Authors:Zhiyao Hu, Qixian Li, Xuanchen Zhang, He-bin Zhang, Long-Gang Huang, Yong-Chun Liu

Abstract: Atomic nonlinear interferometry has wide applications in quantum metrology and quantum information science. Here we propose a nonlinear time-reversal interferometry scheme with high robustness and metrological gain based on the spin squeezing generated by arbitrary quadratic collective-spin interaction, which could be described by the Lipkin-Meshkov-Glick (LMG) model. We optimize the squeezing process, encoding process, and anti-squeezing process, finding that the two particular cases of the LMG model, one-axis twisting and two-axis twisting outperform in robustness and precision, respectively. Moreover, we propose a Floquet driving method to realize equivalent time reverse in the atomic system, which leads to high performance in precision, robustness, and operability. Our study sets a benchmark in achieving high precision and robustness in atomic nonlinear interferometry.

Authors:B. S. Ham

Abstract: Quantum superposition is normally sustained in a microscopic regime governed by Heisenberg uncertainty principle applicable to a single particle. Quantum correlation between paired particles implies the violation of local realism governed by classical physics. Over the last decades, quantum features have been implemented in various quantum technologies including quantum computing, communications, and sensing. Such quantum features are generally known to be impossible by any classical means. Here, a macroscopic quantum correlation is presented for coherence manipulations of polarization-path correlations of a continuous wave laser, satisfying the joint-parameter relation in an inseparable product-basis form. For the coherence control of the polarization-path correlation, a pair of electro-optic modulators is used in a noninterfering Mach-Zehnder interferometer for deterministic switching between paired polarization bases, resulting in the polarization product-basis superposition in a selective product-basis choice manner by a followed pair of acousto-optic modulators. This unprecedented macroscopic quantum feature opens the door to a new understanding of quantum mechanics beyond the microscopic regime for future classical optics-compatible quantum information.

Authors:Florian J. Kiwit, Marwa Marso, Philipp Ross, Carlos A. Riofrío, Johannes Klepsch, Andre Luckow

Abstract: Benchmarking of quantum machine learning (QML) algorithms is challenging due to the complexity and variability of QML systems, e.g., regarding model ansatzes, data sets, training techniques, and hyper-parameters selection. The QUantum computing Application benchmaRK (QUARK) framework simplifies and standardizes benchmarking studies for quantum computing applications. Here, we propose several extensions of QUARK to include the ability to evaluate the training and deployment of quantum generative models. We describe the updated software architecture and illustrate its flexibility through several example applications: (1) We trained different quantum generative models using several circuit ansatzes, data sets, and data transformations. (2) We evaluated our models on GPU and real quantum hardware. (3) We assessed the generalization capabilities of our generative models using a broad set of metrics that capture, e.g., the novelty and validity of the generated data.

Authors:Qi-Ming Ding, Yi-Ming Huang, Xiao Yuan

Abstract: Molecular docking plays a pivotal role in drug discovery and precision medicine, enabling us to understand protein functions and advance novel therapeutics. Here, we introduce a potential alternative solution to this problem, the digitized-counterdiabatic quantum approximate optimization algorithm (DC-QAOA), which utilizes counterdiabatic driving and QAOA on a quantum computer. Our method was applied to analyze diverse biological systems, including the SARS-CoV-2 Mpro complex with PM-2-020B, the DPP-4 complex with piperidine fused imidazopyridine 34, and the HIV-1 gp120 complex with JP-III-048. The DC-QAOA exhibits superior performance, providing more accurate and biologically relevant docking results, especially for larger molecular docking problems. Moreover, QAOA-based algorithms demonstrate enhanced hardware compatibility in the noisy intermediate-scale quantum era, indicating their potential for efficient implementation under practical docking scenarios. Our findings underscore quantum computing's potential in drug discovery and offer valuable insights for optimizing protein-ligand docking processes.

Authors:Nayere Saberian, Seyed Javad Akhtarshenas, Fereshte Shahbeigi

Abstract: Obtaining information from a quantum system through a measurement typically disturbs its state. The post-measurement states for a given measurement, however, are not unique and highly rely on the chosen measurement model, complicating the puzzle of information-disturbance. Two distinct questions are then in order. Firstly, what is the minimum disturbance a measurement may induce? Secondly, when a fixed disturbance occurs, how informative is the possible measurement in the best-case scenario? Here, we propose various approaches to tackle these questions and provide explicit solutions for the set of unbiased binary qubit measurements and post-measurement state spaces that are equivalent to the image of a unital qubit channel. In particular, we show there are different trade-off relations between the sharpness of this measurement and the average fidelity of the pre-measurement and post-measurement state spaces as well as the sharpness and quantum resources preserved in the post-measurement states in terms of coherence and discord-like correlation once the measurement is applied locally.

Authors:X. N. Feng, M. Zhang, L. F. Wei

Abstract: Quantum entanglement and quantum squeezing are two most typical approaches to beat the standard quantum limit (SQL) of the sensitive phase estimations in quantum metrology. Each of them has already been utilized individually to improve the sensitivity of electric field sensing with the trapped ion platform, but the upper bound of the demonstrated sensitivity gain is very limited, i.e., the experimental 3dB and theoretical 6dB, over the SQL. Here, by simultaneously using the internal (spin)-external (oscillator) state entanglements and the oscillator squeezings to effectively amplify the accumulation phase and compress the mean excited phonon number at the same time, we show that these sensitivity gains can be effectively surpassed, once the relevant parameters can be properly set. Hopefully, the proposal provides a novel approach to the stronger beaten of the SQL for the sensitive sensings of the desired electric field and also the other metrologies.

Authors:Michele N. Notarnicola, Stefano Olivares

Abstract: We propose a hybrid feed-forward receiver (HFFRE) for the discrimination of binary phase-shift-keyed coherent states based on the appropriate combination of the displacement feed-forward receiver (DFFRE) and a homodyne-like setup employing a low-intensity local oscillator and photon-number-resolving detectors. We investigate the performance of the proposed scheme addressing also realistic scenarios in the presence of non-unit quantum detection efficiency, dark counts and a visibility reduction. The present HFFRE outperforms the DFFRE in all conditions, beating the standard quantum limit in particular regimes.

Authors:Ling Lin, Yongguan Ke, Li Zhang, Chaohong Lee

Abstract: Chern number is a crucial invariant for characterizing topological feature of two-dimensional quantum systems. Real-space Chern number allows us to extract topological properties of systems without involving translational symmetry, and hence plays an important role in investigating topological systems with disorder or impurity. On the other hand, the twisted boundary condition (TBC) can also be used to define the Chern number in the absence of translational symmetry. Here we study the relation between these different definitions of Chern number. Through analyzing the TBC formula and two real-space formulae (the non-commutative Chern number and the Bott index formula), we show that these approaches are equivalent in the thermodynamic limit. The equivalence is also numerically confirmed via the Haldane model.

Authors:Rene Allerstorfer, Llorenç Escolà-Farràs, Arpan Akash Ray, Boris Škorić, Florian Speelman, Philip Verduyn Lunel

Abstract: In this work we study quantum position verification with continuous-variable quantum states. In contrast to existing discrete protocols, we present and analyze a protocol that utilizes coherent states and its properties. Compared to discrete-variable photonic states, coherent states offer practical advantages since they can be efficiently prepared and manipulated with current technology. We prove security of the protocol against any unentangled attackers via entropic uncertainty relations, showing that the adversary has more uncertainty than the honest prover about the correct response as long as the noise in the quantum channel is below a certain threshold. Additionally, we show that attackers who pre-share one continuous-variable EPR pair can break the protocol.

Authors:Arnaldo Spalvieri

Abstract: The paper analyzes the entropy of a system composed by an arbitrary number of indistinguishable particles at the equilibrium, defining entropy as a function of the quantum state of the system, not of its phase space representation. Our crucial observation is that the entropy of the system is the Shannon entropy of the random occupancy numbers of the quantum states allowed to system's particles. We consider the information-theoretic approach, which is based on Jaynes' maximum entropy principle, and the empirical approach, which leads to canonical typicality in modern quantum thermodynamics. In the information-theoretic approach, the occupancy numbers of particles' quantum states are multinomially distributed, while in the empirical approach their distribution is multivariate hypergeometric. As the number of samples of the empirical probability tends to infinity, the multivariate hypergeometric distribution tends to the multinomial distribution. This reconciles, at least in the limit, the two approaches. When regarded from the perspective of quantum measurement, our analysis suggests the existence of another kind of subjectivism than the well-known subjectivism that characterizes the maximum entropy approach. This form of subjectivity is responsible for the collapse of entropy to zero after the quantum measurement, both in the information-theoretic and in the empirical approaches.

Authors:Xin Xie, Junlong Tian, Jie Peng

Abstract: We have found the special dark state solutions of the anisotropic two-qubit quantum Rabi model (QRM), which has at most one photon, and constant eigenenergy in the whole coupling regime. Accordingly, we propose a scheme to deterministically generate two kinds of the two-qubit Bell states through adiabatic evolution along the dark states. With the assistance of the Stark shift, the generation time can be reduced to subnanosecond scales, proportional to the reverse of the resonator frequency, with fidelity reaching 99%. Furthermore, the other two kinds of Bell states can also be ultrafast generated.

Authors:Brendan Pankovich, Angus Kan, Kwok Ho Wan, Maike Ostmann, Alex Neville, Srikrishna Omkar, Adel Sohbi, Kamil Brádler

Abstract: We propose fault-tolerant architectures based on performing projective measurements in the Greenberger-Horne-Zeilinger (GHZ) basis on constant-sized, entangled resource states. We present linear-optical constructions of the architectures, where the GHZ-state measurements are encoded to suppress the errors induced by photon loss and the probabilistic nature of linear optics. Simulations of our constructions demonstrate high single-photon loss thresholds compared to the state-of-the-art linear-optical architecture realized with encoded two-qubit fusion measurements performed on constant-sized resource states. We believe this result shows a resource-efficient path to achieving photonic fault-tolerant quantum computing.

Authors:Tuck C. Choy

Abstract: Heisenberg's breakthrough in his July 1925 paper that set in motion the development of Quantum Mechanics through subsequent papers by Born, Jordan, Heisenberg and also Dirac (from 1925 to 1927) is reexamined through a modern lens. In this paper, we shall discuss some new perspectives on (i) what could be the guiding intuitions for his discoveries and (ii) the origin of the Born-Jordan-Heisenberg canonical quantization rule. From this vantage point we may get an insight into Einstein's Quantum Riddle (Lande1974,Sommerfeld1918,Born1926) and a possible glimpse of what might come next after the last 100 years of Heisenberg's quantum mechanics.

Authors:Marek Czachor

Abstract: Standard architecture of quantum information processing is based on bottom-up design: One begins with a one-digit one-particle system, while multi-digit quantum registers demand multi-particle configurations, mathematically modeled by tensor products of single quantum digits. Here we show that any single quantum system is automatically equipped with hidden tensor structures that allow for single-particle top-down designs of quantum information processing. Hidden tensor structures imply that any quantum system, even as simple as a single one-dimensional harmonic oscillator, can be decomposed into an arbitrary number of subsystems. The resulting structure is rich enough to enable quantum computation, violation of Bell's inequalities, and formulation of universal quantum gates. In principle, a single-particle quantum computer is possible. Moreover, it is shown that these hidden structures are at the roots of some well known theoretical constructions, such as the Brandt-Greenberg multi-boson representation of creation-annihilation operators, intensively investigated in the context of higher-order or fractional-order squeezing. In effect, certain rather tedious standard proofs known from the literature can be simplified to literally one line. The general construction is illustrated by concrete examples.

Authors:Yuwei Lu, Haoxiang Jiang, Renming Liu

Abstract: Strong dissipation of plasmonic resonances is detrimental to quantum manipulation. To enhance the quantum coherence, we propose to tailor the local density of states (LDOS) of plasmonic resonances by integrating with a photonic cavity operating at a chiral exceptional point (CEP), where the phase of light field can offer a new degree of freedom to flexibly manipulate the quantum states. A quantized few-mode theory is employed to reveal that the LDOS of the proposed hybrid cavity can evolve into sub-Lorentzian lineshape, with order-of-magnitude linewidth narrowing and additionally a maximum of eightfold enhancement compared to the usual plasmonic-photonic cavity without CEP. This results in the enhanced coherent light-matter interaction accompanied by the reduced dissipation of polaritonic states. Furthermore, a scattering theory based on eigenmode decomposition is present to elucidate two mechanisms responsible for the significant improvement of quantum yield at CEP, the reduction of plasmonic absorption by the Fano interference and the enhancement of cavity radiation through the superscattering. Importantly, we find the latter allows achieving a near-unity quantum yield at room temperature; in return, high quantum yield is beneficial to experimentally verify the enhanced LDOS at CEP by measuring the fluorescence lifetime of a quantum emitter. Therefore, our work demonstrates that the plasmonic resonances in CEP-engineered environment can serve as a promising platform for exploring the quantum states control by virtue of the non-Hermiticity of open optical resonators and building the high-performance quantum devices for sensing, spectroscopy, quantum information processing and quantum computing.

Authors:Yuwei Lu, Jingfeng Liu, Haoxiang Jiang, Zeyang Liao

Abstract: Cavity polaritons derived from the strong light-matter interaction at the quantum level provide a basis for efficient manipulation of quantum states via cavity field. Polaritons with narrow linewidth and long lifetime are appealing in applications such as quantum sensing and storage. Here, we propose a prototypical arrangement to implement a whispering-gallery-mode resonator with topological mirror moulded by one-dimensional atom array, which allows to boost the lifetime of cavity polaritons over an order of magnitude. This considerable enhancement attributes to the coupling of polaritonic states to dissipationless edge states protected by the topological bandgap of atom array that suppresses the leakage of cavity modes. When exceeding the width of Rabi splitting, topological bandgap can further reduce the dissipation from polaritonic states to bulk states of atom array, giving arise to subradiant cavity polaritons with extremely sharp linewidth. The resultant Rabi oscillation decays with a rate even below the free-space decay of a single quantum emitter. Inheriting from the topologically protected properties of edge states, the subradiance of cavity polaritons can be preserved in the disordered atom mirror with moderate perturbations involving the atomic frequency, interaction strengths and location. Our work opens up a new paradigm of topology-engineered quantum states with robust quantum coherence for future applications in quantum computing and network.

Authors:Miguel Frías-Pérez, Luca Tagliacozzo, Mari Carmen Bañuls

Abstract: In the out-of-equilibrium evolution induced by a quench, fast degrees of freedom generate long-range entanglement that is hard to encode with standard tensor networks. However, local observables only sense such long-range correlations through their contribution to the reduced local state as a mixture. We present a tensor network method that identifies such long-range entanglement and efficiently transforms it into mixture, much easier to represent. In this way, we obtain an effective description of the time-evolved state as a density matrix that captures the long-time behavior of local operators with finite computational resources.

Authors:Xiao-Jie Tan, Mankei Tsang

Abstract: To investigate the fundamental limit to far-field incoherent imaging, the prequels to this work [M. Tsang, Phys. Rev. A 99, 012305 (2019); 104, 052411 (2021)] have studied a quantum lower bound on the error of estimating an object moment and proved a scaling law for the bound with respect to the object size. As the scaling law was proved only in the asymptotic limit of vanishing object size, this work performs a numerical analysis of the quantum bound to verify that the law works well for nonzero object sizes in reality. We also use the numerical bounds to study the optimality of a measurement called spatial-mode demultiplexing or SPADE, showing that SPADE not only follows the scaling but is also numerically close to being optimal, at least for low-order moments.

Authors:Eduardo O. Dias

Abstract: In conventional quantum mechanics (QM), time is treated as a parameter, $t$, and the evolution of the quantum state with respect to time is described by ${\hat {H}}|\psi(t)\rangle=i\hbar \frac{d}{dt}|\psi(t)\rangle$. In a recently proposed space-time-symmetric (STS) extension of QM, position becomes the parameter and a new quantum state, $|\phi(x)\rangle$, is introduced. This state describes the particle's arrival time at position $x$, and the way the arrival time changes with respect to $x$ is governed by ${\hat {P}}|\phi(x)\rangle=-i\hbar \frac{d}{dx} |\phi(x)\rangle$. In this work, we generalize the STS extension to a particle moving in three-dimensional space. By combining the conventional QM with the three-dimensional STS extension, we have a ``full'' STS QM given by the dynamic equation ${\hat { P}}^{\mu}|{\phi }^\mu(x^{\mu})\rangle=- i \hbar~\eta^{\mu\nu}\frac{d}{dx^{\nu}}|{\phi}^\mu (x^{\mu})\rangle$, where $x^{\mu}$ is the coordinate chosen as the parameter of the state. Depending on the choice of $x^\mu$, we can recover either the Schr\"odinger equation (with $x^\mu=x^0=t$) or the three-dimensional STS extension (with $x^\mu=x^i=$ either $x$, $y$, or $z$). By selecting $x^\mu=x$, we solve the dynamic equation of the STS QM for a free particle and calculate the wave function $\langle t,y,z|\phi^1(x)\rangle$. This wave function represents the probability amplitude of the particle arriving at position ($y$,$z$) at instant $t$, given that the detector occupies the entire $yz$-plane located at position $x$. Remarkably, we find that the integral of $|\langle t,y,z|\phi (x)\rangle|^2$ in $y$ and $z$ takes the form of the three-dimensional version of the axiomatic Kijowski distribution.

Authors:Nicolò Piccione

Abstract: Spontaneous collapse models are modifications of standard quantum mechanics in which a physical mechanism is responsible for the collapse of the wavefunction, thus providing a way to solve the so-called "measurement problem". However, they present great challenges when one tries to make them relativistic. Here, we propose a new kind of non-relativistic spontaneous collapse model whose relativistic version could be easier to obtain. In the non-relativistic regime, we show that this model can lead to a dynamics quite similar to that of the Ghirardi-Rimini-Weber model, by also naturally solving the problem of indistinguishable particles. Moreover, we can also obtain the same master equation of the well-known Continuous Spontaneous Localization models. Finally, we show how our proposed model solves the measurement problem in a manner conceptually similar to the Ghirardi-Rimini-Weber model.

Authors:Alexandra Bergmayr, Florian Kanitschar, Matej Pivoluska, Marcus Huber

Abstract: High-dimensional entanglement has shown to have significant advantages in quantum communication. It is available in many degrees of freedom and in particular in the time-domain routinely produced in down-conversion (SPDC). While advantageous in the sense that only a single detector channel is needed locally, it is notoriously hard to analyze, especially in an assumption-free manner that is required for quantum key distribution applications. We develop the first complete analysis of high-dimensional entanglement in the polarization-time-domain and show how to efficiently certify relevant density matrix elements and security parameters for Quantum Key Distribution (QKD). In addition to putting past experiments on rigorous footing, we also develop physical noise models and propose a novel setup that can further enhance the noise resistance of free-space quantum communication.

Authors:Roland C. Farrell, Marc Illa, Anthony N. Ciavarella, Martin J. Savage

Abstract: The vacuum of the lattice Schwinger model is prepared on up to 100 qubits of IBM's Eagle-processor quantum computers. A new algorithm to prepare the ground state of a gapped translationally-invariant system on a quantum computer is presented, which we call Scalable Circuits ADAPT-VQE (SC-ADAPT-VQE). This algorithm uses the exponential decay of correlations between distant regions of the ground state, together with ADAPT-VQE, to construct quantum circuits for state preparation that can be scaled to arbitrarily large systems. SC-ADAPT-VQE is applied to the Schwinger model, and shown to be systematically improvable, with an accuracy that converges exponentially with circuit depth. Both the structure of the circuits and the deviations of prepared wavefunctions are found to become independent of the number of spatial sites, $L$. This allows for a controlled extrapolation of the circuits, determined using small or modest-sized systems, to arbitrarily large $L$. The circuits for the Schwinger model are determined on lattices up to $L=14$ (28 qubits) with the qiskit classical simulator, and subsequently scaled up to prepare the $L=50$ (100 qubits) vacuum on IBM's 127 superconducting-qubit quantum computers ibm_brisbane and ibm_cusco. After applying an improved error-mitigation technique, which we call Operator Decoherence Renormalization, the chiral condensate and charge-charge correlators obtained from the quantum computers are found to be in good agreement with classical Matrix Product State simulations.