Quantum Physics (quant-ph)

Abstract: We show that for a class of quantum light spectroscopy (QLS) experiments using n = 0,1,2,$\cdots$ classical light pulses and an entangled photon pair (a biphoton state) where one photon acts as a reference without interacting with the matter sample, identical signals can be obtained by replacing the biphotons with classical-like coherent states of light, where these are defined explicitly in terms of the parameters of the biphoton states. An input-output formulation of quantum nonlinear spectroscopy is used to prove this equivalence. We demonstrate the equivalence numerically by comparing a classical pump - quantum probe experiment with the corresponding classical pump - classical probe experiment. This analysis shows that understanding the equivalence between entangled biphoton probes and carefully designed classical-like coherent state probes leads to quantum-inspired classical experiments and provides insights for future design of QLS experiments that could provide a true quantum advantage.

Abstract: A quantum state can be correlated in either a classical or a quantum way. Conventionally, the total correlations within the quantum system are quantified in a geometrical way through distance-based expressions such as the relative entropy or the square-norm. In this work, we provide an alternative method to quantify total correlations through the statistical measure of Pearson correlation coefficient. The two methods can be considered reciprocal to each other, given that they approach the notion of correlations from a different perspective. We also illustrate that, at least for the case of two-qubit systems, the distribution of the correlations among pairs of observables provides insight in regards to whether a system contains classical or quantum correlations. Finally, we show how correlations in quantum systems are connected to the general entropic uncertainty principle.

Abstract: Optical Schr\"odinger "cat" states created by superpositions of coherent light states, correspond to an optical analog of the Schr\"odinger's cat in his $\textit{Gedankenexperiment}$. These non-classical light states are generated by means of quantum state engineering methods, and they are considered as one of the main resources for fundamental tests of quantum theory and the development of new quantum technologies. However, the power of existing optical "cat" state sources is limited by their low average photon number, which prevents their use in nonlinear optics. Here, we demonstrate the generation of a femtosecond duration optical "cat" state in the infrared spectral range, with mean photon number orders of magnitude higher than those delivered by current available sources. These states exhibit intensities sufficient to induce nonlinear processes in matter. This is shown using the process of second harmonic generation in an optical crystal, in which the infrared-frequency photons of an optical "cat" state are up-converted into blue-frequency photons. We create the light states driving the second harmonic generation process, by means of conditioning operations applied on the quantum state of an intense infrared femtosecond laser field after its nonlinear interaction with atoms. Due to the presence of quantum interference between the coherent states composing the optical "cat" state, the quantum properties of the state are imprinted in the measured second-order interferometric autocorrelation traces. The findings introduce the optical "cat" states into the realm of nonlinear quantum optics, opening up exciting new paths in quantum information science.

Abstract: Bayesian experimental design is a technique that allows to efficiently select measurements to characterize a physical system by maximizing the expected information gain. Recent developments in deep neural networks and normalizing flows allow for a more efficient approximation of the posterior and thus the extension of this technique to complex high-dimensional situations. In this paper, we show how this approach holds promise for adaptive measurement strategies to characterize present-day quantum technology platforms. In particular, we focus on arrays of coupled cavities and qubit arrays. Both represent model systems of high relevance for modern applications, like quantum simulations and computing, and both have been realized in platforms where measurement and control can be exploited to characterize and counteract unavoidable disorder. Thus, they represent ideal targets for applications of Bayesian experimental design.

Abstract: Motivated by recent experimental observations carried out in superconducting transmon circuits, we compare two different charging protocols for three-level quantum batteries based on time dependent classical pulses. In the first case the complete charging is achieved through the application of two sequential pulses, while in the second the charging occurs in a unique step applying the two pulses simultaneously. Both protocols are analytically solvable leading to a complete control on the dynamics of the quantum system. According to this it is possible to determine that the latter approach is characterized by a shorter charging time, and consequently by a greater charging power. We have then tested these protocols on IBM quantum devices based on superconducting circuits in the transmon regime. The minimum achieved charging time represents the fastest stable charging reported so far in solid state quantum batteries.

Abstract: Quantum Monte Carlo (QMC) algorithms have proven extremely effective at lowering the computational overhead of electronic structure calculations in a classical setting. In the current noisy intermediate scale quantum (NISQ) era of quantum computation, there are several limitations on the available hardware resources, such as low qubit count, decoherence times and gate noise, which preclude the application of many current hybrid quantum-classical algorithms to non-trivial quantum chemistry problems. Here, we propose combining some of the fundamental elements of conventional QMC algorithms -- stochastic sampling of both the wavefunction and the Hamiltonian of interest -- with an imaginary-time propagation based projective quantum eigensolver. At the cost of increased noise, which can be easily averaged over in a classical Monte Carlo estimation, we obtain a method with quantum computational requirements that are both generally low and highly tunable.

Abstract: The recently developed Projective Quantum Eigensolver (PQE) offers an elegant procedure to evaluate the ground state energies of molecular systems on quantum computers. However, the noise in available quantum hardware can result in significant errors in computed outcomes, limiting the realization of quantum advantage. Although PQE comes equipped with some degree of inherent noise resilience, any practical implementation with apposite accuracy would require additional routines to suppress the errors further. In this work, we propose a way to enhance the efficiency of PQE by developing an optimal framework for introducing Zero Noise Extrapolation (ZNE) in the nonlinear iterative procedure that outlines the PQE; leading to the formulation of ZNE-PQE. For this method, we perform a detailed analysis of how various components involved in it affect the accuracy and efficiency of the reciprocated energy convergence trajectory. Moreover, we investigate the reasons behind the improvements observed in ZNE-PQE over conventional PQE by performing a comparative analysis of their residue norm landscape. This approach is expected to facilitate practical applications of quantum computing in fields related to molecular sciences, where it is essential to determine molecular energies accurately.

Abstract: We propose a method of quantum machine learning called quantum mutual information neural estimation (QMINE) for estimating von Neumann entropy and quantum mutual information, which are fundamental properties in quantum information theory. The QMINE proposed here basically utilizes a technique of quantum neural networks (QNNs), to minimize a loss function that determines the von Neumann entropy, and thus quantum mutual information, which is believed more powerful to process quantum datasets than conventional neural networks due to quantum superposition and entanglement. To create a precise loss function, we propose a quantum Donsker-Varadhan representation (QDVR), which is a quantum analog of the classical Donsker-Varadhan representation. By exploiting a parameter shift rule on parameterized quantum circuits, we can efficiently implement and optimize the QNN and estimate the quantum entropies using the QMINE technique. Furthermore, numerical observations support our predictions of QDVR and demonstrate the good performance of QMINE.

Abstract: We investigate multipartite quantum correlation (MQC), spatially anisotropic coupling, and finite temperature effects in a triangular Ising system with tunable interactions using the exact diagonalization method. We demonstrate that spatially anisotropic coupling serves as an effective means to modulate MQC in the antiferromagnetic ground state, which is achievable with current experimental technologies. Moreover, we explore the interplay between MQC and spatially anisotropic coupling in the Ising system at finite temperatures. Our findings reveal a three-way trade-off relationship among high MQC, robust thermal stability, and anisotropic strength in the triangular Ising system with antiferromagnetic interactions, though the MQC in the ferromagnetic case is quite susceptible to temperature changes. These insights contribute to our understanding of ground state properties and MQC modulation in quantum many-body systems.

Abstract: The quantum walk is a quantum counterpart of the classical random walk. On the other hand, the absolute zeta function can be considered as a zeta function over F_1. This paper presents a connection between the quantum walk and the absolute zeta function. First we deal with a zeta function determined by a time evolution matrix of the Grover walk on a graph. The Grover walk is a typical model of the quantum walk. Then we prove that the zeta function given by the quantum walk is an absolute automorphic form of weight depending on the number of edges of the graph. Furthermore we consider an absolute zeta function for the zeta function based on a quantum walk. As an example, we compute an absolute zeta function for the cycle graph and show that it is expressed as the multiple gamma function of order 2.

Abstract: Here we prove that the classical (respectively, quantum) system, consisting of a particle moving in a static electromagnetic field, is canonically (respectively, unitarily) equivalent to a harmonic oscillator perturbed by a spatially homogeneous force field. This system is canonically and unitarily equivalent to a standard oscillator. Therefore, by composing the two transformations we can integrate the initial problem. Actually, the eigenstates of the initial problem turn out to be entangled states of the harmonic oscillator. When the magnetic field is spatially homogeneous but time-dependent, the equivalent harmonic oscillator has a time-varying frequency. This system can be exactly integrated only for some particular cases of the time dependence of the magnetic field. The unitary transformations between the quantum systems are a representation of the canonical transformations by unitary transformations of the corresponding Hilbert spaces.

Abstract: Quantum entanglement between particles is expected to allow one to perform tasks that would otherwise be impossible. In quantum sensing and metrology, entanglement is often claimed to enable a precision that cannot be attained with the same number of particles and time, forgoing entanglement. Two distinct approaches exist: creation of entangled states that either i) respond quicker to the signal, or ii) are associated with lower noise and uncertainty. Here we show that if our definition of success is -- a precision that is impossible to achieve without entanglement -- then the second approach cannot succeed.

Abstract: The emergence of an objective classical picture is the core question of quantum Darwinism. How does this reconstructed classical picture depends on the resources available to observers? In this Letter, we develop an experimentally relevant model of a qubit coupled dispersively to a transmission line and use time-frequency signal processing techniques to understand if and how the emergent classical picture is changed when we have the freedom to choose the fragment decomposition and the type of radiation sent to probe the system. We show the crucial role of correlations in the reconstruction procedure and point to the importance of studying the type of measurements that must be done to access an objective classical data.

Abstract: Strongly-interacting Rydberg atomic ensembles have shown intense collective excitation effects due to the inclusion of single Rydberg excitation shared by multiple atoms in the ensemble. In this paper we investigate a counter-intuitive Rydberg excitation facilitation with a strongly-interacting atomic ensemble in the strong probe-field regime, which is enabled by the role of a control atom nearby. Differing from the case of a single ensemble, we show that, the control atom's excitation adds to a second two-photon transition onto the doubly-excited Rydberg state, arising an excitation facilitation for the ensemble atoms. Our numerical studies depending on the method of quantum Monte Carlo wavefunction, exhibit the observation constraints of this excitation facilitation effect under practical experimental conditions. The results obtained can provide a flexible control for the excitation of Rydberg atomic ensembles and participate further uses in developing mesoscopic Rydberg gates for multiqubit quantum computation.

Abstract: The scalar spin chirality is a three-body physical observable that plays an outstanding role both in classical magnetism, characterizing non-coplanar spin textures, and in quantum magnetism, as an order parameter for chiral spin liquids. In quantum information, the scalar spin chirality is a witness of genuine tripartite entanglement. Here we propose an indirect measurement scheme, based on the Hadamard test, to estimate the scalar spin chirality for general quantum states. We apply our method to study chirality in two types of quantum states: generic one-magnon states of a ferromagnet, and the ground state of a model with competing symmetric and antisymmetric exchange. We show a single-shot determination of the scalar chirality is possible for chirality eigenstates, via quantum phase estimation with a single auxiliary qutrit. Our approach provides a unified theory of chirality in classical and quantum magnetism.

Abstract: Lithium niobate is a promising material for developing quantum acoustic technologies due to its strong piezoelectric effect and availability in the form of crystalline thin films of high quality. However, at radio frequencies and cryogenic temperatures, these resonators are limited by the presence of decoherence and dephasing due to two-level systems. To mitigate these losses and increase device performance, a more detailed picture of the microscopic nature of these loss channels is needed. In this study, we fabricate several lithium niobate acoustic wave resonators and apply different processing steps that modify their surfaces. These treatments include argon ion sputtering, annealing, and acid cleans. We characterize the effects of these treatments using three surface-sensitive measurements: cryogenic microwave spectroscopy measuring density and coupling of TLS to mechanics, x-ray photoelectron spectroscopy and atomic force microscopy. We learn from these studies that, surprisingly, increases of TLS density may accompany apparent improvements in the surface quality as probed by the latter two approaches. Our work outlines the importance that surfaces and fabrication techniques play in altering acoustic resonator coherence, and suggests gaps in our understanding as well as approaches to address them.

Abstract: The famous CHSH game can be interpreted with Boolean functions while understanding the success probability in the classical scenario. In this paper, we have exhaustively studied all the Boolean functions on four variables to express binary input binary output two-party nonlocal games and explore their performance in both classical and quantum scenarios. Our analysis finds out some other games (other than the CHSH game) which offer a higher success probability in the quantum scenario as compared to the classical one. Naturally, our study also notes that the CHSH game (and the games corresponding to the similar partition) is the most efficient in terms of separation between quantum and classical techniques.

Abstract: We train a generative language model on the randomized local measurement data collected from Schr\"odinger's cat quantum state. We demonstrate that the classical reality emerges in the language model due to the information bottleneck: although our training data contains the full quantum information about Schr\"odinger's cat, a weak language model can only learn to capture the classical reality of the cat from the data. We identify the quantum-classical boundary in terms of both the size of the quantum system and the information processing power of the classical intelligent agent, which indicates that a stronger agent can realize more quantum nature in the environmental noise surrounding the quantum system. Our approach opens up a new avenue for using the big data generated on noisy intermediate-scale quantum (NISQ) devices to train generative models for representation learning of quantum operators, which might be a step toward our ultimate goal of creating an artificial intelligence quantum physicist.

Abstract: Near-term quantum simulators are mostly based on qubit-based architectures. However, their imperfect nature significantly limits their practical application. The situation is even worse for simulating fermionic systems, which underlie most of material science and chemistry, as one has to adopt fermion-to-qubit encodings which create significant additional resource overhead and trainability issues. Thanks to recent advances in trapping and manipulation of neutral atoms in optical tweezers, digital fermionic quantum simulators are becoming viable. A key question is whether these emerging fermionic simulators can outperform qubit-based simulators for characterizing strongly correlated electronic systems. Here, we perform a comprehensive comparison of resource efficiency between qubit and fermionic simulators for variational ground-state emulation of fermionic systems in both condensed matter systems and quantum chemistry problems. We show that the fermionic simulators indeed outperform their qubit counterparts with respect to resources for quantum evolution (circuit depth), as well as classical optimization (number of required parameters and iterations). In addition, they show less sensitivity to the random initialization of the circuit. The relative advantage of fermionic simulators becomes even more pronounced as interaction becomes stronger, or tunneling is allowed in more than one dimension, as well as for spinful fermions. Importantly, this improvement is scalable, i.e., the performance gap between fermionic and qubit simulators only grows for bigger system sizes.

Abstract: We investigate the energetic advantage of accelerating a quantum harmonic oscillator Otto engine by use of shortcuts to adiabaticity (for the power and compression strokes) and to equilibrium (for the hot isochore), by means of counter-diabatic (CD) driving. By comparing various protocols with and without CD driving, we find that, applying both type of shortcuts leads to enhanced power and efficiency even after the driving costs are taken into account. The hybrid protocol not only retains its advantage in the limit cycle, but also recovers engine functionality (i.e., a positive power output) in parameter regimes where an uncontrolled, finite-time Otto cycle fails. We show that controlling three strokes of the cycle leads to an overall improvement of the performance metrics compared with controlling only the two adiabatic strokes. Moreover, we numerically calculate the limit cycle behavior of the engine and show that the engines with accelerated isochoric and adiabatic strokes display a superior power output in this mode of operation.

Abstract: Breaking of the discrete time-translation symmetry leads to the emergence of the discrete time-crystalline (DTC) phase in quantum many-body systems. In this phase, system observables exhibit a robust sub-harmonic response. DTC has been experimentally realized in the driven dipolar systems, which are usually analyzed using a Floquet formalism applicable to closed systems. Here, we extend the analysis to a realistic open quantum system, a dipolar coupled two-spin dissipative system subjected to a periodic drive. To this end, we use a fluctuation-regulated quantum master equation (FRQME) for our analysis. The dissipators of this master equation are regularized by thermal fluctuations and play a central role in stabilizing the DTC phase against perturbations. Our results are in excellent agreement with the published experiments. Moreover, we show the temperature dependence of the DTC phase in open quantum systems.

Abstract: For the efficient simulation of open quantum systems we often use quantum jump trajectories given by pure states that evolve stochastically to unravel the dynamics of the underlying master equation. In the Markovian regime, when the dynamics is described by a Lindblad master equation, this procedure is known as Monte Carlo wavefunction (MCWF) approach. However, beyond ultraweak system-bath coupling, the dynamics of the system is not described by an equation of Lindblad type, but rather by the Redfield equation, which can be brought into pseudo-Lindblad form. Here negative dissipation strengths prohibit the conventional approach. To overcome this problem, we propose a pseudo-Lindblad quantum trajectory (PLQT) unravelling. It does not require an effective extension of the state space, like other approaches, except for the addition of a single classical bit. We test the PLQT for the eternal non-Markovian master equation for a single qubit and an interacting Fermi Hubbard chain coupled to a thermal bath and discuss its computational effort compared to solving the full master equation.

Abstract: We report an accurate, memory and time efficient classical simulation of a 127-qubit kicked Ising quantum system on the heavy-hexagon lattice. A simulation of this system on a quantum processor was recently performed using noise mitigation techniques to enhance accuracy (Nature volume 618, p. 500-505 (2023)). Here we show that, by adopting a tensor network approach that reflects the qubit connectivity of the device, we can perform a classical simulation that is significantly more accurate than the results obtained from the quantum device in the verifiable regime and comparable to the quantum simulation results for larger depths. The tensor network approach used will likely have broader applications for simulating the dynamics of quantum systems with tree-like correlations.

Abstract: One of the central tasks in many-body physics is the determination of phase diagrams, which can be cast as a classification problem. Typically, classification problems are tackled using discriminative classifiers that explicitly model the conditional probability of labels given a sample. Here we show that phase-classification problems are naturally suitable to be solved using generative classifiers that are based on probabilistic models of the measurement statistics underlying the physical system. Such a generative approach benefits from modeling concepts native to the realm of statistical and quantum physics, as well as recent advances in machine learning. This yields a powerful framework for mapping out phase diagrams of classical and quantum systems in an automated fashion capable of leveraging prior system knowledge.