The 100 most-cited articles in Physical Review A, 2024

The authors prove the monotonicity of stabilizer entropies under general stabilizer operations. This result establishes stabilizer entropies as the only known family of monotones that are experimentally measurable and computationally tractable.

The authors establish a connection between nonstabilizerness and a readily measurable property -- the entanglement spectrum. This connection not only provides a deeper understanding of quantum complexity but also offers a practical way to probe nonstabilizerness even in noisy environments.

We explore the role of exceptional points and complex eigenvalues on the occurrence of the quantum Mpemba effect. To this end, we study a two-level driven dissipative system subjected to an oscillatory electric field and dissipative coupling with the environment. We find that both exceptional points and complex eigenvalues can lead to multiple quantum Mpemba effect. It occurs in an observable when time-evolved copies corresponding to two different initial conditions, one initially having higher observable value compared to the other and both relaxing towards the same steady state, intersect each other more than once during their relaxation process. Each of the intersections denotes a quantum Mpemba effect and marks the reversal of identities between the two copies, i.e., the copy with higher observable value before the intersection becomes the lower-valued copy (and vice versa) after the intersection. Such multiple intersections originate from additional algebraic time dependence at the exceptional points and due to oscillatory relaxation in the case of complex eigenvalues. We…

The photon blockade based on destructive quantum interference is called the unconventional photon blockade (UPB), which has been intensively studied in Markovian systems but barely explored in the non-Markovian ones. In this paper, we construct a coupled-cavities system to achieve UPB with the non-Markovian effect, where the dissipationless left cavity and Markovian dissipative right cavity are respectively mediated by two-photon pump and single-photon driving field. Through the equivalence between the Markovian master equation and Heisenberg-Langevin equation for the environment initialization in the vacuum state, we can derive the exact non-Markovian Heisenberg-Langevin equation and reduced master equation for the left cavity, which contains the two-photon pump and effective single-photon driving field. In the non-Markovian regime (the dissipation falling below a threshold), the effective single-photon driving field holds nonzero, which can lead to UPB occurring due to a closed quantum interference path forming. When the dissipation exceeds the threshold, the system enters the…

Cavity magnomechanics has shown great potential in studying macroscopic quantum effects, especially for quantum entanglement, which is a key resource for quantum information science. Here we propose to realize magnon mediated nonreciprocal photon-phonon entanglement, which exhibits asymmetry when opposite magnetic or driving fields are respectively applied to the magnons with the Kerr effect or the photons with the Sagnac effect. We find that the mean magnon number can selectively exhibit nonreciprocal linear or nonlinear (bistable) behavior with the strength of the strong driving field on the cavity. Assisted by this driving field, the magnon-phonon coupling is greatly enhanced, leading to the nonreciprocal photon-phonon entanglement via the swapping interaction between the magnons and photons. This nonreciprocal entanglement can be significantly enhanced with the magnon Kerr and Sagnac effects. Given the available parameters, the nonreciprocal photon-phonon entanglement can be preserved at $\ensuremath{\approx}3$ K, showing remarkable resilience against the bath temperature. The…

We study the non-Markovian dynamics of a two-level giant atom interacting with a one-dimensional semi-infinite waveguide through multiple coupling points, where a perfect mirror is located at the endpoint of the waveguide. The system enters a non-Markovian process when the travel time of the photon between adjacent coupling points is sufficiently large compared to the inverse of the bare relaxation rate of the giant atom. The photon released by the spontaneous emission of the atom transfers between multiple coupling points through the waveguide or is reabsorbed by the atom with the photon emitted via the atom having completed the round trip after reflection of the mirror, which leads to the photon being trapped and forming bound states. We find that three different types of bound states can be formed in the system, containing the static bound states with no inversion of population, the periodic equal amplitude oscillation with two bound states, and the periodic nonequal amplitude oscillation with three bound states. The physical origins of the three bound states' formation are…

Among the various types of quantum entanglement, the most fascinating and interesting one is genuine entanglement undoubtedly. In this paper, the unique genuine decomposition theorem with respect to any pure state is given and the relations between battery capacity gap and ergtropic gap are studied. By exploiting the battery capacity of multipartite systems, we introduce a vector-valued genuine measure compared with other existing measures and find that the one is superior to some other existing measures in confirming whether the deterministic LOCC transformation between two multipartite pure states is prohibited. Through some proper constructions, one can derive many GME measures. We also give the explicit expression of bipartite battery capacity gap with respect to mixed states by the convex roof extension and investigate some monogamy relations related to the battery capacity gap.

Quantum enhanced optimization of classical cost functions is a central theme of quantum computing due to its high potential value in science and technology. The variational quantum eigensolver (VQE) and the quantum approximate optimization algorithm (QAOA) are popular variational approaches that are considered the most viable solutions in the noisy-intermediate scale quantum (NISQ) era. Here, we study the scaling of the quantum resources, defined as the required number of circuit repetitions, to reach a fixed success probability as the problem size increases, focusing on the role played by measurement shot noise, which is unavoidable in realistic implementations. Simple and reproducible problem instances are addressed, namely, the ferromagnetic and disordered Ising chains. Our results show that: (1) VQE with the standard heuristic Ansatz scales comparably to direct brute-force search when energy-based optimizers are employed. The performance improves at most quadratically using a gradient-based optimizer. (2) When the parameters are optimized from random guesses, also the scaling of…

Device-independent (DI) quantum secret sharing (QSS) can relax the security assumptions about the devices' internal workings and provide QSS the highest level of security in theory. The original DI QSS protocol proved its correctness and completeness under a causal independence assumption regarding measurement devices. However, there has been a lack of DI QSS's performance characterization in practical communication situations, which impedes its experimental demonstration and application in the future. Here we propose a three-partite DI QSS protocol with noise preprocessing and postselection strategies and develop the numerical methods to implement its performance characterization in practical communication situations. The adoption of the noise preprocessing and postselection can reduce DI QSS's global detection efficiency threshold from 96.32% to 94.30% and increase the noise threshold from 7.148% to 8.072%. Our DI QSS protocol has two advantages. First, it is a DI QSS protocol with performance characterization in practical communication situations. Second, the adoption of noise…

The authors investigate the use of time-crystal phases for metrology, providing a concrete proposal as quantum sensors of AC fields. Their performance shows several useful advantages due to their long-range spatiotemporal ordering, overcoming the shot noise limit along with long interrogation times and robustness.

The Tavis-Cummings (TC) model, which serves as a natural physical realization of a quantum battery, comprises ${N}_{b}$ atoms as battery cells that collectively interact with a shared photon field, functioning as the charger, initially containing ${n}_{0}$ photons. In this paper, we introduce the invariant subspace method to effectively represent the quantum dynamics of the TC battery. Our findings indicate that in the limiting case of ${n}_{0}\ensuremath{\gg}{N}_{b}$ or ${N}_{b}\ensuremath{\gg}{n}_{0}$, a distinct SU(2) symmetry emerges in the dynamics, thereby ensuring the realization of optimal energy storage. We also establish a negative relationship between the battery-charger entanglement and the energy storage capacity. As a result, we demonstrate that asymptotically optimal energy storage can be achieved in the scenario where ${N}_{b}={n}_{0}\ensuremath{\gg}1$. Our approach not only enhances our comprehension of the algebraic structure inherent in the TC model but also contributes to the broader theoretical framework of quantum batteries. Furthermore, it provides crucial…

A quantum system which can store energy, and from which one can extract useful work, is known as a quantum battery. Such a device raises interesting issues surrounding how quantum physics can provide certain advantages in the charging, energy storage, or discharging of the quantum battery as compared to their classical equivalents. However, the pernicious effect of dissipation degrades the performance of any realistic battery. Here, we show how one can circumvent this problem of energy loss by proposing a quantum battery model which benefits from quantum squeezing. Namely, charging the battery quadratically with a short temporal pulse induces a hyperbolic enhancement in the stored energy, such that the dissipation present becomes essentially negligible in comparison. Furthermore, we show that when the driving is strong enough, the useful work which can be extracted from the quantum battery, that is the ergotropy, is exactly equal to the stored energy. These impressive properties imply a highly efficient quantum energetic device with abundant amounts of ergotropy. Our theoretical…

In this paper, the authors use a fully quantum-mechanical description to study the effect of electron-electron correlations on the quantum state of light emitted via high-harmonic generation. The work represents a crucial step toward a fully quantized description of the interaction between intense laser fields and complex systems, with potential applications in quantum light engineering.

Quantum batteries (QBs) are energy storage and transfer microdevices that open up new possibilities in energy technology. Here, we derive a resonator--multiple-qutrit quantum battery (QB) model consisting of a multimode resonator and $N$ superconducting transmon qutrits. We investigate the charging and self-discharging performances of the QBs and discuss the roles of quantum coherence and quantum entanglement. The results show that environment noise is not always detrimental for QB systems. The QB with efficient charging, stable energy-storage, and slow self-discharging processes can be realized by considering the dephasing noise and manipulating the energy gap. We find that the charging energy is positively related to coherence and entanglement while the stable energy and the self-discharging energy are negatively related to coherence. The phenomenon of the vanishing entanglement corresponds to the dynamic decoupling behavior of the QB's steady states. Our results provide a way to realize many-body QBs on a superconducting circuit platform.

A variational quantum algorithm for numerically solving partial differential equations (PDEs) on a quantum computer was proposed by Lubasch et al. [Phys. Rev. A 101, 010301(R) (2020)]. In this paper we generalize the method introduced by Lubasch et al. to cover a broader class of nonlinear PDEs as well as multidimensional PDEs and study the performance of the variational quantum algorithm on several example equations. Specifically, we show via numerical simulations that the algorithm can solve instances of the single-asset Black-Scholes equation with a nontrivial nonlinear volatility model, the double-asset Black-Scholes equation, the Buckmaster equation, and the deterministic Kardar-Parisi-Zhang equation. Our simulations use up to $n=12$ Ansatz qubits, computing PDE solutions with ${2}^{n}$ grid points. We also perform proof-of-concept demonstrations with a trapped-ion quantum processor from IonQ [Nat. Commun. 10, 5464 (2019)], showing accurate computation of two representative expectation values needed for the calculation of a single time step of the nonlinear Black-Scholes…

Optical cat states, the nonclassical superposition of two quasi-classical coherent states, serve as a basis for gedankenexperiments testing quantum physics on mesoscopic scales and are increasingly recognized as a resource for quantum information processing. Here, we report the first experimental realization of optical cat states by adding a photon to a squeezed vacuum state; so far only photon-subtraction protocols have been realized. Photon addition gives us the advantage of using heralded signal photons as experimental triggers, and we can generate cat states at rates exceeding $2.3\ifmmode\times\else\texttimes\fi{}{10}^{5}$ counts per second. Our most highly squeezed vacuum input state shows \ensuremath{-}8.9 dB squeezing, but such squeezing entails some degradation---in this case, 15.1 dB antisqueezing. Even so, our approach enables us to synthesize a state with a maximum cat amplitude of $|\ensuremath{\alpha}|\ensuremath{\approx}1.77$ whose Wigner distribution still shows pronounced negative parts. Our experimental implementation with controlled photon addition demonstrates a…

We propose to achieve and enhance the magnon blockade effect in an optomechanical-magnetic system based on a quantum destructive interference mechanism between three indirect transition pathways. By introducing a degenerate parametric amplifier, we analytically obtain the optimal parametric gain and phase for achieving the magnon blockade. Under the optimized parameter conditions, the driving detunings of cavity and magnon modes can be flexibly controlled to achieve the smallest second-order magnon correlation function. Moreover, the magnon blockade can exhibit fascinating features by proper driving detuning and weaker driving strength. Our scheme combines the benefits of destructive interference-induced magnon blockade and energy-level anharmonicity-induced magnon blockade, which results in a reduction of equal-time second-order magnon correlation while simultaneously avoiding time-delay second-order magnon correlation with rapid oscillation. Our work provides an alternative and experimentally feasible platform for manipulating few-magnon states and generating single-magnon sources.

Probabilistic error cancellation (PEC) is a technique that generates error-mitigated estimates of expectation values from ensembles of quantum circuits. In this work we extend the application of PEC from unitary-only circuits to dynamic circuits with midcircuit measurements and classically controlled (feedforward) Clifford operations. Our approach extends the sparse Pauli-Lindblad noise model from characterizing gates to characterizing target channels containing midcircuit measurements and feedforward while accounting for nonlocal measurement crosstalk in superconducting processors. Our mitigation and monitoring experiments provide a holistic view for the performance of the protocols developed in this work. These capabilities will be a crucial tool in the exploration of near-term dynamic circuit applications.

Ultracold diatomic molecules find application in quantum studies ranging from controlled chemistry and precision measurement physics to quantum many-body simulation and potentially quantum computing. Accurate knowledge of molecular properties is required to guide and explain ongoing experiments. Here, in an extensive and comparative study, we theoretically investigate the electronic properties of the ground-state diatomic molecules composed of alkali-metal (Li, Na, K, Rb, Cs, Fr) and alkaline-earth-metal (Be, Mg, Ca, Sr, Ba, Ra) atoms. We study 78 heteronuclear and homonuclear diatomic combinations, including 21 alkali-metal molecules in the ${X}^{1}{\mathrm{\ensuremath{\Sigma}}}^{+}$ and ${a}^{3}{\mathrm{\ensuremath{\Sigma}}}^{+}$ electronic states, 36 alkali-metal--alkaline-earth-metal molecules in the ${X}^{2}{\mathrm{\ensuremath{\Sigma}}}^{+}$ electronic state, and 21 alkaline-earth-metal molecules in the ${X}^{1}{\mathrm{\ensuremath{\Sigma}}}^{+}$ electronic state. We calculate potential energy curves, permanent electric dipole moments, and polarizabilities using the hierarchy…

Quantum batteries are energy storage devices that satisfy quantum-mechanical principles. How to obtain analytical solutions for quantum battery systems and achieve a full charging is a crucial element of the quantum battery. Here, we investigate the Rosen-Zener quantum battery with $N$ two-level systems, which includes atomic interactions and external driving field. The analytical solutions of the stored energy, changing power, energy quantum fluctuations, and von Neumann entropy (diagonal entropy) are derived by employing the gauge transformation. We demonstrate that a full charging process can be achieved when the external driving field strength and scanning period conforms to a quantitative relationship. The local maximum value of the final stored energy corresponds to the local minimum values of the final energy fluctuations and diagonal entropy. Moreover, we find that the atomic interaction induces the quantum phase transition and the maximum stored energy of the quantum battery reaches the maximum value near the quantum phase transition point. Our result provides an insightful…

Scrambling of quantum information is an important feature at the root of randomization and benchmarking protocols, the onset of quantum chaos, and black-hole physics. Unscrambling this information is possible given perfect knowledge of the scrambler [arXiv:1710.03363]. We show that one can retrieve the scrambled information even without any previous knowledge of the scrambler, by a learning algorithm that allows the building of an efficient decoder. Remarkably, the decoder is classical in the sense that it can be efficiently represented on a classical computer as a Clifford operator. It is striking that a classical decoder can retrieve with fidelity one all the information scrambled by a random unitary that cannot be efficiently simulated on a classical computer, as long as there is no full-fledged quantum chaos. This result shows that one can learn the salient properties of quantum unitaries in a classical form and sheds a new light on the meaning of quantum chaos. Furthermore, we obtain results concerning the algebraic structure of $t$-doped Clifford circuits, i.e., Clifford…

NP-hard problems are not believed to be exactly solvable through general polynomial time algorithms. Hybrid quantum-classical algorithms to address such combinatorial problems have been of great interest in the past few years. Such algorithms are heuristic in nature and aim to obtain an approximate solution. Significant improvements in computational time and/or the ability to treat large problems are some of the principal promises of quantum computing in this regard. The hardware, however, is still in its infancy and the current noisy intermediate-scale quantum (NISQ) computers are not able to optimize industrially relevant problems. Moreover, the storage of qubits and introduction of entanglement require extreme physical conditions. An issue with quantum optimization algorithms such as the quantum approximate optimization algorithm is that they scale linearly with problem size. In this paper, we build upon a proprietary methodology which scales logarithmically with problem size---opening an avenue for treating optimization problems of unprecedented scale on gate-based quantum…

We consider quantum two-block group algebra (2BGA) codes, a family of smallest lifted-product (LP) codes. These codes are related to generalized-bicycle codes, except a cyclic group is replaced with an arbitrary finite group, generally non-Abelian. As special cases, 2BGA codes include a subset of square-matrix LP codes over Abelian groups, including quasicyclic codes, and all square-matrix hypergraph-product codes constructed from a pair of classical group codes. We establish criteria for permutation equivalence of 2BGA codes and give bounds for their parameters, both explicit and in relation to other quantum and classical codes. We also enumerate the optimal parameters of all inequivalent binary connected 2BGA codes with stabilizer generator weights $W\ensuremath{\le}8$, of length $n\ensuremath{\le}100$ for Abelian groups, and $n\ensuremath{\le}200$ for non-Abelian groups.

We propose a design of a quantum battery exploiting the non-Hermitian Hamiltonian as a charger. In particular, starting with the ground or the thermal state of the interacting (noninteracting) Hamiltonian as the battery, the charging of the battery is performed via parity-time ($\mathcal{PT}$)- and rotational-time ($\mathcal{RT}$)-symmetric Hamiltonians to store energy. We report that such a quenching with a non-Hermitian Hamiltonian leads to an enhanced power output compared to a battery with a Hermitian charger. We identify the region in the parameter space which provides the gain in performance. We also demonstrate that the improvements persist with the increase of system size for batteries with both $\mathcal{PT}$- and $\mathcal{RT}$-symmetric chargers. In the $\mathcal{PT}$-symmetric case, although the anisotropy of the $\mathrm{XY}$ model does not help in the performance, we show that the $\mathrm{XXZ}$ model as a battery with a non-Hermitian charger performs better than that of the $\mathrm{XX}$ model having certain interaction strengths. We also exhibit that the advantage of…

Making and using polaritonic states (i.e., hybrid electron-photon states) for chemical applications has recently become one of the most prominent and active fields that connects the communities of chemistry and quantum optics. Modeling of such polaritonic phenomena using ab initio approaches calls for new methodologies, leading to the reinvention of many commonly used electronic structure methods, such as Hartree-Fock, density functional, and coupled cluster theories. In this work, we explore the formally exact diffusion quantum Monte Carlo approach to obtain numerical solutions to the polaritonic ground state during the dissociation of the ${\mathrm{H}}_{2}$ molecular system. We examine various electron-nuclear-photon properties throughout the dissociation, such as changes to the minimum of the cavity Born-Oppenheimer surface, the localization of the electronic wave function, and the average mode occupation. Finally, we directly compare our results to that obtained with state-of-the-art, yet approximate, polaritonic coupled cluster approaches.

The authors explain the spectral shape of electromagnetically induced transparency via Rydberg states in the presence of a strong magnetic field. They also demonstrate experimentally that Autler-Townes splitting in a strong magnetic field can be performed with a single participating transition dipole moment, providing a method for electrometry with robust calibration.

Non-Markovianity, the intricate dependence of an open quantum system on its temporal evolution history, holds tremendous implications across various scientific disciplines. However, accurately characterizing the complex non-Markovian effects has posed a formidable challenge for numerical simulations. While quantum computing technologies show promise, a universal theory enabling practical quantum algorithm implementation has been elusive. We address this gap by introducing the dissipaton-embedded quantum master equation in second quantization (DQME-SQ). This exact and compact theory offers two key advantages: representability by quantum circuits and universal applicability to any Gaussian environment. We demonstrate these capabilities through digital quantum simulations of non-Markovian dissipative dynamics in both bosonic and fermionic environments. The DQME-SQ framework opens a new horizon for the efficient exploration of complex open quantum systems by leveraging the rapidly advancing quantum computing technologies.

The preparation of an equilibrium thermal state of a quantum many-body system on noisy intermediate-scale quantum (NISQ) devices is an important task in order to extend the range of applications of quantum computation. Faithful Gibbs state preparation would pave the way to investigate protocols such as thermalization and out-of-equilibrium thermodynamics and provide useful resources for quantum algorithms, where sampling from Gibbs states constitutes a key subroutine. We propose a variational quantum algorithm (VQA) to prepare Gibbs states of a quantum many-body system. The novelty of our VQA consists of implementing a parameterized quantum circuit acting on two distinct, yet connected (via cnot gates), quantum registers. The VQA evaluates the Helmholtz free energy, where the von Neumann entropy is obtained via postprocessing of computational-basis measurements on one register, while the Gibbs state is prepared on the other register via a unitary rotation in the energy basis. Finally, we benchmark our VQA by preparing Gibbs states of the transverse-field Ising and Heisenberg XXZ…

In the photonic spin Hall effect (SHE), also known as the transverse shift, incident light photons with opposite spins are spatially separated in the transverse direction due to the spin-orbit interaction of light. Here, we propose a gain-assisted model to control the SHE in the reflected probe light. In this model, a probe light is incident on a cavity containing a three-level dilute gaseous atomic medium, where the interaction between the atom and the control field follows two-photon Raman transitions. We show that the direction of photonic spin accumulations can be switched between positive and negative values across the Brewster angle in both the anomalous and normal dispersion regimes. For the same magnitude of control fields, the peak value of the photonic SHE is higher in the anomalous dispersion region compared to the normal dispersion regime. Additionally, the angular range around the Brewster angle is wider in the normal dispersion regime than in the anomalous dispersion region. Furthermore, the peak value of the photonic SHE and the angular range can be controlled by…

The generation of a logical qubit called the Gottesman-Kitaev-Preskill (GKP) qubit in an optical traveling wave is a major challenge for realizing large-scale universal fault-tolerant optical quantum computers. Recently, probabilistic generation of elementary GKP qubits has been demonstrated using photon number measurements and homodyne measurements. However, the generation rate is only a few Hz, and it will be difficult to generate fault-tolerant GKP qubits at a practical rate unless success probability is significantly improved. Here, we propose a method to efficiently synthesize GKP qubits from several quantum states by adaptive Gaussian operations. In the initial-state preparation that utilizes photon number measurements, an adaptive operation allows any measurement outcome above a certain threshold to be considered as a success. This threshold is lowered by utilizing the generalized photon subtraction method. The initial states are synthesized into a GKP qubit by homodyne measurements and a subsequent adaptive operation. As a result, the single-shot success probability of…

Quantum-electrodynamical density-functional theory (QEDFT) provides a promising avenue for exploring complex light-matter interactions in optical cavities for real materials. Similar to conventional density-functional theory, the Kohn-Sham formulation of QEDFT needs approximations for the generally unknown exchange-correlation functional. In addition to the usual electron-electron exchange-correlation potential, an approximation for the electron-photon exchange-correlation potential is needed. A recent electron-photon exchange functional [C. Schäfer , ], derived from the equation of motion of the nonrelativistic Pauli-Fierz Hamiltonian, shows robust performance in one-dimensional systems across weak- and strong-coupling regimes. Yet, its performance in reproducing electron densities in higher dimensions remains unexplored. Here we consider this QEDFT functional approximation from one- to three-dimensional finite systems and across weak to strong light-matter couplings. The electron-photon exchange approximation provides excellent results in the ultrastrong-coupling regime. However,…

Quantum batteries have significant potential applications for future industry and daily life. The capacity is an important indicator for a battery. Methods to improve the capacity of quantum batteries are important. We consider quantum batteries given by bipartite quantum systems and study the enhancement of the battery capacity under local projective measurements on a subsystem of the quantum state. By using two-qubit Bell-diagonal states and $X$-type states as examples, we show that quantum battery capacity with respect to the whole system or a subsystem can be improved by local projective measurements. Our theoretical analysis will provide ideas for the experimental development of quantum batteries.

Magnon blockade is an effective physical mechanism to generate a single magnon, which is of great importance in quantum information processing. We present a scheme to realize the unconventional magnon blockade effect by using Kerr nonlinearity in a cavity electromagnonic system. Under weak driving conditions, we give the optimal parameter conditions for magnon antibunching through analytical calculations, which are in good agreement with numerical results. Furthermore, we find that Kerr nonlinearity is necessary to create magnon blockade. In particular, unconventional magnon blockade can be achieved in both weak nonlinearity and weak coupling regimes. The present scheme indicates a method for generating unconventional magnon blockade effect and provides a feasible and flexible platform for the realization of a single-magnon source with high purity.

Neutral atoms trapped in microscopic optical tweezers have emerged as a growing platform for quantum science. Achieving homogeneity over the tweezers array is an important technical requirement, and our research focuses on improving it for holographic arrays generated with a spatial light modulator. We present a series of optimization methods to calculate better holograms, fueled by precise measurement schemes. These innovations enable to achieve intensity homogeneity with a relative standard deviation of 0.3%, shape variations below 0.5%, and positioning errors within 70 nm. Such ultraprecise holographic optical tweezers arrays allow for the most demanding applications in quantum science with atomic arrays.

This study examines the performance of finite spin quantum batteries (QBs) using Heisenberg $XX, XY, XXZ$, and $XYZ$ spin models with Dzyaloshinsky-Moriya (DM) and Kaplan--Shekhtman--Entin-Wohlman--Aharony (KSEA) interactions. The QBs are modeled as interacting quantum spins in local inhomogeneous magnetic fields, inducing variable Zeeman splitting. We derive analytical expressions for the maximal extractable work, ergotropy and the capacity of QBs, as recently examined by Yang et al. [Phys. Rev. Lett. 131, 030402 (2023)]. These quantities are analytically linked through certain quantum correlations, as posited in the aforementioned study. Different Heisenberg spin-chain models exhibit distinct behaviors under varying conditions, emphasizing the importance of model selection for optimizing QB performance. In antiferromagnetic (AFM) systems, maximum ergotropy occurs with a Zeeman splitting field applied to either spin, while ferromagnetic (FM) systems benefit from a uniform Zeeman field. Temperature significantly impacts QB performance such that ergotropy in the AFM case is generally…

The authors present an experimental study of entangled two-photon absorption in solvated rhodamine 6G, providing strong evidence that the orders-of-magnitude increases in two-photon-absorption efficiency by using entangled light reported in previous studies cannot be explained by the community's current understanding of the process. Thus, the sought-after advantages of using two-photon absorption of time-frequency-entangled photon pairs as a practical tool for enhancing molecular spectroscopy and biological imaging remains elusive.

A quantum algorithm for solving the advection equation by embedding the discrete time-marching operator into Hamiltonian simulations is presented. One-dimensional advection can be simulated directly since the central finite-difference operator for first-order derivatives is anti-Hermitian. Here this is extended to industrially relevant multidimensional flows with realistic boundary conditions and arbitrary finite-difference stencils. A single copy of the initial quantum state is required and the circuit depth grows linearly with the required number of time steps, the sparsity of the time-marching operator, and the inverse of the allowable error. State-vector simulations of a scalar transported in a two-dimensional channel flow and lid-driven cavity configuration are presented as a proof of concept of the proposed approach. Published by the American Physical Society 2024

We present a scheme to generate distant bipartite and tripartite entanglement between an atomic ensemble and an yttrium iron garnet (YIG) sphere in coupled microwave cavities. We consider an atomic ensemble in a single-mode microwave cavity which is coupled with a second single-mode cavity having a YIG sphere. Our system, therefore, has five excitation modes, namely, cavity-1 photons, an atomic ensemble, cavity-2 photons, a magnon and a phonon mode in the YIG sphere. We show that significant bipartite entanglement exists between indirectly coupled subsystems in the cavities which is robust against temperature. Moreover, we present suitable parameters for a significant tripartite entanglement of the ensemble, magnon, and phonon modes. We also demonstrate the existence of tripartite entanglement between the magnon and phonon modes of the YIG sphere with indirectly coupled cavity photons. Interestingly, this distant tripartite entanglement is of the same order as that previously found for a single-cavity system. We show that cavity-cavity coupling strength affects both the degree and…

An approximate quantum state preparation method is introduced, called the Walsh series loader (WSL). The WSL approximates quantum states defined by real-value functions of single real variables with a depth independent of the number $n$ of qubits. Two approaches are presented. The first one approximates the target quantum state by a Walsh series truncated at $O(1/\sqrt{\ensuremath{\epsilon}})$, where $\ensuremath{\epsilon}$ is the precision of the approximation in terms of infidelity. The circuit depth is also $O(1/\sqrt{\ensuremath{\epsilon}})$, the size is $O(n+1/\sqrt{\ensuremath{\epsilon}})$, and only one ancilla qubit is needed. The second method accurately represents quantum states with sparse Walsh series. The WSL loads $s$-sparse Walsh series into $n$ qubits with a depth doubly sparse in $s$ and $k$, the maximum number of bits with value 1 in the binary decomposition of the Walsh function indices. The associated quantum circuit approximates the sparse Walsh series up to an error $\ensuremath{\epsilon}$ with a depth $O(sk)$, a size $O(n+sk)$, and one ancilla qubit. In both…

We propose a heralded protocol for implementing nontrivial quantum gates on two stationary qubits coupled to spatially separated cavities. By dynamically controlling the evolution of the composite system, nonlocal two-qubit quantum (e.g., cphase and cnot) gates can be achieved without real excitations of either cavity modes or atoms. The success of our protocol is conditioned on projecting an auxiliary atom onto a postselected state, which simultaneously removes various detrimental effects of dissipation on the gate fidelity. In principle, the success probability of the gate can approach unity as the single-atom cooperativity becomes sufficiently large. Furthermore, we show its application for implementing single- and two-qubit gates within a decoherence-free subspace that is immune to a collective dephasing noise. This faithful, heralded, and nonlocal protocol could, therefore, be useful for distributed quantum computation and scalable quantum networks.

It has been a long-standing goal to bring massive objects into a superposition of different locations in real space, not only to confirm quantum theory in new regimes, but also to explore the interface with gravity. The main challenge is usually thought to arise from forces or scattering due to environmental fields and particles that decohere the large object's wave function into a statistical mixture. We unveil a decoherence channel which cannot be eliminated by improved isolation from the environment. It originates from sound waves within the object, which are excited as part of any splitting process and carry partial information. This puts stringent constraints on future spatial superpositions of large objects. Published by the American Physical Society 2024

Typical measures of nonstabilizerness of a system of $N$ qubits require computing ${4}^{N}$ expectation values, one for each Pauli string in the Pauli group, over a state of dimension ${2}^{N}$. For permutationally invariant systems, this exponential overhead can be reduced to just $O({N}^{3})$ expectation values on a state with a dimension $O(N)$. We exploit this simplification to study the nonstabilizerness phase transitions of systems with hundreds of qubits.

In many states of the quantum electromagnetic field, including the vacuum state, entanglement exists between different space-time regions---even spacelike separated ones. These correlations can be harvested and, thereby, detected by quantum systems which locally interact with the field. Here, we propose an experimental implementation of such an entanglement-harvesting scheme which is based on electro-optic sampling (EOS). We demonstrate that state-of-the-art EOS experiments enable one to harvest entanglement from the vacuum field and to study quantum correlations within general THz fields. We further show how Bell nonlocality present in the vacuum field can be probed. Finally, we introduce an approach to mitigate shot noise in single-beam EOS configurations. These findings pave the way for experimental inquiries into foundational properties of relativistic quantum field theory, and empower EOS as a diagnostic tool in THz quantum optics.

In this paper, we employ two two-level atoms and a $\mathrm{\ensuremath{\Lambda}}$-type atom to connect two one-dimensional semi-infinite coupled resonator waveguides, respectively. The first configuration is a chiral setup, where incident photons undergo an elastic scattering process. We investigate the influence on the single-photon transfer rate of spatial coupling points between the small or giant atom and the waveguides. Our numerical simulations demonstrate that additional coupling points in the giant atom system will modify the transmission rule observed in the small atom system due to their unique interference effects. The second configuration is a nonreciprocal setup, where the conduction direction depends entirely on the initial state of the $\mathrm{\ensuremath{\Lambda}}$-type atom. The incident photon undergoes an inelastic scattering process accompanied by frequency conversion.

The authors consider a fermionic lattice under quasiperiodic modulation and demonstrate that the localization transition can be a useful quantum resource for designing quantum sensors capable of high-precision quantum measurements.

We show how to achieve strong squeezing of a microwave output field by reservoir engineering a cavity magnomechanical system, consisting of a microwave cavity, a magnon mode, and a mechanical vibration mode. The magnon mode is simultaneously driven by two microwave fields at the blue and red sidebands associated with the vibration mode. The two-tone drive induces a squeezed magnonic reservoir for the intracavity field, leading to a squeezed cavity mode due to the cavity-magnon state swapping, which further yields a squeezed cavity output field. The squeezing of the output field is stationary and substantial by using the currently available parameters in cavity magnomechanics. The work indicates the potential of the cavity magnomechanical system in preparing squeezed microwave fields, and may find promising applications in quantum information science and quantum metrology.

Orbital angular momentum (OAM), as a physical dimension of light, has been demonstrated to enhance the channel capacity and turbulence resistance of free-space optical (FSO) communication. However, the channel crosstalk in OAM-based FSO communication inevitably increases with transmission distance and turbulence intensity. Here, we propose a deep-learning-based recognition of a composite vortex beam to extend the regime of moderate-to-strong turbulence and long-distance FSO links. The composite vortex beam is generated by a coherent combination of two subbeams carrying different helical charges and phase delays, providing its helical charges and phase delay as new multiplexing dimensions and exhibiting better turbulence resistance compared to a single subbeam. We also developed a modified regular network to achieve the high-accuracy recognition of a composite vortex beam over a long distance at moderate-to-strong atmospheric turbulence. We believe that our approach has potential in deep-learning-based OAM high-capacity communication systems.

We study single-photon scattering spectra of a giant atom chirally coupled to a one-dimensional waveguide at multiple connection points, and examine chirality-induced effects in the scattering spectra. We show that the transmission spectra typically possess an anti-Lorentzian lineshape with a nonzero minimum, but by engineering the chirality of the multipoint coupling, the transmission spectrum of an incident photon can undergo a transition from complete transmission to total reflection at multiple frequency ``windows,'' where the width of the anti-Lorentzian lineshape for each of the window can be flexibly tuned at a fixed frequency detuning. Moreover, we show that a perfect nonreciprocal photon scattering can be achieved due to the interplay between internal atomic spontaneous emission and the chirally external decay to the waveguide, in contrast to that induced by the non-Markovian retardation effect. We also consider the non-Markovian retardation effect on the scattering spectra, which allows for a photonic band gap even with only two chiral coupling points. The…

In this article we present a benchmark for resource characterization in the process of controlled quantum state reconstruction and secret sharing for general three-qubit states. This is achieved by providing a closed expression for the reconstruction fidelity, which relies on the genuine tripartite correlation and the bipartite channel between the dealer and the reconstructor characterized by the respective correlation parameters. We formulate the idea of quantum advantage in approximate state reconstruction as surpassing the classical limit set at $\frac{2}{3}$. This article introduces new interoperability between teleportation and state reconstruction. This is detailed through a case-by-case analysis of relevant correlation matrices. We reformulate the idea of quantum secret sharing by setting up additional constraints on the teleportation capacity of the bipartite channels between dealer and shareholders by ensuring that, individually, the shareholders cannot reconstruct the secret. We believe that this will give us an ideal picture of how quantum secret sharing should be.

Matter-wave interferometry with nanoparticles will enable the development of quantum sensors capable of probing ultraweak fields with unprecedented applications for fundamental physics. The high sensitivity of such devices, however. makes them susceptible to several noise and decoherence sources and, as such, can operate only when sufficient isolation from the environment is achieved. It is thus imperative to model and characterize the interaction of nanoparticles with the environment and to estimate its deleterious effects. This paper will aim to study the decoherence of the matter-wave interferometer due to dipole-dipole interactions, which is one of the unavoidable channels for decoherence, even for a neutral micro-crystal. We will use the scattering model for decoherence, characterized by the differential cross section, in order to obtain simple expressions for the decoherence rate due to dipole-dipole interactions in the short- and long-wavelength limits that can be readily applied to estimate the available coherence time. We will conclude by applying the formulas we obtained…

Stabilizer entropy (SE) quantifies the spread of a state in the basis of Pauli operators. It is a computationally tractable measure of nonstabilizerness and thus a useful resource for quantum computation. SE can be moved around a quantum system, effectively purifying a subsystem from its complex features. We show that there is a phase transition in the residual subsystem SE as a function of the density of non-Clifford resources. This phase transition has important operational consequences: it marks the onset of a subsystem purity estimation protocol that requires $poly(n)exp(t)$ many queries to a circuit containing $t$ non-Clifford gates that prepares the state from a stabilizer state. Then, for $t=O({log}_{2}n)$, it estimates the purity with polynomial resources, and, for highly entangled states, attains an exponential speed-up over the known state-of-the-art algorithms.

Qudit, a high-dimensional quantum system, can provide a larger Hilbert space, and it has been shown that the larger Hilbert space has remarkable advantages over the smaller one in quantum information processing. However, it is a great challenge to realize the high-fidelity quantum gates with qudits. Here we theoretically propose and experimentally demonstrate the four-dimensional quantum gates (including the generalized Pauli ${X}_{4}$ gate, Pauli ${Z}_{4}$ gate, and all of their integer powers) with optical qudits based on the polarization-spatial degree of freedom of the single photon. Furthermore, we also realize the polarization-controlled eight-dimensional controlled-${X}_{4}$ gate and all of its integer powers. The experimental results achieve both the ultrahigh average gate fidelity $99.73%$ and efficiency $99.47%$, which are above the error threshold for fault-tolerant quantum computation. Our work paves the way for the large-scale high-dimensional fault-tolerant quantum computation with a polynomial resource cost.

Quantum imaginarity and quantum nonlocality capture the essence of quantumness of a physical system from different aspects. We establish a connection between the imaginarity and quantum nonlocality in any two-qubit states. Based on the imaginarity ${l}_{1}$-norm and relative entropy, we derive complementary relations among the quantum imaginarities with respect to any set of mutually unbiased bases for arbitrary qubit states. Based on these complementary relations, we introduce the nonlocal advantage of quantum imaginarity (NAQI). We show that any two-qubit state with NAQI is quantum steerable, but not vice versa. In addition, we investigate the exclusion property of NAQI in three-qubit pure states.

The authors investigate the dynamics of an interacting bosonic chain which can be made non-reciprocal through engineered coupling to the environment. They show the presence of directional motion both for single bosons and repulsively bound pairs, which can be exploited to make the two move in opposite directions.

Scalable quantum computers hold the promise to solve hard computational problems, such as prime factorization, combinatorial optimization, simulation of many-body physics, and quantum chemistry. While being key to understanding many real-world phenomena, simulation of nonconservative quantum dynamics presents a challenge for unitary quantum computation. In this work, we focus on simulating nonunitary parity-time-symmetric systems, which exhibit a distinctive symmetry-breaking phase transition as well as other unique features that have no counterpart in closed systems. We show that a qutrit, a three-level quantum system, is capable of realizing this nonequilibrium phase transition. By using two physical platforms, an array of trapped ions and a superconducting transmon, and by controlling their three energy levels in a digital manner, we experimentally simulate the parity-time-symmetry-breaking phase transition. Our results indicate the potential advantage of multilevel (qudit) processors in simulating physical effects, where additional accessible levels can play the role of a…

We classify different ways to passively protect classical and quantum information, i.e., we do not allow for syndrome measurements, in the context of local Lindblad models for spin systems. Within this family of models, we suggest that passive error correction is associated with nontrivial phases of matter and propose a definition for dissipative phases based on robust steady-state degeneracy of a Lindbladian in the thermodynamic limit. We study three thermalizing models in this context: the two-dimensional (2D) Ising model, the 2D toric code, and the 4D toric code. In the low-temperature phase, the 2D Ising model hosts a robust classical steady-state degeneracy, while the 4D toric code hosts a robust quantum steady-state degeneracy. We perturb the models with terms that violate detailed balance and observe that qualitative features remain unchanged, suggesting that ${\mathbb{Z}}_{2}$ symmetry breaking in a Lindbladian is useful to protect a classical bit while intrinsic topological order protects a qubit.

We investigate the ground state and dynamics of one-dimensional spin-orbit coupled (SOC) quantum droplets within the extended Gross--Pitaevskii approach. As the SOC wave number increases, stripe droplet patterns emerge, with a flat-top background, for larger particle numbers. The surface energy decays following a power-law with respect to the interactions. At small SOC wave numbers, a transition from Gaussian to flat-top droplets occurs for either a larger number of atoms or reduced intercomponent attraction. The excitation spectrum shows that droplets for relatively small SOC wave numbers are stable, otherwise stripe droplets feature instabilities as a function of the particle number or the interactions. We also witness rich droplet dynamical features using velocity imprinting and abrupt changes in the intercomponent interaction or the SOC parameters. Characteristic responses include breathing oscillations, expansion, symmetric and asymmetric droplet fragmentation, admixtures of single and stripe droplet branches, and erratic spatial distributions suggesting the triggering of…

The Mpemba effect is a counterintuitive relaxation phenomenon that a system which begins hot can cool down faster than the identical system which is initially cold. Here, we investigate heat and work in a quantum open system with being alternatively driven by hot-cold heat reservoirs, which operates as an Otto cycle in finite time, either with or without the Mpemba effect. Based on the quantum master equation, the time duration of the cooling relaxation process is determined by exploring the distance function which can be used to observe the appearance of the Mpemba effect. We then numerically evaluate and compare the average and variance of the power of two scenarios with and without the Mpemba effect. We demonstrate that, given the efficiency, the Markovian Mpemba effect leads to improved performance by significantly increasing the power without sacrificing the stability.

The authors explore interactions between quasiparticles mediated by a surrounding environment in different experimental platforms, such as quantum degenerate atomic gases, cavity QED, and two-dimensional semiconductors. They also discuss theoretical frameworks for describing interactions between quasiparticles and approaches for calculating them.

We present several improvements to the recently developed ground-state preparation algorithm based on the quantum eigenvalue transformation for unitary matrices (QETU), apply this algorithm to a lattice formulation of U(1) gauge theory in $(2+1)$ dimensions, as well as propose an alternative application of QETU, a highly efficient preparation of Gaussian distributions. The QETU technique was originally proposed as an algorithm for nearly optimal ground-state preparation and ground-state energy estimation on early fault-tolerant devices. It uses the time-evolution input model, which can potentially overcome the large overall prefactor in the asymptotic gate cost arising in similar algorithms based on the Hamiltonian input model. We present modifications to the original QETU algorithm that significantly reduce the cost for the cases of both exact and Trotterized implementation of the time evolution circuit. We use QETU to prepare the ground state of a U(1) lattice gauge theory in two spatial dimensions, explore the dependence of computational resources on the desired precision and…

Quantum entanglement is a fundamental resource for various quantum applications and generation of large-scale entanglement is a key quantum technology. In recent years, continuous-variable optical systems have shown promising results in this direction, thanks to deterministic generation and multiplexing via rich degrees of freedom naturally occurring in the optical system. In this paper, we review the generation and applications of multipartite optical quantum entanglement. We begin with a theoretical overview of how to represent and verify multimode continuous-variable quantum states and entanglement. Then, we discuss the multiplexing technique in time, frequency, and spatial domains, and their applications in generation of large-scale entangled states. Afterward, we review the current status and development of the basic technology used in the generation of multipartite quantum entanglement. Finally, we discuss the applications of large-scale quantum entanglement and possible future perspectives. Published by the American Physical Society 2024

We investigate the steady-state charging process of a single-cell quantum battery embedded in an $N$-cell star network of qubits, each interacting with a fermion reservoir, collectively and individually in equilibrium and nonequilibrium scenarios, respectively. We find an optimal steady-state charging in both scenarios, which grows monotonically with the reservoirs' chemical potential and chemical potential difference, where the high base temperature of the reservoirs has a destructive role in all parameter regimes. We indicate that regardless of the strength of the nonequilibrium condition, the high base chemical potential of the battery's corresponding reservoir can significantly enhance the charging process. On the other hand, a weak-coupling strength can strongly suppress the charging. Consequently, our results could counteract the detrimental effects of self-discharging and provide valuable guidelines for enhancing the stable charging of open quantum batteries in the absence of an external charging field.

The nonadiabatic holonomic quantum computation (NHQC) has received great attention for decades, however, there are many challenges to its implementation in experiments. To further shorten the evolution time is the first challenge to be conquered to realize high-fidelity quantum gates in NHQC. In this paper, we propose a controlled two-qubit model in Rydberg atoms to realize nonadiabatic holonomic quantum gates, where the evolution time is extremely decreased and the influences of several kinds of noises are minimized by utilizing the time-optimal-control technique on the target atom. In addition, we can construct arbitrary geometric gates by selecting the appropriate parameters in our model. Furthermore, numerical simulations for the $\mathrm{C}\ensuremath{-}T$ gate and $\mathrm{C}\ensuremath{-}\sqrt{H}$ gate based on the master equation show that the fidelities of geometric gates obtained in our model are still very high even though noises are considered, which demonstrates the robustness of our protocol. It is worth noting that the controlled two-qubit model may pave the way to…

Recently, there has been an upsurge of interest in quantum thermodynamic devices, notably quantum batteries. Quantum batteries serve as energy storage devices governed by the rules of quantum thermodynamics. Here, we propose a model of a quantum battery wherein the system of interest can be envisaged as a battery, and the ambient environment acts as a charger (dissipation) mechanism, modeled along the ubiquitous quantum Brownian motion. We employ quantifiers such as ergotropy and its (in)coherent manifestations, as well as instantaneous and average powers, to characterize the performance of the quantum battery. We investigate the influence of the bath's temperature and the system's coupling with the environment via momentum and position coordinates on the discharging and recharging dynamics. Moreover, we probe the memory effects of the system's dynamics and obtain a relationship between the system's non-Markovian evolution and the battery's recharging process.

We investigate one- and two-photon scattering in a one-dimensional waveguide coupled to a giant atom within the non-Markovian regime using the resolvent approach. The non-Markovian behavior gives rise to an atom-photon bound state that cannot be excited by a single incident photon. However, the bound state can be excited via the two-photon scattering process described by multichannel scattering theory, from which an analytical trapping probability of a photon in the bound state can be achieved. Additionally, we analyze the two-photon scattering process, obtaining the analytical expressions for scattered states. As non-Markovian effects strengthen, two peaks appear in the incoherent power spectrum, attributed to the system behaving as a leaky cavity formed by the giant atom's coupling points. Through the analysis of second-order correlation functions, we observe the bunching behavior for transmitted photons, antibunching behavior for reflected photons, and the distinctive retrieval behavior at the coupling points' separation.

The non-Hermitian skin effect (NHSE), characterized by the collapse of bulk-band eigenstates into the localized boundary modes of the systems, is one of most striking properties in the field of non-Hermitian physics. Unique physical phenomena related to the NHSE have attracted a great deal of interest; however, their experimental realization usually requires nonreciprocal hopping, which faces a great challenge in ultracold-atom systems. In this work we propose the realization of the NHSE in a one-dimensional optical lattice by periodically driven ultracold atoms in the presence of staggered atomic loss. By studying the effective Floquet Hamiltonian in the high-frequency approximation, we reveal the underlying mechanism for the periodic-driving-induced NHSE. We find that the robust NHSE can be tuned by the driving phase, which is manifested by the dynamical localization. Most remarkably, we uncover the periodic-driving-induced critical skin effect for two coupled chains with different driving phases, accompanied by the appearance of size-dependent topological in-gap modes. Our study…

Counterintuitively, the authors show that critical metrology not only remains robust in the presence of finite temperature but can also benefit from it. Moreover, in certain cases, excited states exhibit significantly more non-classical behavior than ground states, further contributing to quantum-enhanced metrology.

Parameterized quantum circuits as machine learning models are typically well described by their representation as a partial Fourier series of the input features, with frequencies uniquely determined by the feature map's generator Hamiltonians. Ordinarily, these data-encoding generators are chosen in advance, fixing the space of functions that can be represented. In this work we consider a generalization of quantum models to include a set of trainable parameters in the generator, leading to a trainable-frequency (TF) quantum model. We numerically demonstrate how TF models can learn generators with desirable properties for solving the task at hand, including nonregularly spaced frequencies in their spectra and flexible spectral richness. Finally, we showcase the real-world effectiveness of our approach, demonstrating an improved accuracy in solving the Navier-Stokes equations using a TF model with only a single parameter added to each encoding operation. Since TF models encompass conventional fixed-frequency models, they may offer a sensible default choice for variational quantum…

Quantum-phase-estimation algorithms are critical subroutines in many applications for quantum computers and in quantum-metrology protocols. These algorithms estimate the unknown strength of a unitary evolution. By using coherence or entanglement to sample the unitary N tot times, the variance of the estimates can scale as O ( 1 / N tot 2 ) , compared to the best “classical” strategy with O ( 1 / N tot ) . The original algorithm for quantum phase estimation cannot be implemented on near-term hardware as it requires large-scale entangled probes and fault-tolerant quantum computing. Therefore, alternative algorithms have been introduced that rely on coherence and statistical inference. These algorithms produce quantum-boosted phase estimates without interprobe entanglement. This family of phase-estimation algorithms have, until now, never exhibited the possibility of achieving optimal scaling O ( 1 / N tot 2 ) . Moreover, previous works have not considered the effect of noise on these algorithms. Here, we present a coherence-based phase-estimation algorithm which can achieve the…

Quantum coherence, nonlocality, and contextuality are key resources for quantum advantage in metrology, communication, and computation. We introduce a graph-based approach to derive classicality inequalities that bound local, noncontextual, and coherence-free models, offering a unified description of these seemingly disparate quantum resources. Our approach generalizes recently proposed basis-independent coherence witnesses, and it recovers all noncontextuality inequalities of the exclusivity graph approach. Moreover, violations of certain classicality inequalities witness preparation contextuality. We describe an algorithm to find all such classicality inequalities, and we use it to analyze some of the simplest scenarios.

We investigate parametrized multipartite entanglement measures from the perspective of $k$ nonseparability in this paper. We present two types of entanglement measures in $n$-partite systems, $q\text{\ensuremath{-}}k\text{\ensuremath{-}}\mathrm{ME}$ concurrence $(q>1,\phantom{\rule{0.28em}{0ex}}2\ensuremath{\le}k\ensuremath{\le}n)$ and $\ensuremath{\alpha}\text{\ensuremath{-}}k\text{\ensuremath{-}}\mathrm{ME}$ concurrence $(0\ensuremath{\le}\ensuremath{\alpha}<1,\phantom{\rule{0.28em}{0ex}}2\ensuremath{\le}k\ensuremath{\le}n)$, which unambiguously detect all $k$-nonseparable states in arbitrary $n$-partite systems. Rigorous proofs show that the proposed $k$-nonseparable measures satisfy all the requirements for being an entanglement measure including the entanglement monotone, strong monotone, convexity, vanishing on all $k$-separable states, and being strictly greater than zero for all $k$-nonseparable states. In particular, the $q\text{\ensuremath{-}}\mathrm{2}\text{\ensuremath{-}}\mathrm{ME}$ concurrence and…

Kirkwood-Dirac representations of quantum states are increasingly finding use in many areas within quantum theory. Usually, representations of this sort are only applied to provide a representation of quantum states (as complex functions over some set). We show how standard Kirkwood-Dirac representations can be extended to a fully compositional representation of all of quantum theory (including channels, measurements, and so on), and prove that this extension satisfies the essential features of functoriality (namely, that the representation commutes with composition of channels), linearity, and quasistochasticity. Interestingly, the representation of a positive operator-valued measure (POVM) element is uniquely picked out to be the collection of weak values for it relative to the bases defining the representation. We then prove that if one can find any Kirkwood-Dirac representation that is everywhere real and non-negative for a given experimental scenario or fragment of quantum theory, then the scenario or fragment is consistent with the principle of generalized noncontextuality, a…

Cavity magnomechanics using mechanical degrees of freedom in ferromagnetic crystals provides a powerful platform for observing many interesting classical and quantum nonlinear phenomena in the emerging field of magnon spintronics. However, to date, the generation and control of chaotic motion in a cavity magnomechanical system remain an outstanding challenge due to the inherently weak nonlinear interaction of magnons. Here, we present an efficient mechanism for achieving magnomechanical chaos, in which the magnomechanical system is coherently driven by a two-tone microwave field consisting of a pump field and a probe field. Numerical simulations show that the relative phase of the two input fields plays an important role in controlling the appearance of chaotic motion and, more importantly, the threshold power of chaos is reduced by 6 orders of magnitude from watts to microwatts. In addition to providing insight into the nonlinear interaction of magnons, cavity magnomechanical chaos will always be of interest because of its significance in both fundamental physics and potential…

We consider the problem of maximizing the stored energy for a given charging duration in a quantum battery composed of a pair of spins-$1/2$ with Ising coupling starting from the spin-down state, using bounded transverse field control. We map this problem to an optimal control problem on a single qubit and by using optimal control theory we show that, although a single bang pulse can quickly achieve considerable charging levels for relatively large upper control bounds, higher levels of stored energy including complete charging are accomplished by a bang-singular-bang pulse sequence, where the intermediate singular pulse is an off pulse. If the control is restricted between zero and a maximum value, the initial and final bang pulses attain the maximum bound but have different durations, while if it is restricted between symmetric negative and positive boundaries, the bang pulses have the same duration but opposite boundary values. For both cases we provide transcendental equations from which the durations of the individual pulses in the optimal pulse-sequence can be calculated. For…

We propose a scheme to achieve the simultaneous nonreciprocal photon blockade of two microring resonators in an all-optical system without any rotating parts. By unidirectionally parametrically pumping a ${\ensuremath{\chi}}^{(2)}$-nonlinear resonator with a classical coherent field, the counterclockwise mode in the resonator is exposed to the parametric amplification process when a forward signal field is input in another resonator, but not the case for a backward signal field. This leads to different quantum interference effects among distinct excitation paths for two optical modes, which is the essential reason for simultaneous nonreciprocal photon blockade. We analytically give the optimal parameter conditions to achieve simultaneous strong photon blockade with the parametric amplification. Furthermore, the nonreciprocity is simultaneously enhanced through tuning the amplitude of input signal. Our work provides an avenue to realize simultaneous nonreciprocal single-photon devices without moving parts and may have potential applications in many-body quantum information processing…

We investigate the properties of the mobility edge in an Aubry-Andr\'e-Harper model with nonreciprocal long-range hopping. The results reveal that there can be a type of mobility edge featuring both strength-dependent and scale-free properties. By calculating the fractal dimension, we find that the positions of mobility edges are robust to the strength of nonreciprocal long-range hopping. Furthermore, through scale analysis of the observables such as fractal dimension, eigenenergy, eigenstate, etc., we show that four different specific mobility edges can be observed in the system. This paper extends the family tree of mobility edges and hopefully it will shed more light on the related theory and experiment.

We give an explicit construction to perform any $n$-qubit computational-basis-state transposition using $\mathrm{\ensuremath{\Theta}}(n)$ gates. This nearly coincides with the lower bound $\mathrm{\ensuremath{\Omega}}[n/log(nd)]$ on worst-case and average-case gate complexities to perform transpositions using a $d$-element gate set, which we also prove.

The phase transition between gapped topological phases represents a class of unconventional criticality beyond the Landau paradigm. However, recent research has shifted attention to topological phases without a bulk gap, where the phase transitions between them are still elusive. In this work, based on large-scale density-matrix renormalization-group techniques, we investigate the critical behaviors of the extended quantum $XXZ$ model obtained by the Kennedy-Tasaki transformation. Using fidelity susceptibility as a diagnostic, we obtain a complete phase diagram, which includes both topological nontrivial and trivial gapless phases. Furthermore, as the $XXZ$-type anisotropy parameter $\mathrm{\ensuremath{\Delta}}$ varies, both the critical points ${h}_{c}$ and correlation length exponent $\ensuremath{\nu}$ remain the same as in the $\mathrm{\ensuremath{\Delta}}=0$ case, characterized by $c=3/2$ (Ising plus free boson) conformal field theory. Our results indicate that fidelity susceptibility can effectively detect and reveal a stable unconventional critical line between the…

Here we employ the quasibound state in the continuum (quasi-BIC) resonance in all-dielectric metasurfaces for efficient nonlinear processes in consideration of the backward frequency conversion. We theoretically study second-harmonic generation (SHG) from symmetry-broken AlGaAs metasurfaces and reveal the efficiency enhancement empowered by high-$Q$ quasi-BIC resonances. By introducing the correction term of nonlinear polarization at the fundamental wave field to the conventional undepleted approximation, we uncover the effect of backward frequency conversion on the nonlinear conversation efficiency. The SHG efficiency of $2.45\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}2}$ with the developed depleted model shows a $14.3%$ decrease compared with $2.86\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}2}$ in the conventional undepleted approximation, under an incident intensity of $10\phantom{\rule{4pt}{0ex}}\mathrm{MW}/{\mathrm{cm}}^{2}$. Our results are of importance for designing efficient nonlinear metasurfaces supporting high-$Q$ resonances.

In this paper, we propose a one-step scheme for implementing the nonadiabatic holonomic swap gate with Rydberg atoms. By applying invariant-based reverse engineering to design the effective Hamiltonian of the system, a suitable evolution path for implementing nonadiabatic holonomic quantum computation is found. In addition, the systematic-error-sensitivity nullified optimal control method is considered in the parameter selections, so that the scheme is insensitive to the systematic error of pulses. We also estimate the effects of random noise, the random initial phase of the pulses, the Doppler shift, and decoherence on the scheme. The numerical results show that the scheme exhibits fairly good performance against these negative factors. Finally, we generalize the scheme to realize the non-Clifford swap gates. Therefore, this scheme can provide a feasible framework for implementing high-fidelity and robust swap gates and non-Clifford swap gates with Rydberg atoms.

It is well known that non-Hermitian, nonreciprocal systems may harbor exponentially localized skin modes. However, in this work, we find that, generically, non-Hermiticity gives rise to abrupt and prominent kinks in the semiclassical wave packet trajectories of quantum gases, despite the absence of sudden physical impulses. This physically stems from a hitherto underappreciated intrinsic nonlocality from non-Hermitian pumping, even if all physical couplings are local, thereby resulting in enigmatic singularities in the band structure that lead to discontinuous band geometry and Berry curvature. Specifically, we focus on the realization of the kinked response in an ultracold atomic setup. For a concrete experimental demonstration, we propose an ultracold atomic setup in a two-dimensional optical lattice with laser-induced loss such that response kinks can be observed without fine-tuning in the physical atomic cloud dynamics. Our results showcase unique nonmonotonic behavior from non-Hermitian pumping beyond the non-Hermitian skin effect and suggest new avenues for investigating…

Coupling of a quantum battery (QB) to two thermal reservoirs brings it to a nonequilibrium steady-state (NESS). In this situation, although, the QB can be charged through the NESS heat current, no work may be extracted from it through unitary cyclic processes. We exemplify this statement by studying the two- and three-qubit $XXZ$ spin-chain QBs coupled weakly to two different thermal reservoirs in a symmetric configuration wherein each end of chain interacts with an individual reservoir. We show that the work can be extracted at positive temperature bias when two ends of a $XXZ$ chain are collectively coupled to the hot reservoir.

Fabry-Perot microcavities can strongly enhance interactions between light and molecules, leading to the formation of hybrid light-matter states known as polaritons. Polaritons possess much smaller effective masses and much larger group velocities when the molecules are resonant with cavity modes that have finite (nonzero) in-plane wave vectors, giving rise to the possibilities of long-range and ultrafast ballistic transport. In this paper, we present the results of numerical simulations of the ultrafast ballistic transport phenomenon in real space and time during and after initialization with a short, spatially localized pulse. We address this problem with two approaches: the standard transfer-matrix method (TMM) for planar structures and a second simulation based on the numerical solution of the Maxwell-Bloch equations that can be used for general configurations (complex metasurfaces, for instance) and excitation modes. The agreement between the TMM and the full numerical calculation with the Maxwell-Bloch equations when applied to multilayer planar configurations provides proof of…

Entanglement is a key property of quantum computing that separates it from its classical counterpart; however, its exact role in the performance of quantum algorithms, especially variational quantum algorithms, is not well understood. In this work, we utilize tensor network methods to systematically probe the role of entanglement in the working of two variational quantum algorithms, the quantum approximate optimization algorithm (QAOA) and quantum neural networks (QNNs), on prototypical problems under controlled entanglement environments. We find that for the max-cut problem solved using QAOA, the fidelity as a function of entanglement is highly dependent on the number of layers, layout of edges in the graph, and edge density, generally exhibiting that a high number of layers indicates a higher resilience to truncation of entanglement. This is in contrast to previous studies based on no more than four QAOA layers which show that the fidelity of QAOA follows a scaling law for circuits. Contrarily, in the case of QNNs, trained circuits with high test accuracies are underpinned by…

We present a theoretical study of the excitations of the two-dimensional supersolid state of a Bose-Einstein condensate with either dipole-dipole interactions or soft-core interactions. This supersolid state has three gapless excitation branches arising from the spontaneously broken continuous symmetries. Two of these branches are related to longitudinal sound waves, similar to those in one-dimensional supersolids. The third branch is a transverse wave arising from the nonzero shear modulus of the two-dimensional crystal. We present the results of numerical calculations for the excitations and dynamic structure factor characterizing the density fluctuations and study their behavior across the discontinuous superfluid to supersolid transition. We show that the speeds of sound are described by a hydrodynamic theory that incorporates generalized elastic parameters, including the shear modulus. Furthermore, we establish that dipolar and soft-core supersolids manifest distinct characteristics, falling into the bulk-incompressible and rigid-lattice limits, respectively.

Alongside its companion in $P\phantom{\rule{0}{0ex}}h\phantom{\rule{0}{0ex}}y\phantom{\rule{0}{0ex}}s\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}c\phantom{\rule{0}{0ex}}a\phantom{\rule{0}{0ex}}l$ $R\phantom{\rule{0}{0ex}}e\phantom{\rule{0}{0ex}}v\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}e\phantom{\rule{0}{0ex}}w$ $L\phantom{\rule{0}{0ex}}e\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}e\phantom{\rule{0}{0ex}}r\phantom{\rule{0}{0ex}}s$, this paper reports on the similarities and differences between the information-theoretic and relativistic notions of causality, bringing them together under a single framework. This sheds light on a long-standing debate on whether experimentally realizable processes are truly indefinite causal ordered processes.

Long-range interactions can fundamentally alter properties in gapped topological phases such as emergent massive edge modes. However, recent research has shifted attention to topological nontrivial critical points or phases, and it is natural to explore how long-range interactions influence them. In this work, we investigate the topological behavior at the quantum critical point of extended Kitaev chains with long-range interactions, which can be derived from the critical Ising model via the Jordan-Wigner transformation in the short-range limit. Specifically, we analytically find that the critical edge modes at the critical point remain stable against long-range interactions. More importantly, we observe that these critical edge modes remain massless even when long- range interactions become very strong. As a by-product, we numerically find that the critical behavior of the long-range model belongs to the free-Majorana-fermion universality class, which is entirely different from the long-range universality class in the usual long-range spin models. Our work could shed new light on…

Variational quantum algorithms, inspired by neural networks, have become a novel approach in quantum computing. However, designing efficient parameterized quantum circuits remains a challenge. Quantum architecture search tackles this by adjusting circuit structures along with gate parameters to automatically discover high-performance circuit structures. In this study, we propose an end-to-end distributed quantum architecture search framework, where we aim to automatically design distributed quantum circuit structures for interconnected quantum processing units with specific qubit connectivity. We devise a circuit generation algorithm which incorporates TeleGate and TeleData methods to enable nonlocal gate implementation across quantum processing units. While taking into account qubit connectivity, we also incorporate qubit assignment from logical to physical qubits within our quantum architecture search framework. A two-stage progressive training-free strategy is employed to evaluate extensive circuit structures without circuit training costs. Through numerical experiments on three…

We delve into the use of photonic quantum computing to simulate quantum mechanics and extend its application towards quantum field theory. We develop and prove a method that leverages this form of continuous-variable quantum computing (CVQC) to reproduce the time evolution of quantum-mechanical states under arbitrary Hamiltonians, and we demonstrate the method's remarkable efficacy with various potentials. Our method centers on constructing an , a specially prepared quantum state that induces the desired time evolution on the target state. This is achieved by introducing a non-Gaussian operation using a measurement-based quantum computing approach, enhanced by machine learning. Furthermore, we propose a framework in which these methods can be extended to encode field theories in CVQC without discretizing the field values, thus preserving the continuous nature of the fields. This opens new avenues for quantum computing applications in quantum field theory. Published by the American Physical Society 2024

The authors study First-Passage-Time distributions, and how they can be computed for stochastic measurement currents. They gain insights into signal-to-noise-ratio bounds and false positive reduction strategies, and charge-resolved master equations.

Going beyond the standard quantum limit in noisy quantum metrology is an important and challenging task. Here we show how Dicke states can be used to surpass the standard quantum limit and achieve the Heisenberg limit in open quantum systems. The system we study has qubits symmetrically coupled to a resonator, and our objective is to estimate the coupling between the qubits and the resonator. The time-dependent quantum Fisher information with respect to the coupling is studied for this open quantum system where the same decay rates are assumed on all qubits. We show that when the system is initialized to a Dicke state with an optimal excitation number one can go beyond the standard quantum limit and achieve the Heisenberg limit even for finite values of the decays on the qubit and the resonator, particularly when the qubits and resonator are strongly coupled. We compare our results against the highly entangled GHZ state and a completely separable state and show that the GHZ state performs quite poorly, whereas under certain noise conditions the separable state is able to go beyond…

The authors theoretically analyze the performance of long-distance quantum communication protocols, specifically quantum repeaters based on Gottesman-Kitaev-Preskill (GKP) qudits. Previously, only the qubit case has been studied. They construct three quantum repeater schemes and find that, while in most cases any benefits of using higher dimensions is negated by worse error correction, there are some regimes where the use of qubits does increase the secret key rate.

We unravel stationary states in the form of dark soliton stripes, bubbles, and kinks embedded in a two-dimensional droplet-bearing setting emulated by an extended Gross-Pitaevskii approach. The existence of these configurations is corroborated through an effectively reduced potential picture demonstrating their concrete parametric regions of existence. The excitation spectra of such configurations are analyzed within the Bogoliubov--de Gennes framework exposing the destabilization of dark soliton stripes and bubbles, while confirming the stability of droplets, and importantly unveiling spectral stability of the kink against transverse excitations. Additionally, a variational approach is constructed providing access to the transverse stability analysis of the dark soliton stripe for arbitrary chemical potentials and widths of the structure. This is subsequently compared with the stability analysis outcome demonstrating very good agreement at small wave numbers. Dynamical destabilization of dark soliton stripes via the snake instability is showcased, while bubbles are found to feature…

We provide an architecture for a multimode quantum battery based on the framework of continuous-variable systems. We examine the performance of the battery by using a generic class of multimode initial states whose parameters can be tuned to produce separable as well as entangled states and that can be charged locally as well as globally by Gaussian unitary operations. Analytical calculations show that a separable state is equally advantageous as an entangled one for two- and three-mode batteries when taking the figures of merit as the second moments of the change in energy. In order to produce a stable quantum battery consisting of an arbitrary number of modes, we derive compact analytical forms of the energy fluctuations and prove that for a multimode separable Gaussian initial state, fluctuations decrease as the number of modes increases, thereby obtaining a scaling analysis. Moreover, we demonstrate that local displacement as a charger is better for minimizing the fluctuations in energy than that involving the squeezing unitary operation.

We investigate the controlled manipulation of the Goos-H\"anchen (GH) shift in probe light beams, encompassing both plane and Gaussian beams, when introduced into a cavity housing a highly resonant five-level atomic system exhibiting a combined tripod and $\mathrm{\ensuremath{\Lambda}}$ (CTL) configuration. This complex scheme arises from the interaction of three atomic ground states with two excited states through five distinct light fields. Such a system effectively reduces to a $\mathrm{\ensuremath{\Lambda}}$- or N-shaped configuration by manipulating the light fields, resulting in the alteration of the dispersion behavior of the probe beam, ultimately inducing either a positive or negative GH shift. This unique behavior is a direct consequence of the closed-loop structure inherent to the five-level atomic scheme. For both plane and Gaussian beams, we demonstrate the superiority of CTL over $\mathrm{\ensuremath{\Lambda}}$ and $N$ schemes in achieving substantial positive GH shifts. When considering Gaussian probe light, we observe a critical role played by the beam width in…

From the perspective of quantum information theory, the effect of Unruh radiation on a two-level accelerated detector can be modeled as a quantum channel. In this work we employ the tools of channel-position finding to locate Unruh channels. The signal-idler and idler-free protocols are explored to determine the position of the target Unruh channel within a sequence of background channels. We derive the fidelity-based bounds for the ultimate error probability of each strategy and obtain the conditions where the signal-idler protocol is superior to the protocol involving idler-free states. It is found that the lower bound of the error probability for the signal-idler scheme exhibits clear advantages in all cases, while the idler-free scheme can only be implemented when the temperatures of the two channels are very close and the number of initial states is insufficient. Interestingly, it is shown that the optimal detection protocol relies on the residual correlations shared between the emitted probe state and the retained idler modes.

Quantum signal processing provides an optimal procedure for simulating Hamiltonian evolution on a quantum computer using calls to a block encoding of the Hamiltonian. In many situations it is possible to control between forward and reverse steps with almost identical cost to a simple controlled operation. We show that it is then possible to reduce the cost of Hamiltonian simulation by a factor of 2 using the recent results of generalized quantum signal processing.

Quantum computing has a potential to accelerate data processing efficiency, especially in machine learning, by exploiting special features such as the quantum interference. The major challenge in this application is that, in general, the task of loading a classical data vector into a quantum state requires an exponential number of quantum gates. The approximate amplitude encoding (AAE) method, which uses a variational means to approximately load a given real-valued data vector into the amplitude of a quantum state, was recently proposed as a general approach to this problem mainly for near-term devices. However, AAE cannot load a complex-valued data vector, which narrows its application range. In this work, we extend AAE so that it can handle a complex-valued data vector. The key idea is to employ the fidelity distance as a cost function for optimizing a parametrized quantum circuit, where the classical shadow technique is used to efficiently estimate the fidelity and its gradient. We apply this algorithm to realize the complex-valued-kernel binary classifier called the compact…

We construct a central-spin battery model affected by noise or thermal bath and consider the impact of environment on battery performance. In a noisy environment, we show that the steady-state energy and ergotropy are related to the number of spins exposed to the noisy channel. In addition, we reveal the relationship between the steady-state energy and ergotropy with the quantum information lost in the battery. In a thermal bath, we find that the higher the mean occupation number of the thermal bath, the greater the steady-state energy. In particular, we find that, in two different environments, the steady-state energy and ergotropy are related to the effective magnetic-field strength of battery $B$, but not charger $h$. It is worth noting that, in the central-spin model, when $B$ is a constant and $|h/B\ensuremath{-}1|$ reaches a certain value, the steady-state energy and ergotropy with noise or thermal bath are always greater than or equal to the maximum energy and ergotropy of the corresponding closed battery. At the same time, we generalize the conclusions to the two-qubit,…

We perform a systematic comparison between photoelectron momentum distributions computed with the rescattered quantum-orbit strong-field approximation (RQSFA) and the Coulomb quantum-orbit strong-field approximation (CQSFA). We exclude direct, hybrid, and multiply scattered CQSFA trajectories and focus on the contributions of trajectories that undergo a single act of rescattering. For this orbit subset, one may establish a one-to-one correspondence between the RQSFA and CQSFA contributions for backscattered and forward-scattered trajectory pairs. We assess the influence of the Coulomb potential on the ionization and rescattering times of specific trajectory pairs, kinematic constraints determined by rescattering, and quantum interference between specific pairs of trajectories. We analyze how the Coulomb potential alters their ionization and return times, and their interference in photoelectron momentum distributions. We show that Coulomb effects are not significant for high or medium photoelectron energies and shorter orbits, while for lower momentum ranges or longer electron…