Quantum Physics (quant-ph)

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.

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.

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.

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 $O\left(\mathrm{poly}(n,1/\epsilon)\right)$.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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).

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.

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.

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.