Showing 1–50 of 167 results for author: Yang, M

Practical Error Suppression and Mitigation for Reliable Quantum Computing

Authors: Han-Ze Li, Mengjie Yang, Xianquan Yan, Dax Enshan Koh, Ching Hua Lee, Ruizhe Shen

Abstract: Quantum computing is entering a transitional regime between noisy intermediate-scale quantum (NISQ) processing and early fault-tolerant quantum computation (FTQC), in which increasingly capable hardware is beginning to support repeated syndrome measurements, partial error correction, and logical-qubit operations, while residual physical and logical errors remain non-negligible. In this regime, err… ▽ More Quantum computing is entering a transitional regime between noisy intermediate-scale quantum (NISQ) processing and early fault-tolerant quantum computation (FTQC), in which increasingly capable hardware is beginning to support repeated syndrome measurements, partial error correction, and logical-qubit operations, while residual physical and logical errors remain non-negligible. In this regime, error suppression, error mitigation, and quantum error correction are increasingly better viewed as complementary layers of a unified error-reduction strategy rather than as separate approaches, with each acting at a different stage of the quantum computation to improve simulation reliability. Thus, in this review, we provide a practical and forward-looking overview of the principal hardware error sources and the corresponding error suppression and mitigation methods for reducing their impact across the current NISQ-FTQC transition. We discuss hardware-aware circuit design, coherent-error suppression, readout mitigation, noise extrapolation, classical inference, and software-supported workflows, with particular emphasis on their implementation on actual quantum processors. We further examine how error mitigation techniques can be adapted to encoded and logical-qubit settings so that they can operate alongside quantum error correction to suppress residual logical errors and improve the accuracy of computation in the early fault-tolerant regime. △ Less

Submitted 20 August, 2026; originally announced August 2026.

Comments: 35 figures in total. Comments welcome

Exceptional activated mode theory for generalized real-complex transitions

Authors: Mengjie Yang, Alexander N. Poddubny, Ching Hua Lee

Abstract: Real-to-complex spectral transitions mark the onset of amplification in non-Hermitian systems, but their thresholds are often treated as model-specific quantities. Here we develop a general, non-perturbative activated-mode principle that governs the real-to-complex threshold across broad classes of non-Hermitian systems. A central insight is that only a small Hilbert subspace is ``activated" at th… ▽ More Real-to-complex spectral transitions mark the onset of amplification in non-Hermitian systems, but their thresholds are often treated as model-specific quantities. Here we develop a general, non-perturbative activated-mode principle that governs the real-to-complex threshold across broad classes of non-Hermitian systems. A central insight is that only a small Hilbert subspace is ``activated" at the transition onset, which can be variationally determined through the competition between spectral detuning and mode-level projected non-Hermitian couplings. The result is a closed-form exceptional-activation condition for arbitrarily large ``disturbances", rather than a perturbative estimate. We apply our framework to three contrasting illustrative problems, establishing (i) a closed-form threshold for critical non-Hermitian skin amplification at \emph{all} system sizes; (ii) a new link between impurity tunneling threshold and exceptional point switching; and (iii) activation channel switching without underlying topological phase transition. Overall, our findings recast real-to-complex transitions as generic mode-selection problems independent of any specific symmetry. △ Less

Submitted 12 August, 2026; originally announced August 2026.

Comments: Any comments are welcome

Quantum Coordination Advantages in AI State-Tracking Tasks: Semantic Compilation and Latent Memory

Authors: Ming Yang

Abstract: We prove inference-time quantum coordination advantages for specified AI state-tracking tasks. A solver compresses semantic history into a future-accessible boundary state and later answers a query. We count communication $B$, persistent instance-dependent memory $M$, and local work $D$; classical recurrence, caches, tools, and recomputation are allowed and charged. The central result is a boundar… ▽ More We prove inference-time quantum coordination advantages for specified AI state-tracking tasks. A solver compresses semantic history into a future-accessible boundary state and later answers a query. We count communication $B$, persistent instance-dependent memory $M$, and local work $D$; classical recurrence, caches, tools, and recomputation are allowed and charged. The central result is a boundary-preserving semantic-compilation theorem. It maps a finite one-way, streaming, or adaptive causal task into a semantic AI interface while preserving event order and access to past input. Classical boundary-state lower bounds and quantum-memory upper bounds transfer up to explicit compiler overhead, independently of the finite-precision recurrent architecture. Two applications have classical semantics. Matched-entity synopsis QA inherits the hidden-matching separation between $O(\log N)$ qubits and $Ω(\sqrt{N})$ classical boundary bits. Continual requirements auditing inherits a Max-$k$SAT streaming separation: a recurrent solver uses $O(\log^5 n\log(1/δ))$ qubits and polylogarithmic classical workspace to obtain a $0.7172$-approximation, whereas every classical one-pass finite-information solver attaining that ratio requires $Ω(\sqrt{n})$ coordination width. As a quantum-native compiler test, a stabilizer latent-state dialogue uses $n$ qubits, while every exact finite-state classical causal online realization satisfies $B+M \ge \frac{1}{2}n^2+(\frac{3}{2}-\log_2 3)n+O(1)$. The source protocols, streaming algorithms, and stabilizer witness are imported; the new result is their architecture-independent semantic transfer. These are memory and coordination separations, not runtime or empirical advantages for present-day language models. The stabilizer result assumes exact simulation and ideal noiseless quantum memory. △ Less

Submitted 11 August, 2026; originally announced August 2026.

Comments: Comments and suggestions are welcome on alphaXiv

Nonlocality-induced critical-length hierarchy from non-Hermitian competition

Authors: Mengjie Yang, Alexander N. Poddubny, Ching Hua Lee

Abstract: Spectral transitions in non-Hermitian lattices often arise from the competition between non-reciprocal skin accumulation and inter-component hybridization. In short-range systems formed by two coupled chains, this competition conventionally leads to the logarithmic critical-length law $N_c\sim\ln D$, where $D$ is the transverse separation between the chains. Here we show that long-range hoppings f… ▽ More Spectral transitions in non-Hermitian lattices often arise from the competition between non-reciprocal skin accumulation and inter-component hybridization. In short-range systems formed by two coupled chains, this competition conventionally leads to the logarithmic critical-length law $N_c\sim\ln D$, where $D$ is the transverse separation between the chains. Here we show that long-range hoppings fundamentally reorganizes this critical behavior, producing a hierarchy of distinct scaling laws. When only the hybridization couplings are power-law decaying with exponent $α$, the onset becomes algebraic, $N_c\sim D^{α/3}$. When the hoppings within each chain are themselves also power-law decaying, in addition to the hybridization couplings, the system enters a scale-covariant regime for $α<2$, in which the criticality threshold equation depends only on the system aspect ratio $N_c/D$. At $α=2$ and beyond, this regime is followed by a marginal logarithmically corrected and algebraically corrected regimes, respectively. We identify two new non-local mechanisms that enable this unconventional critical hierarchy: a nonanalytic band-edge dispersion from long-range intra-chain hoppings, and parity-mixing hybridization induced by non-reciprocity. Our results show that nonlocality systematically removes the physical length scales i.e. skin depth underlying conventional critical non-Hermitian skin behavior, offering a platform-independent framework testable in programmable topoelectrical circuits, photonic lattices and digital quantum simulators. △ Less

Submitted 3 August, 2026; originally announced August 2026.

Comments: Any comments are welcome

Detecting quantum phase transitions via shallow variational quantum circuits

Authors: Ching-Yu Huang, Min-Fong Yang

Abstract: Mapping quantum phase diagrams through classical simulation is notoriously resource-intensive, as even small systems far from the thermodynamic limit demand prohibitive computational effort. The variational quantum eigensolver (VQE) offers a compelling alternative, exploiting approximate ground states to distinguish phases. An appealing proposal, dubbed as Delta-VQE, determines critical points by… ▽ More Mapping quantum phase diagrams through classical simulation is notoriously resource-intensive, as even small systems far from the thermodynamic limit demand prohibitive computational effort. The variational quantum eigensolver (VQE) offers a compelling alternative, exploiting approximate ground states to distinguish phases. An appealing proposal, dubbed as Delta-VQE, determines critical points by contrasting variational energies optimized from reference states of distinct phases. Intriguingly, the diagnostic sharpens as circuit depth decreases, highlighting its promise as a resource-conscious probe of quantum criticality. To probe the broader applicability and underlying mechanisms of this approach, we investigate the one-dimensional transverse-field Ising model with a three-spin cluster interaction, a setting in which the Ising transitions are generally situated beyond the self-dual line. We demonstrate that, whenever dual ansätze are employed, Delta-VQE invariably detects the self-dual points rather than the true criticality. In contrast, when ansätze are carefully tailored to embody the competing phases across the boundary, the genuine Ising critical point can be successfully identified with only minor finite-size effects. Our results establish that, while Delta-VQE provides a resource-efficient probe of quantum criticality without requiring precise ground-state preparation, its diagnostic power is fundamentally contingent upon the judicious selection of physically representative ansätze. △ Less

Submitted 28 July, 2026; originally announced July 2026.

Comments: 9 pages, 6 figures

Layer-Resolved Topological Metals in the Bilayer Lieb Lattice

Authors: Mengjie Yang, S Rahul, Giandomenico Palumbo

Abstract: We identify a two-dimensional time-reversal-invariant topological metallic phase on a bilayer Lieb lattice, characterized by a quantized layer--resolved pseudo-spin Chern number. Without the orbital-angular-momentum-dependent (OAM-dependent) coupling, the system gives rise to a time-reversal-invariant topological semimetal with a zero indirect gap and quantized pseudo-spin Chern number. Opposite-s… ▽ More We identify a two-dimensional time-reversal-invariant topological metallic phase on a bilayer Lieb lattice, characterized by a quantized layer--resolved pseudo-spin Chern number. Without the orbital-angular-momentum-dependent (OAM-dependent) coupling, the system gives rise to a time-reversal-invariant topological semimetal with a zero indirect gap and quantized pseudo-spin Chern number. Opposite-sign intralayer OAM-dependent coupling immediately converts the zero-indirect-gap semimetal into a metal, in which the global spectrum is metallic while the layer--resolved pseudo-spin Chern number remains well defined as long as the direct gap at each crystal momentum and the pseudo-spin gap remain open. The model also exhibits asymmetric boundary states: in the semimetallic regime, one edge hosts perfectly flat bands, whereas the opposite edge supports gapless counter-propagating modes forming a one-dimensional Dirac cone. An edge-localized interlayer coupling gaps only the counter-propagating edge states, leaving the flat-band edge essentially intact, while intralayer OAM-dependent coupling bends the exact flat band into a dispersive boundary mode without affecting the gapped Dirac edge. These results open a route toward the controlled engineering of layer--resolved topological gapless phases in synthetic and quantum materials. △ Less

Submitted 12 July, 2026; originally announced July 2026.

Comments: Any comments are welcome

An Iterative Dual-Channel Neural Quantum State Algorithm for Selected Configuration Interaction

Authors: Jen-Yu Chang, Yi-Chun Chang, Yu-Jui Lin, Ming-Chun Yang, Hsiu-Chi Tsai, Tai-Yue Li, Nan Yow Chen, Tsung-Wei Huang, En-Jui Kuo

Abstract: Accurately solving the electronic Schrödinger equation for strongly correlated systems remains a central challenge in quantum chemistry, where the exponential growth of configuration space limits the applicability of exact methods. Selected Configuration Interaction (SCI) algorithms address this challenge by adaptively constructing compact determinantal expansions, yet their efficiency depends cri… ▽ More Accurately solving the electronic Schrödinger equation for strongly correlated systems remains a central challenge in quantum chemistry, where the exponential growth of configuration space limits the applicability of exact methods. Selected Configuration Interaction (SCI) algorithms address this challenge by adaptively constructing compact determinantal expansions, yet their efficiency depends critically on the quality of the sampling strategy used to identify chemically important configurations. Here we introduce the Handover Iterative Neural Quantum State (HI-NQS) algorithm, which embeds a classically trained autoregressive Transformer neural quantum state within the iterative sample--diagonalize--update framework of Sample-Based Quantum Diagonalization. A dual-channel Transformer architecture with explicit spin-up/spin-down cross-attention encodes fermionic spin structure as an architectural inductive bias, enabling expressive and physically informed wavefunction representations. After each subspace diagonalization, the resulting eigenvector is distilled back into the network through a factorized spin-marginal teacher signal, establishing a closed feedback loop between generative sampling and exact diagonalization. Benchmarks across a range of small molecules and a systematic nitrogen active-space series demonstrate that HI-NQS achieves chemical accuracy on all systems tested, with determinant-count scaling substantially more favorable than conventional CIPSI-based SCI for all but the smallest active spaces. All calculations are performed on GPU hardware without quantum computing resources, establishing HI-NQS as an efficient and scalable purely classical approach to the selected configuration interaction problem. △ Less

Submitted 25 June, 2026; originally announced June 2026.

Genuine Global Kochen-Specker Contextuality as Classical Coordination Cost

Authors: Ming Yang

Abstract: We formulate classical simulation of quantum correlations as coordination across a spacetime separator. All past-dependent information capable of influencing the future must cross the separator either through communicated messages or through a persistent classical state. We prove a general lower bound on this classical coordination cost in terms of the nonnegative rank of the separator correlation… ▽ More We formulate classical simulation of quantum correlations as coordination across a spacetime separator. All past-dependent information capable of influencing the future must cross the separator either through communicated messages or through a persistent classical state. We prove a general lower bound on this classical coordination cost in terms of the nonnegative rank of the separator correlation matrix, with approximate and amortized extensions governed by approximate nonnegative rank and Wyner common information. For sequential processes, we introduce a causal positive-realization rank that enforces consistent response and state-update maps, and we explain how bounded local computation and support-covering models arise as restricted cases. We apply the framework to genuine global Kochen--Specker contextuality, in which local subsystems are noncontextual and the tested multipartite blocks are generalized-Bell-local, while the complete empirical model has no global noncontextual explanation. At the symmetric point of a polarization--path Hardy construction, exact classical simulation requires two coordination bits, whereas the corresponding quantum separator requires one qubit. Under exact repetition, the classical lower bound is the base-two logarithm of three bits per copy; with asymptotically vanishing error, it becomes one and a half classical bits per copy, compared with one qubit per copy. We further construct a finite stabilizer--Peres--Mermin family for which the exact classical coordination cost grows quadratically with system size, while the quantum boundary cost grows only linearly. The separation applies to arbitrary finite-state causal online simulators. Establishing a constant-error quadratic separation and an intrinsic construction without flag conditioning remain open. △ Less

Submitted 26 July, 2026; v1 submitted 22 June, 2026; originally announced June 2026.

Comments: 44 pages. Added general nonnegative-rank coordination bounds, exact and asymptotic Hardy separations, an exact quadratic separation, and clarified the GLHV assumptions and support-covering framework. Comments and suggestions are welcome on alphaXiv

Global Kochen-Specker Contextuality Without Local Contextuality and Generalized Bell Nonlocality

Authors: Ming Yang

Abstract: A set of quantum data can look classical in every local test and still fail to admit a single classical explanation of the whole composite system. We formulate this failure as global contextuality. Here global means global in the physical sense of the whole multipartite system, not the local/global terminology of sheaf theory. Each party's local statistics are noncontextual and each measured multi… ▽ More A set of quantum data can look classical in every local test and still fail to admit a single classical explanation of the whole composite system. We formulate this failure as global contextuality. Here global means global in the physical sense of the whole multipartite system, not the local/global terminology of sheaf theory. Each party's local statistics are noncontextual and each measured multipartite context admits a generalized local hidden-variable description, but the GLHV block descriptions cannot be promoted to a single noncontextual hidden-variable model for the whole system. Three bipartite constructions exhibit this separation. A polarization-path construction gives a direct global obstruction. A qubit-qutrit KCBS construction gives an algebraic scenario-level example, with explicit formulas for the unconditional KCBS operator, the correlation-polytope constraints, and the postselected violation. A flagged qutrit Werner-local state gives a state-level example: the state is entangled and local for all projective measurements, its local qutrit marginals do not violate KCBS, yet postselection rules out a single GNCHV model. We also spell out the classical composition lemma: classical conditional hidden variables can be absorbed into a larger hidden variable, whereas quantum contextual data need not allow such a factorization. Within the general Bell-type framework considered here, with arbitrary parties and arbitrary local compatible contexts, but no cross-party joint measurements, the absence of local contextuality and GLHV-type generalized Bell nonlocality does not imply the existence of a global noncontextual hidden-variable model. Global contextuality is thus a compositional obstruction to classical explanation. △ Less

Submitted 8 June, 2026; v1 submitted 27 May, 2026; originally announced May 2026.

Comments: Welcome to comment on alphaXiv

Quantizing gravitational fields with an entropy-corrected action principle

Authors: Jianhao M. Yang

Abstract: A variational framework for the quantization of gravitational fields is developed based on an extension of the stationary action principle. Within this framework, the Wheeler-DeWitt equation for the gravitational wave functional is recovered without assuming operator promotion of the canonical momentum, thus avoiding the ambiguity of operator ordering in canonical quantization. The derivation is b… ▽ More A variational framework for the quantization of gravitational fields is developed based on an extension of the stationary action principle. Within this framework, the Wheeler-DeWitt equation for the gravitational wave functional is recovered without assuming operator promotion of the canonical momentum, thus avoiding the ambiguity of operator ordering in canonical quantization. The derivation is based on three main ingredients. First, motivated by information-theoretic considerations, the classical stationary action principle is generalized by incorporating a correction term constructed from the relative entropy associated with field fluctuations. Second, an ensemble formulation on superspace is enhanced to incorporate this entropy correction. Third, the formalism is further refined to provide a unified treatment of quantization and constraints, thereby addressing the long-standing ambiguity concerning the ordering of quantization and constraint reduction. The framework is then applied to gravitational fields coupled to a massless scalar field. Using an emergent time parameter defined via the rate equation of the gravitational fields, a Schrodinger equation for the scalar-field wave functional is recovered, supplemented by an additional quantum correction term suppressed at order $G\hbar^2$. Finally, we comment on possible connections between the notion of relative entropy employed here and holographic dualities in quantum gravity. △ Less

Submitted 2 May, 2026; originally announced May 2026.

Comments: 22 pages. This paper is related to arXIv:2302.14619, arXiv:2310.02274, arXiv:2503.02362, and arXiv:2509.22747

Ontic Dynamical Locality Reduces to Bell Locality

Authors: Ming Yang

Abstract: Bell inequalities exclude a broad class of local hidden-variable explanations of quantum correlations. A recurring objection is that the usual Bell form is static, whereas real measuring devices may contain local memory, stochastic dynamics, and measurement-induced disturbances of their hidden variables. We formulate this objection as a general transition-kernel model for dynamical hidden variable… ▽ More Bell inequalities exclude a broad class of local hidden-variable explanations of quantum correlations. A recurring objection is that the usual Bell form is static, whereas real measuring devices may contain local memory, stochastic dynamics, and measurement-induced disturbances of their hidden variables. We formulate this objection as a general transition-kernel model for dynamical hidden variables. The only locality assumption is imposed at the ontic level: conditional on the pre-measurement ontic state, the transition kernel and response function in each wing depend on the local setting but not on the distant setting or on distant post-measurement variables. Under measurement independence, all such dynamics can be absorbed into effective local response functions. The resulting probabilities have exactly the static Bell-local form and therefore obey the CHSH inequality. The result identifies the available escape routes for reproducing quantum correlations: violating ontic dynamical locality, violating measurement independence, or abandoning a classical hidden-variable ontology. As a consequence, local classical dynamical complexity cannot by itself spoof Bell-nonlocal statistics in device-independent protocols. △ Less

Submitted 25 May, 2026; v1 submitted 20 April, 2026; originally announced April 2026.

Geometric Optimization over Quantum State Spaces: Tight Uncertainty Relations and Resource Certification

Authors: Ma-Cheng Yang, Cong-Feng Qiao

Abstract: Determining the fundamental limits of nonlinear functionals of quantum measurement statistics is a crucial yet generally intractable non-convex optimization problem. We introduce a generic support-function-based outer-approximation framework for solving concave-minimization (or convex-maximization) problems over the quantum state space. By mapping the problem onto a reduced $\mathcal{Z}$-space, we… ▽ More Determining the fundamental limits of nonlinear functionals of quantum measurement statistics is a crucial yet generally intractable non-convex optimization problem. We introduce a generic support-function-based outer-approximation framework for solving concave-minimization (or convex-maximization) problems over the quantum state space. By mapping the problem onto a reduced $\mathcal{Z}$-space, we characterize the exact quantum boundary through supporting half-spaces derived from the largest eigenvalues of effective observables. This yields an effective method that produces tight bounds for general measurements in finite-dimensional quantum systems with preassigned numerical precision. As an initial application, we recover the exact variance-based uncertainty relations of [PRL \textbf{119}, 170404 (2017)] and efficiently compute optimal entropic uncertainty relations (EURs). Our results reveal that standard analytical and majorization-based EUR bounds are fundamentally loose for generic measurements, and we show that the resulting exact bounds directly enhance quantum steering detection under asymmetric settings. We further apply the framework to determine the maximal athermality resource certifiable from a restricted measurement scenario. Our method thus provides a universal computational tool for exploring the boundaries of quantum state space and the limits of quantum resources. △ Less

Submitted 2 July, 2026; v1 submitted 31 January, 2026; originally announced February 2026.

Comments: 13 pages, 4 figures, revised version with reorganized presentation and improved structure; main results and conclusions remain unchanged, with one new application added

Extracting conserved operators from a projected entangled pair state

Authors: Wen-Tao Xu, Miguel Frías Pérez, Mingru Yang

Abstract: Given a tensor network state, how can we determine conserved operators (including Hamiltonians) for which the state is an eigenstate? We answer this question by presenting a method to extract geometrically $k$-local conserved operators that have the given infinite projected entangled pair state (iPEPS) in 2D as an (approximate) eigenstate. The key ingredient is the evaluation of the static structu… ▽ More Given a tensor network state, how can we determine conserved operators (including Hamiltonians) for which the state is an eigenstate? We answer this question by presenting a method to extract geometrically $k$-local conserved operators that have the given infinite projected entangled pair state (iPEPS) in 2D as an (approximate) eigenstate. The key ingredient is the evaluation of the static structure factors of multi-site operators through differentiating the generating function. These generating functions define a manifold of the given tensor network state deformed by some parameters, endowed with a quantum geometry, where conserved operators correspond to vanishing fidelity susceptibility. Despite the approximation errors, we show that our method is still able to extract from exact or variational iPEPS to good precision both frustration-free and non-frustration-free parent Hamiltonians that are beyond the standard construction and obtain better locality. In particular, we find a 4-site-plaquette local Hamiltonian that approximately has the short-range RVB state as the ground state. Moreover, we find a Hamiltonian for which the deformed toric code state at arbitrary string tension is an excited eigenstate with the same energy, thereby potentially realizing quantum many-body scars. △ Less

Submitted 14 April, 2026; v1 submitted 25 November, 2025; originally announced November 2025.

Comments: 5+12 pages

Excited states from local effective Hamiltonians of matrix product states and their entanglement spectrum transition

Authors: Denise Cocchiarella, Mingru Yang, Yueshui Zhang, Mari Carmen Bañuls, Hong-Hao Tu, Yuhan Liu

Abstract: Solving excited states is a challenging task for interacting systems. For one-dimensional critical systems, however, excited states can be directly accessed from the eigenvectors of the local effective Hamiltonian that is constructed from the ground state obtained by variational matrix product state (MPS) optimization. Despite its numerical success, the theoretical mechanism underlying this method… ▽ More Solving excited states is a challenging task for interacting systems. For one-dimensional critical systems, however, excited states can be directly accessed from the eigenvectors of the local effective Hamiltonian that is constructed from the ground state obtained by variational matrix product state (MPS) optimization. Despite its numerical success, the theoretical mechanism underlying this method has remained largely unexplored. In this work, we provide a conformal field theory (CFT) perspective that helps elucidate this connection. The key insight is that this construction effectively uses a truncated basis of ground-state Schmidt vectors to represent excited states, where the contribution of each Schmidt vector can be expressed as a CFT correlation function and shown to decay with increasing Schmidt index. The CFT analysis further predicts an entanglement-spectrum transition of excited states as the ratio of the subsystem size to the total system size is varied. Our numerical results support this picture and demonstrate a reorganization of the entanglement spectrum into distinct conformal towers as this ratio changes. △ Less

Submitted 27 May, 2026; v1 submitted 20 November, 2025; originally announced November 2025.

Comments: 13 pages, 4 figures

Journal ref: Phys. Rev. B 113, 205145 (2026)

Observation of Non-Hermitian Spectral Deformation in Complex Momentum Space

Authors: Mu Yang, Yue Li, Mingtao Xu, Wei Yi, Jin-Shi Xu, Chuan-Feng Li, Guang-Can Guo

Abstract: Open systems feature a variety of phenomena that arise from non-Hermitian physics. Recent theoretical studies have offered much insights into these phenomena through the non-Bloch band theory, though many of the theory's key features are experimentally elusive. For instance, the correspondence between complex momenta and non-Hermitian bands, while central to non-Bloch band theory, has so far defie… ▽ More Open systems feature a variety of phenomena that arise from non-Hermitian physics. Recent theoretical studies have offered much insights into these phenomena through the non-Bloch band theory, though many of the theory's key features are experimentally elusive. For instance, the correspondence between complex momenta and non-Hermitian bands, while central to non-Bloch band theory, has so far defied direct experimental observation. Here we experimentally study the non-Hermitian spectral deformation in complex-momentum space, by implementing a non-Hermitian lattice with long-range couplings in the synthetic orbital-angular-momentum (OAM) dimension of photons inside a degenerate cavity. Encoding the complex momenta in the phase and amplitude modulations of the OAM modes, and devising a complex-momentum-resolved projective detection, we reconstruct the spectral deformation in momentum space, where the eigenspectrum on the complex plane morphs through distinct geometries. This enables us to experimentally extract key information of the system under the non-Bloch band theory, including exceptional points in the complex-momentum space, the open-boundary spectra, and the generalized Brillouin zone. Our work demonstrates a versatile platform for exploring non-Hermitian physics and non-Bloch band theory, and opens the avenue for direct experimental investigation of non-Bloch features in the complex-momentum space. △ Less

Submitted 10 November, 2025; originally announced November 2025.

Quantum Fisher Information With General Quantum Coherence in multi-dimensional quantum systems

Authors: Jun-Long Zhao, Li Yu, Ming Yang, Chui-Ping Yang

Abstract: Quantum metrology is a science about quantum measurements and it plays a key role in precision of quantum parameter estimation. Meanwhile, quantum coherence is an important quantum feature and quantum Fisher information (QFI) is an important indicator for precision of quantum parameter estimation. In this paper, we explore the relationship between QFI and quantum coherence in multi-dimensional qua… ▽ More Quantum metrology is a science about quantum measurements and it plays a key role in precision of quantum parameter estimation. Meanwhile, quantum coherence is an important quantum feature and quantum Fisher information (QFI) is an important indicator for precision of quantum parameter estimation. In this paper, we explore the relationship between QFI and quantum coherence in multi-dimensional quantum systems. We introduce a new concept referred to as General Quantum Coherence (GQC), which characterizes the quantum coherence and the eigenenergies of the Hamiltonian in the interaction processes. GQC captures quantum nature of high-dimensional quantum states and addresses shortcomings in coherence measurement. Additionally, we observe a stringent square relationship between GQC and QFI. This finding provides a crucial guideline for improving the precision of parameter estimation. △ Less

Submitted 29 October, 2025; originally announced October 2025.

Comments: 10 pages, 5 figures

Excitonic Insulator and Possible Superfluid Based on Two-Dimensional Diamond

Authors: Shisheng Lin, Shaoqi Huang, Minhui Yang, Xin Chen, Hongjia Bi, Kangchen Xiong

Abstract: Recent research on excitonic insulator has progressed mainly based on narrow bandgap semiconductor or semimetal. Herein, we realize excitonic insulator based on two-dimensional (2D) wide band gap diamond with transition temperature as high as 220K. The resistance rises dramatically by more than three orders, which can be explained by the Bose-Einstein condensation (BEC) of excitons. While cooling… ▽ More Recent research on excitonic insulator has progressed mainly based on narrow bandgap semiconductor or semimetal. Herein, we realize excitonic insulator based on two-dimensional (2D) wide band gap diamond with transition temperature as high as 220K. The resistance rises dramatically by more than three orders, which can be explained by the Bose-Einstein condensation (BEC) of excitons. While cooling down below transition temperature, the wavelength of the bound excitons caused by boron and nitrogen centers becomes highly overlapped, leading to BEC process. Furthermore, the variable range hopping mechanism is used to simulate the resistance as a function of temperature, which reveals the formation of excitonic insulator. When temperature drops down further, a sudden drop of resistance over three orders was observed around 60K, possibly due to the formation of non-equilibrium excitonic superfluid resulting from highly overlap of wavelength of the large density bound excitons at lower temperature. This study provides evidences for excitonic insulator and possible superfluid phase based on wide bandgap semiconductor. △ Less

Submitted 7 October, 2025; originally announced October 2025.

Beyond the non-Hermitian skin effect: scaling-controlled topology from Exceptional-Bound Bands

Authors: Mengjie Yang, Ching Hua Lee

Abstract: We establish a novel mechanism for topological transitions in non-Hermitian systems that are controlled by the system size. Based on a new paradigm known as exceptional-bound (EB) band engineering, its mechanism hinges on the unique critical scaling behavior near an exceptional point, totally unrelated to the well-known non-Hermitian skin effect. Through a series of ansatz models, we analytically… ▽ More We establish a novel mechanism for topological transitions in non-Hermitian systems that are controlled by the system size. Based on a new paradigm known as exceptional-bound (EB) band engineering, its mechanism hinges on the unique critical scaling behavior near an exceptional point, totally unrelated to the well-known non-Hermitian skin effect. Through a series of ansatz models, we analytically derive and numerically demonstrate how topological transitions depend on the system size with increasingly sophisticated topological phase boundaries. Our approach can be generically applied to design scaling-dependent bands in multi-dimensional lattices, gapped or gapless, challenging established critical and entanglement behavior. It can be experimentally demonstrated in any non-Hermitian platform with versatile couplings or multi-orbital unit cells, such as photonic crystals, as well as classical and quantum circuits. The identification of this new EB band mechanism provides new design principles for engineering band structures through scaling-dependent phenomena unique to non-Hermitian systems. △ Less

Submitted 8 March, 2026; v1 submitted 7 October, 2025; originally announced October 2025.

Comments: any comments are welcome

Journal ref: Advanced Science (2026): e23989

Simultaneous Quantization and Reduction of Constrained Systems

Authors: Jianhao M. Yang

Abstract: We present a novel framework for quantizing constrained quantum systems in which the processes of quantization and constraint enforcement are performed simultaneously. The approach is based on an extension of the stationary action principle, incorporating an information-theoretic term arising from vacuum fluctuations. Constraints are included directly in the Lagrangian via Lagrange multipliers, al… ▽ More We present a novel framework for quantizing constrained quantum systems in which the processes of quantization and constraint enforcement are performed simultaneously. The approach is based on an extension of the stationary action principle, incorporating an information-theoretic term arising from vacuum fluctuations. Constraints are included directly in the Lagrangian via Lagrange multipliers, allowing the subsequent variational procedure to yield the quantum dynamics without ambiguity regarding the order of quantization and reduction. We demonstrate the method through two examples: (i) a one-dimensional system with vanishing local momentum, where the simultaneous approach produces the time-independent Schrödinger equation while conventional reduced and Dirac quantization yield only trivial states, and (ii) a bipartite system with global translational invariance, where all three methods agree. These results show that the proposed framework generalizes standard quantization schemes and provides a consistent treatment of systems with constraints that cannot be expressed as linear operators acting on the wave function. In addition to a unified variational principle for constrained quantum systems, the formalism also offers an information-theoretic perspective on quantum effects arising from vacuum fluctuations. △ Less

Submitted 24 December, 2025; v1 submitted 26 September, 2025; originally announced September 2025.

Comments: 15 pages, 1 figure. Added Appendix A to provide more background on extended canonical transformation. The article is related to arXiv:2302.14619

Quantum Topological Analysis on Digraphs

Authors: Yunpeng Zi, Muchun Yang, D. L. Zhou

Abstract: Quantum algorithms for topological data analysis provide significant advantages over the best known classical algorithms. Unlike previous work on simplicial complexes built from point clouds, path homology on digraphs is defined for directed graphs and provides a natural topological framework for analyzing data with intrinsic directional structures. Path homology has become an emerging area in Top… ▽ More Quantum algorithms for topological data analysis provide significant advantages over the best known classical algorithms. Unlike previous work on simplicial complexes built from point clouds, path homology on digraphs is defined for directed graphs and provides a natural topological framework for analyzing data with intrinsic directional structures. Path homology has become an emerging area in Topological Data Analysis (TDA), attracting increasing attention in recent years. We propose a quantum algorithm for path homology on digraphs that offers a significant advantage over the best known classical algorithms. We design a universal encoding protocol for the paths and boundary operators of digraphs on quantum systems. We prove a property of path homology that provides the theoretical guarantee for the algorithm. The speedup of the quantum algorithm for path homology depends on input-access assumptions. The exponential speedup arises when the path space can be efficiently accessed, while for standard digraph input the algorithm provides polynomial speedup. △ Less

Submitted 20 August, 2026; v1 submitted 17 September, 2025; originally announced September 2025.

Comments: 26 pages, 2 figures, To be appeared on Quantum Science and Technology

Mitigating the sign problem by quantum computing

Authors: Kwai-Kong Ng, Min-Fong Yang

Abstract: The notorious sign problem severely limits the applicability of quantum Monte Carlo (QMC) simulations, as statistical errors grow exponentially with system size and inverse temperature. A recent proposal of a quantum-computing stochastic series expansion (qc-SSE) method suggested that the problem could be avoided by introducing constant energy shifts into the Hamiltonian. Here we critically examin… ▽ More The notorious sign problem severely limits the applicability of quantum Monte Carlo (QMC) simulations, as statistical errors grow exponentially with system size and inverse temperature. A recent proposal of a quantum-computing stochastic series expansion (qc-SSE) method suggested that the problem could be avoided by introducing constant energy shifts into the Hamiltonian. Here we critically examine this framework and show that it does not strictly resolve the sign problem for Hamiltonians with non-commuting terms. Instead, it provides a practical mitigation strategy that suppresses the occurrence of negative weights. Using the antiferromagnetic anisotropic XY chain as a test case, we analyze the dependence of the average sign on system size, temperature, anisotropy, and shift parameters. An operator contraction method is introduced to improve efficiency. Our results demonstrate that moderate shifts optimally balance sign mitigation and statistical accuracy, while large shifts amplify errors, leaving the sign problem unresolved but alleviated. △ Less

Submitted 25 February, 2026; v1 submitted 16 September, 2025; originally announced September 2025.

Comments: 9 pages, 5 figures. Accepted for publication in Physical Review A

Journal ref: Phys. Rev. A 113, 032611 (2026)

Reversing non-Hermitian skin accumulation with a non-local transverse switch

Authors: Mengjie Yang, Ching Hua Lee

Abstract: Asymmetrically directed couplings in non-Hermitian systems cause directional amplification that leads to boundary skin state accumulation. However, counter-intuitively, the direction of accumulation may not follow that of the directed couplings. In this work, we demonstrate new mechanisms where this accumulation can be systematically reversed without modifying the couplings at all, just by adjusti… ▽ More Asymmetrically directed couplings in non-Hermitian systems cause directional amplification that leads to boundary skin state accumulation. However, counter-intuitively, the direction of accumulation may not follow that of the directed couplings. In this work, we demonstrate new mechanisms where this accumulation can be systematically reversed without modifying the couplings at all, just by adjusting the system size or boundary conditions in a different, transverse direction. Moreover, the reversed skin dynamics can be made very robust by suppressing the amplification in the original non-reversed direction. We motivate our approach through a series of warm-up models, culminating in a designed non-Hermitian Kagome lattice whereby wavepacket simulations demonstrate how robust reversed skin dynamics can be switched on/off in a non-local transverse manner. Our findings highlight how the non-trivial entanglement between the spectral loops of different PBC directions can be harnessed as a directional amplification switch, paving the way for new avenues of non-Hermitian sensing and lasing. △ Less

Submitted 2 September, 2025; originally announced September 2025.

Comments: Any comments are welcome

Journal ref: Commun Phys 9, 210 (2026)

A hybrid-frequency on-chip programmable synthetic-dimension simulator with arbitrary couplings

Authors: Xiao-Dong Zeng, Zhao-An Wang, Jia-Ming Ren, Yi-Tao Wang, Chun Ao, Wei Liu, Nai-Jie Guo, Lin-Ke Xie, Jun-You Liu, Yu-Hang Ma, Ya-Qi Wu, Shuang Wang, Pei-Yun Li, Zong-Quan Zhou, Mu Yang, Jin-Shi Xu, Xi-Wang Luo, Jian-Shun Tang, Chuan-Feng Li, Guang-Can Guo

Abstract: High-performance photonic chips provide a powerful platform for analog computing, enabling the simulation of high-dimensional physical systems using low-dimensional devices with additional synthetic dimensions. The realization of large-scale complex simulations necessitates an architecture capable of arbitrary coupling configurations (encompassing symmetric, asymmetric and long-range coupling sche… ▽ More High-performance photonic chips provide a powerful platform for analog computing, enabling the simulation of high-dimensional physical systems using low-dimensional devices with additional synthetic dimensions. The realization of large-scale complex simulations necessitates an architecture capable of arbitrary coupling configurations (encompassing symmetric, asymmetric and long-range coupling schemes) which is also crucial for scaling up. Previous approaches rely on excessive physical components to introduce asymmetric coupling, however, are restricted in reconfiguring and scaling by the relatively complicated structures. Here, to solve this problem, we propose a hybrid-frequency synthetic-dimension simulator architecture that combines both intra-resonant and inter-resonant frequency-lattice sites, and experimentally demonstrate it using the thin-film lithium niobate (TFLN) photonic chip. Employing this hybrid programmable architecture, we are able to simulate both the regular and long-range coupled forms of diverse compound-lattice models, such as the Hall ladder, Creutz ladder (symmetric) and Su-Schrieffer-Heeger (SSH, asymmetric) model, on a single chip, simultaneously reducing the experimental requirements significantly. As results, the direct readout of the bandstructure of the SSH model is able to be achieved, to be distinguished from all previous works, and important phenomena such as spin-momentum locking, topological flat band and Aharonov-Bohm cage effect are also observed with lower experimental requirements. Furthermore, applications like piecewise-continuous optical frequency shifting can be enabled by cascading our devices. Our results offer promising insights for future large-scale complex on-chip simulators with arbitrary couplings. △ Less

Submitted 21 August, 2025; originally announced August 2025.

Comments: 11 pages, 4 figures

Witness High-Dimensional Quantum Steering via Majorization Lattice

Authors: Ma-Cheng Yang, Cong-Feng Qiao

Abstract: Quantum steering enables one party to influence another remote quantum state by local measurement. While steering is fundamental to many quantum information tasks, the existing detection methods in the literature are mainly constrained to either specific measurement scenario or low-dimensional systems. In this work, we propose a majorization lattice framework for steering detection, which is capab… ▽ More Quantum steering enables one party to influence another remote quantum state by local measurement. While steering is fundamental to many quantum information tasks, the existing detection methods in the literature are mainly constrained to either specific measurement scenario or low-dimensional systems. In this work, we propose a majorization lattice framework for steering detection, which is capable of exploring the steering in arbitrary dimension and measurement setting. Steering inequalities for two-qubit states, high-dimensional Werner states and isotropic states are obtained, which set even stringent bars than what has been reached yet. Notably, the known high-dimensional results turn out to be some kind of approximate limits of the new approach. △ Less

Submitted 9 March, 2026; v1 submitted 28 July, 2025; originally announced July 2025.

Comments: 43 pages, 4 figures, 3 tables

Journal ref: npj Quantum Inf (2026)

Beyond symmetry protection: Robust feedback-enforced edge states in non-Hermitian stacked quantum spin Hall systems

Authors: Mengjie Yang, Ching Hua Lee

Abstract: Conventional wisdom holds that, in the simplest time-reversal-symmetric setting, strongly coupling two QSH layers yields a trivial $\mathbb Z_2$ phase and no protected topological edge states. We demonstrate that, in a regime with intermediate inter-layer coupling (neither in the strong or weak coupling regimes) and competitive non-Hermitian directed amplification, bulk modes are rendered with neg… ▽ More Conventional wisdom holds that, in the simplest time-reversal-symmetric setting, strongly coupling two QSH layers yields a trivial $\mathbb Z_2$ phase and no protected topological edge states. We demonstrate that, in a regime with intermediate inter-layer coupling (neither in the strong or weak coupling regimes) and competitive non-Hermitian directed amplification, bulk modes are rendered with negligible gain while arbitrary bulk excitations inevitably accumulate into robust helical edge transport modes -- without relying on any symmetry protection. Our feedback-enforced mechanism persists over broad parameter ranges and remains robust even on fractal or irregular boundaries. These findings challenge the traditional view of stacked QSH insulators as inevitably trivial, and open up new avenues for designing helical topological devices that exploit feedback-enforced non-Hermitian engineering, instead of symmetry-enforced robustness. △ Less

Submitted 16 December, 2025; v1 submitted 23 July, 2025; originally announced July 2025.

Comments: Any comments are welcome

Tangent Space Excitation Ansatz for Quantum Circuits

Authors: Ji-Yao Chen, Bochen Huang, D. L. Zhou, Norbert Schuch, Chenfeng Cao, Muchun Yang

Abstract: Computing excitation spectra of quantum many-body systems is a promising avenue to demonstrate the practical utility of current noisy quantum devices, especially as we move toward the ``megaquop'' regime. For this task, here we introduce a \textit{tangent space excitation ansatz} for quantum circuits, motivated by the quasi-particle picture of many-body systems and the structural similarity betwee… ▽ More Computing excitation spectra of quantum many-body systems is a promising avenue to demonstrate the practical utility of current noisy quantum devices, especially as we move toward the ``megaquop'' regime. For this task, here we introduce a \textit{tangent space excitation ansatz} for quantum circuits, motivated by the quasi-particle picture of many-body systems and the structural similarity between quantum circuits and tensor networks. Increasing circuit depth by one layer to construct tangent space around the variational optimum of a parametrized quantum circuit, we show that massive low-energy single-particle states can be captured. Our ansatz relies on a distinct mechanism from that of excitation ansatz in matrix product state and projected entangled-pair state, and avoids intrinsic limitations of the latter. Comparing our approach with existing quantum excited-state algorithms, we find that with similar computational cost, both the number of excited states and accuracy are significantly improved. We demonstrate our ansatz in both one and two dimensions, and further show that this approach, implementable using Hadamard test, is scalable and suitable for current quantum processors. △ Less

Submitted 14 April, 2026; v1 submitted 10 July, 2025; originally announced July 2025.

Comments: v1: 9+7 pages, 5+10 figures. v2: accepted version

Journal ref: Phys. Rev. Lett. 136, 150601 (2026)

Tunable Antichiral Hinge State in Photonic Synthetic Dimensions

Authors: Xian-Hao Wei, Xi-Wang Luo, Mu Yang, Yu-Wei Liao, Jin-Shi Xu, Guang-Can Guo, Zheng-Wei Zhou

Abstract: Recent research in 2-dimensional (2D) topological matter has generalized the notion of edge states from chiral to antichiral configurations with the same propagating direction at parallel edges, revealing a rich variety of robust transport phenomena. Here, we propose that antichiral hinge states can emerge in a 3D higher-order topological insulator/semimetal, where two surface/bulk Dirac points ar… ▽ More Recent research in 2-dimensional (2D) topological matter has generalized the notion of edge states from chiral to antichiral configurations with the same propagating direction at parallel edges, revealing a rich variety of robust transport phenomena. Here, we propose that antichiral hinge states can emerge in a 3D higher-order topological insulator/semimetal, where two surface/bulk Dirac points are connected by the hinge states. The band dispersion can be controlled and tilted independently for each hinge using properly designed tunnelings, resulting in tunable antichiral hinge states with programmable propagation direction and velocity. Moreover, we propose experimental realization schemes based on a 1D coupled cavity array with additional synthetic dimensions represented by the photonic orbital angular momentum and frequency. We innovatively introduce both longitudinal and transversal electro-optic modulators to generate the desired tunable tunnelings along the synthetic dimensions, which significantly reduce the experimental complexity by eliminating the need for beam splittings and auxiliary cavities. The tunable antichiral hinge states are confirmed by the photonic transmission spectra. Our work presents the robust and tunable antichiral hinge-state transports which paves the way for exploring novel topological matter and their device applications. △ Less

Submitted 21 June, 2025; originally announced June 2025.

Comments: 12 pages, 6 figures

Conditions for Quantum Violation of Macrorealism in Large-spin Limit

Authors: Qi-Hong Cai, Xue-Hao Yu, Ma-Cheng Yang, Ao-Xiang Liu, Cong-Feng Qiao

Abstract: This study investigates the emergence of macroscopic classical behavior from quantum foundations via the entropic Leggett--Garg inequality. We introduce a geometric framework for deriving entropic Leggett--Garg inequalities with higher-order temporal correlations and demonstrate their advantages over conventional formulations. Numerical analyses show that entropic Leggett--Garg inequalities offer… ▽ More This study investigates the emergence of macroscopic classical behavior from quantum foundations via the entropic Leggett--Garg inequality. We introduce a geometric framework for deriving entropic Leggett--Garg inequalities with higher-order temporal correlations and demonstrate their advantages over conventional formulations. Numerical analyses show that entropic Leggett--Garg inequalities offer a robust and complementary criterion to standard approaches, providing a transparent information theoretic interpretation that facilitates the characterization of coherent quantum processes. By applying the WKB approximation, we prove that violations for maximally mixed states remain bounded by a constant in the macroscopic limit, indicating that macrorealism dominates in generic parameter regimes. We further explain previously reported maximal violations at specific parameter regimes as a consequence of the breakdown of the WKB approximation. Our findings indicate that quantum and classical descriptions remain macroscopically incompatible, while violations persist only in fine-tuned regimes, clarifying the conditions for detecting macroscopic quantum phenomena. △ Less

Submitted 28 November, 2025; v1 submitted 19 May, 2025; originally announced May 2025.

Comments: 32 pages, 7 figures

Work statistics and thermal phase transitions

Authors: Kwai-Kong Ng, Min-Fong Yang

Abstract: The investigation of nonequilibrium thermodynamics in quantum many-body systems underscores the importance of quantum work, which differs from its classical counterpart due to its statistical nature. Recent studies have shown that quantum work can serve as an effective indicator of quantum phase transitions in systems subjected to sudden quenches. However, the potential of quantum work to identify… ▽ More The investigation of nonequilibrium thermodynamics in quantum many-body systems underscores the importance of quantum work, which differs from its classical counterpart due to its statistical nature. Recent studies have shown that quantum work can serve as an effective indicator of quantum phase transitions in systems subjected to sudden quenches. However, the potential of quantum work to identify thermal phase transitions remains largely unexplored. In this paper, we examine several types of thermal phase transitions in a sudden-quench hard-core boson model, including Ising, three-state Potts, and Berezinskii-Kosterlitz-Thouless transitions. Through finite-size scaling analysis, we conclude that work statistics can also characterize the critical behaviors of thermal phase transitions in generic many-body systems. Our investigation paves the way for applying work statistics to characterize critical behavior in many-body systems, with implications that may extend to broader contexts. △ Less

Submitted 8 April, 2025; originally announced April 2025.

Comments: 8 pages, 4 figures

Journal ref: Phys. Rev. E 112, 014110 (2025)

Measuring Incompatible Observables with Quantum Neural Networks

Authors: Muchun Yang, Yibin Huang, D. L. Zhou

Abstract: The Heisenberg uncertainty principle imposes a fundamental restriction in quantum mechanics, stipulating that measuring one observable completely erases the information on its conjugate one, thereby preventing simultaneous measurements of incompatible observables. Quantum neural networks (QNNs) is one of the most significant applications on near-term devices in noisy intermediate-scale quantum era… ▽ More The Heisenberg uncertainty principle imposes a fundamental restriction in quantum mechanics, stipulating that measuring one observable completely erases the information on its conjugate one, thereby preventing simultaneous measurements of incompatible observables. Quantum neural networks (QNNs) is one of the most significant applications on near-term devices in noisy intermediate-scale quantum era. Here, we demonstrate that by implementing a multiple-output QNN that emulates a unital quantum channel, one can measure the expectation values of many incompatible observables simultaneously by Pauli-$Z$ measurements on distinct output qubits. We prove the existence of such quantum channel, derive analytical scaling constraints of the measured expectation values, and validate this framework by numerical simulations of observables learning tasks. Notably, our analysis reveals that it requires fewer copies of state when measuring some incompatible observables by the multiple-output QNNs, which demonstrates a resource efficiency advantage compared to separately applying projective measurements. △ Less

Submitted 26 March, 2025; originally announced March 2025.

Comments: 14 pages, 4 figures

Journal ref: Physical Review B 112, 214308 (2025)

Controllable and Continuous Quantum Phase Transitions in Intrinsic Magnetic Topological Insulator

Authors: Shengjie Xu, Zhijian Shi, Ming Yang, Jingwei Zhang, Hang Xu, Haifeng Feng, Ningyan Cheng, Jianfeng Wang, Weichang Hao, Yi Du

Abstract: The intrinsic magnetic topological material MnBi2Te4 has demonstrated great potential to investigate the interplay between topology and magnetism, which opens up new avenues for manipulating non-trivial electronic states and designing quantum devices. However, challenges and controversies remain due to its inevitable n-type antisite defects, hindering the experimental realization of intrinsic magn… ▽ More The intrinsic magnetic topological material MnBi2Te4 has demonstrated great potential to investigate the interplay between topology and magnetism, which opens up new avenues for manipulating non-trivial electronic states and designing quantum devices. However, challenges and controversies remain due to its inevitable n-type antisite defects, hindering the experimental realization of intrinsic magnetic topological phenomena and rendering the precise control of topological phase transitions (TPTs) unachievable. Here, we study a candidate material family, Mn(1-x)GexBi2Te4, in which the heavy n-type doping features are strongly suppressed when the Ge content reaches 0.46, and multiple topological phases are well maintained with the surface Dirac point located near the Fermi level. Based on angle-resolved photoemission spectroscopy, transport measurements, and first-principles calculations, we reveal two magnetism-induced TPTs: the first is antiferromagnetic-ordering-induced transition from strong topological insulator to a magnetic topological insulator as revealed by gap opening of topological surface states; the second is external-magnetic-field-dependent transition from magnetic topological insulator to a Weyl semimetal with the gap reclosed. Our work paves the way for the realization of intrinsic magnetic topological states in MnBi2Te4 family and provides an ideal platform for achieving controllable and continuous TPTs towards future spintronic applications. △ Less

Submitted 7 March, 2025; originally announced March 2025.

Alternative Framework to Quantize Fermionic Fields

Authors: Jianhao M. Yang

Abstract: A variational framework is developed here to quantize fermionic fields based on the extended stationary action principle. From the first principle, we successfully derive the well-known Floreanini-Jackiw representation of the Schrödinger equation for the wave functional of fermionic fields - an equation typically introduced as a postulate in standard canonical quantization. The derivation is accom… ▽ More A variational framework is developed here to quantize fermionic fields based on the extended stationary action principle. From the first principle, we successfully derive the well-known Floreanini-Jackiw representation of the Schrödinger equation for the wave functional of fermionic fields - an equation typically introduced as a postulate in standard canonical quantization. The derivation is accomplished through three key contributions. At the conceptual level, the classical stationary action principle is augmented to include a correction term based on the relative entropy arising from field fluctuations. Then, an extended canonical transformation for fermionic fields is formulated that leads to the quantum version of the Hamilton-Jacobi equation in a form consistent with the Floreanini-Jackiw representation; Third, necessary functional calculus with Grassmann-valued field variables is developed for the variation procedure. The quantized Hamiltonian can generate the Poincaré algebra, thus satisfying the symmetry requirements of special relativity. Concrete calculation of the probability of particle creation for the fermionic field under the influence of constant external field confirms that the results agree with those using standard canonical quantization. We also show that the framework can be applied to develop theories of interaction between fermionic fields and other external fields such as electromagnetic fields, non-Abelian gauge fields, or another fermionic field. These results further establish that the present variational framework is a novel alternative to derive quantum field theories. △ Less

Submitted 26 November, 2025; v1 submitted 4 March, 2025; originally announced March 2025.

Comments: Closely match the version published with IJTP. Added in Section VII the calculation of Schwinger particle creation probability using our framework. Related to articles arXiv:2302.14619 and arXiv:2310.02274

Journal ref: Int. J. Theor. Phys. 65:3 (2026)

Enhancing the coherence time of a neutral atom by an optical quartic trap

Authors: Haobo Chang, Zhuangzhuang Tian, Xin Lv, Mengna Yang, Zhihui Wang, Qi Guo, Pengfei Yang, Pengfei Zhang, Gang Li, Tiancai Zhang

Abstract: The coherence time of an optically trapped neutral atom is a crucial parameter for quantum technologies. We found that optical dipole traps with higher-order spatial forms inherently offer lower decoherence rates compared to those with lower-order spatial forms. We formulated the decoherence rate caused by the variance of the differential energy shift and photon jumping rate. Then, we constructed… ▽ More The coherence time of an optically trapped neutral atom is a crucial parameter for quantum technologies. We found that optical dipole traps with higher-order spatial forms inherently offer lower decoherence rates compared to those with lower-order spatial forms. We formulated the decoherence rate caused by the variance of the differential energy shift and photon jumping rate. Then, we constructed blue-detuned harmonic and quartic optical dipole traps, and experimentally investigated the coherence time of a trapped single cesium atom. The experimental results qualitatively verified our theory. Our approach provides a novel method to enhance the coherence time of optically trapped neutral atoms. △ Less

Submitted 28 February, 2025; originally announced February 2025.

Comments: 4 pages, 3 figures

Non-Hermitian ultra-strong bosonic clustering through interaction-induced caging

Authors: Mengjie Yang, Luqi Yuan, Ching Hua Lee

Abstract: We uncover a new mechanism whereby the triple interplay of non-Hermitian pumping, bosonic interactions and nontrivial band topology leads to ultra-strong bosonic condensation. The extent of condensation goes beyond what is naively expected from the interaction-induced trapping of non-Hermitian pumped states, and is based on an emergent caging mechanism that can be further enhanced by topological b… ▽ More We uncover a new mechanism whereby the triple interplay of non-Hermitian pumping, bosonic interactions and nontrivial band topology leads to ultra-strong bosonic condensation. The extent of condensation goes beyond what is naively expected from the interaction-induced trapping of non-Hermitian pumped states, and is based on an emergent caging mechanism that can be further enhanced by topological boundary modes. Beyond our minimal model with 2 bosons, this caging remains applicable for generic many-boson systems subject to a broad range of density interactions and non-Hermitian hopping asymmetry. Our novel new mechanism for particle localization and condensation would inspire fundamental shifts in our comprehension of many-body non-Hermitian dynamics and opens new avenues for controlling and manipulating bosons. △ Less

Submitted 18 February, 2025; v1 submitted 2 October, 2024; originally announced October 2024.

Comments: Any comments are welcome

Journal ref: Commun Phys 8, 388 (2025)

Microsatellite-based real-time quantum key distribution

Authors: Yang Li, Wen-Qi Cai, Ji-Gang Ren, Chao-Ze Wang, Meng Yang, Liang Zhang, Hui-Ying Wu, Liang Chang, Jin-Cai Wu, Biao Jin, Hua-Jian Xue, Xue-Jiao Li, Hui Liu, Guang-Wen Yu, Xue-Ying Tao, Ting Chen, Chong-Fei Liu, Wen-Bin Luo, Jie Zhou, Hai-Lin Yong, Yu-Huai Li, Feng-Zhi Li, Cong Jiang, Hao-Ze Chen, Chao Wu , et al. (16 additional authors not shown)

Abstract: A quantum network provides an infrastructure connecting quantum devices with revolutionary computing, sensing, and communication capabilities. As the best-known application of a quantum network, quantum key distribution (QKD) shares secure keys guaranteed by the laws of quantum mechanics. A quantum satellite constellation offers a solution to facilitate the quantum network on a global scale. The M… ▽ More A quantum network provides an infrastructure connecting quantum devices with revolutionary computing, sensing, and communication capabilities. As the best-known application of a quantum network, quantum key distribution (QKD) shares secure keys guaranteed by the laws of quantum mechanics. A quantum satellite constellation offers a solution to facilitate the quantum network on a global scale. The Micius satellite has verified the feasibility of satellite quantum communications, however, scaling up quantum satellite constellations is challenging, requiring small lightweight satellites, portable ground stations and real-time secure key exchange. Here we tackle these challenges and report the development of a quantum microsatellite capable of performing space-to-ground QKD using portable ground stations. The quantum microsatellite features a payload weighing approximately 23 kg, while the portable ground station weighs about 100 kg. These weights represent reductions by more than an order and two orders of magnitude, respectively, compared to the Micius satellite. Additionally, we multiplex bidirectional satellite-ground optical communication with quantum communication, enabling key distillation and secure communication in real-time. Using the microsatellite and the portable ground stations, we demonstrate satellite-based QKD with multiple ground stations and achieve the sharing of up to 0.59 million bits of secure keys during a single satellite pass. The compact quantum payload can be readily assembled on existing space stations or small satellites, paving the way for a satellite-constellation-based quantum and classical network for widespread real-life applications. △ Less

Submitted 20 August, 2024; originally announced August 2024.

Comments: 40 pages, 8 figures

Journal ref: Nature 640, 47-54 (2025)

Entanglement Criteria Based on Quantum Fisher Information

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

Abstract: To optimize the entanglement detection, we formulate the metrologically operational entanglement condition in quantum Fisher information by maximizing the QFI on the measurement orbit. Specifically, we consider two classes of typical local observables, i.e. the local orthonormal observables and symmetric informationally complete positive operator-valued measures. Result shows that the symmetric in… ▽ More To optimize the entanglement detection, we formulate the metrologically operational entanglement condition in quantum Fisher information by maximizing the QFI on the measurement orbit. Specifically, we consider two classes of typical local observables, i.e. the local orthonormal observables and symmetric informationally complete positive operator-valued measures. Result shows that the symmetric informationally complete positive operator-valued measures are superior to local orthonormal observables in entanglement detection, which in some sense hints the yet unconfirmed generally superiority of symmetric informationally complete positive operator-valued measures in quantum information processing. △ Less

Submitted 25 October, 2024; v1 submitted 22 July, 2024; originally announced July 2024.

Comments: 20 pages, 3 figures. Close to the published version

Journal ref: Physics Letters A 527 (2024) 130003

Direct generation of multi-photon hyperentanglement

Authors: Peng Zhao, Jia-Wei Ying, Meng-Ying Yang, Wei Zhong, Ming-Ming Du, Shu-Ting Shen, Yun-Xi Li, An-Lei Zhang, Lan Zhou, Yu-Bo Sheng

Abstract: Multi-photon hyperentangement is of fundamental importance in optical quantum information processing. Existing theory and experiment producing multi-photon hyperentangled states have until now relied on the outcome post-selection, a procedure where only the measurement results corresponding to the desired state are considered. Such approach severely limits the usefulness of the resulting hyperenta… ▽ More Multi-photon hyperentangement is of fundamental importance in optical quantum information processing. Existing theory and experiment producing multi-photon hyperentangled states have until now relied on the outcome post-selection, a procedure where only the measurement results corresponding to the desired state are considered. Such approach severely limits the usefulness of the resulting hyperentangled states. We present the protocols of direct production of three- and four-photon hyperentanglement and extend the approach to an arbitrary number of photons through a straightforward cascade of spontaneous parametric down-conversion (SPDC) sources. The generated multi-photon hyperentangled states are encoded in polarization-spatial modes and polarization-time bin degrees of freedom, respectively. Numerical calculation shows that if the average photon number $μ$ is set to 1, the down conversion efficiency is $7.6*10^{-6}$ and the repetition frequency of the laser is $10^9$ Hz, the number of the generation of three-photon and four-photon hyperentanglement after cascading can reach about $5.78*10^{-2}$ and $4.44*10^{-7}$ pairs per second, respectively. By eliminating the constraints of outcome post-selection, our protocols may represent important progresses for multi-photon hyperentangement generation and providing a pivotal role in future multi-party and high-capacity communication networks. △ Less

Submitted 12 June, 2024; originally announced June 2024.

Characterizing Biphoton Spatial Wave Function Dynamics with Quantum Wavefront Sensing

Authors: Yi Zheng, Zhao-Di Liu, Rui-Heng Miao, Jin-Ming Cui, Mu Yang, Xiao-Ye Xu, Jin-Shi Xu, Chuan-Feng Li, Guang-Can Guo

Abstract: With an extremely high dimensionality, the spatial degree of freedom of entangled photons is a key tool for quantum foundation and applied quantum techniques. To fully utilize the feature, the essential task is to experimentally characterize the multiphoton spatial wave function including the entangled amplitude and phase information at different evolutionary stages. However, there is no effective… ▽ More With an extremely high dimensionality, the spatial degree of freedom of entangled photons is a key tool for quantum foundation and applied quantum techniques. To fully utilize the feature, the essential task is to experimentally characterize the multiphoton spatial wave function including the entangled amplitude and phase information at different evolutionary stages. However, there is no effective method to measure it. Quantum state tomography is costly, and quantum holography requires additional references. Here we introduce quantum Shack-Hartmann wavefront sensing to perform efficient and reference-free measurement of the biphoton spatial wave function. The joint probability distribution of photon pairs at the back focal plane of a microlens array is measured and used for amplitude extraction and phase reconstruction. In the experiment, we observe that the biphoton amplitude correlation becomes weak while phase correlation shows up during free-space propagation. Our work is a crucial step in quantum physical and adaptive optics and paves the way for characterizing quantum optical fields with high-order correlations or topological patterns. △ Less

Submitted 16 July, 2024; v1 submitted 7 June, 2024; originally announced June 2024.

Comments: Main text: 6 pages, 4 figures; Supplemental Material: 13 pages, 11 figures. (c) 2024 American Physical Society

Journal ref: Phys. Rev. Lett. 133, 033602 (2024)

Multi-Stage Watermarking for Quantum Circuits

Authors: Min Yang, Xiaolong Guo, Lei Jiang

Abstract: Quantum computing represents a burgeoning computational paradigm that significantly advances the resolution of contemporary intricate problems across various domains, including cryptography, chemistry, and machine learning. Quantum circuits tailored to address specific problems have emerged as critical intellectual properties (IPs) for quantum computing companies, attributing to the escalating com… ▽ More Quantum computing represents a burgeoning computational paradigm that significantly advances the resolution of contemporary intricate problems across various domains, including cryptography, chemistry, and machine learning. Quantum circuits tailored to address specific problems have emerged as critical intellectual properties (IPs) for quantum computing companies, attributing to the escalating commercial value of quantum computing. Consequently, designing watermarking schemes for quantum circuits becomes imperative to thwart malicious entities from producing unauthorized circuit replicas and unlawfully disseminating them within the market. Unfortunately, the prevailing watermarking technique reliant on unitary matrix decomposition markedly inflates the number of 2-qubit gates and circuit depth, thereby compromising the fidelity of watermarked circuits when embedding detectable signatures into the corresponding unitary matrices. In this paper, we propose an innovative multi-stage watermarking scheme for quantum circuits, introducing additional constraints across various synthesis stages to validate the ownership of IPs. Compared to the state-of-the-art watermarking technique, our multi-stage watermarking approach demonstrates, on average, a reduction in the number of 2-qubit gates by 16\% and circuit depth by 6\%, alongside an increase in the fidelity of watermarked circuits by 8\%, while achieving a 79.4\% lower probabilistic proof of authorship. △ Less

Submitted 27 April, 2024; originally announced April 2024.

Spin Theory Based on the Extended Least Action Principle and Information Metrics: Quantization, Entanglement, and Bell Test With Time Delay

Authors: Jianhao M. Yang

Abstract: Quantum theory of electron spin is developed here based on the extended least action principle and assumptions of intrinsic angular momentum of an electron with random orientations. The novelty of the formulation is the introduction of relative entropy for the random orientations of intrinsic angular momentum when extremizing the total actions. Applying recursively this extended least action princ… ▽ More Quantum theory of electron spin is developed here based on the extended least action principle and assumptions of intrinsic angular momentum of an electron with random orientations. The novelty of the formulation is the introduction of relative entropy for the random orientations of intrinsic angular momentum when extremizing the total actions. Applying recursively this extended least action principle, we show that the quantization of electron spin is a mathematical consequence when the order of relative entropy approaches a limit. In addition, the formulation of the measurement probability when a second Stern-Gerlach apparatus is rotated relative to the first Stern-Gerlach apparatus, and the Schrödinger-Pauli equation, are recovered successfully. Furthermore, the principle allows us to provide an intuitive physical model and formulation to explain the entanglement phenomenon between two electron spins. In this model, spin entanglement is the consequence of the correlation between the random orientations of the intrinsic angular momenta of the two electrons. Since spin orientation is an intrinsic local property of the electron, the correlation of spin orientations can be preserved and manifested even when the two electrons are remotely separated. The entanglement of a spin singlet state is represented by two joint probability density functions that reflect the orientation correlation. Using these joint probability density functions, we prove that the Bell-CHSH inequality is violated in a Bell test. To test the validity of the spin-entanglement model, a Bell test experiment with time delay is proposed. As the time delay increases, we predict that the Bell-CHSH inequality changes from being violated to non-violated. △ Less

Submitted 16 December, 2024; v1 submitted 21 April, 2024; originally announced April 2024.

Comments: Closely match the accepted version. Appendix C is added to explain the choice of relative entropy. 25 pages, 7 figures. This paper is related to arXiv:2302.14619 and arXiv:2310.02274

Journal ref: Int. J. Theor. Phys. 64:16 (2025)

Active Quantum Distillation

Authors: Muchun Yang, D. L. Zhou

Abstract: Quantum distillation is a modern technology to decrease the von Neumann entropy of a subsystem by coherent system dynamics. Here we propose an active quantum distillation protocol, in which a bang-bang theme is applied to actively control the coherent dynamics of our system in order to obtain a subsystem with the von Neumann entropy as low as possible. For a bipartite Bosonic system, we derive the… ▽ More Quantum distillation is a modern technology to decrease the von Neumann entropy of a subsystem by coherent system dynamics. Here we propose an active quantum distillation protocol, in which a bang-bang theme is applied to actively control the coherent dynamics of our system in order to obtain a subsystem with the von Neumann entropy as low as possible. For a bipartite Bosonic system, we derive the analytical expression of lower bound of the entropy of subsystem under any unitary transformation with conservation of particles. The lower bound is validated by numerical simulations on the Bose-Hubbard model, where the coherent evolution is controlled by tuning one interaction term of the Hamiltonian. Our protocol can be used to decrease the entropy of one subsystem lower than the total bipartite state and increase the number of Bosons or only distill out very few Bosons in the subsystem. △ Less

Submitted 21 May, 2024; v1 submitted 17 April, 2024; originally announced April 2024.

Comments: 13 pages, 6 figures

Journal ref: Phys. Rev. A 110, 012622 (2024)

Optimized Quantum Autoencoder

Authors: Yibin Huang, Muchun Yang, D. L. Zhou

Abstract: Quantum autoencoder (QAE) compresses a bipartite quantum state into its subsystem by a self-checking mechanism. How to characterize the lost information in this process is essential to understand the compression mechanism of QAE\@. Here we investigate how to decrease the lost information in QAE for any input mixed state. We theoretically show that the lost information is the quantum mutual informa… ▽ More Quantum autoencoder (QAE) compresses a bipartite quantum state into its subsystem by a self-checking mechanism. How to characterize the lost information in this process is essential to understand the compression mechanism of QAE\@. Here we investigate how to decrease the lost information in QAE for any input mixed state. We theoretically show that the lost information is the quantum mutual information between the remaining subsystem and the ignorant one, and the encoding unitary transformation is designed to minimize this mutual information. Further more, we show that the optimized unitary transformation can be decomposed as the product of a permutation unitary transformation and a disentanglement unitary transformation, and the permutation unitary transformation can be searched by a regular Young tableau algorithm. Finally we numerically identify that our compression scheme outperforms the quantum variational circuit based QAE\@. △ Less

Submitted 12 April, 2024; originally announced April 2024.

QuantumLeak: Stealing Quantum Neural Networks from Cloud-based NISQ Machines

Authors: Zhenxiao Fu, Min Yang, Cheng Chu, Yilun Xu, Gang Huang, Fan Chen

Abstract: Variational quantum circuits (VQCs) have become a powerful tool for implementing Quantum Neural Networks (QNNs), addressing a wide range of complex problems. Well-trained VQCs serve as valuable intellectual assets hosted on cloud-based Noisy Intermediate Scale Quantum (NISQ) computers, making them susceptible to malicious VQC stealing attacks. However, traditional model extraction techniques desig… ▽ More Variational quantum circuits (VQCs) have become a powerful tool for implementing Quantum Neural Networks (QNNs), addressing a wide range of complex problems. Well-trained VQCs serve as valuable intellectual assets hosted on cloud-based Noisy Intermediate Scale Quantum (NISQ) computers, making them susceptible to malicious VQC stealing attacks. However, traditional model extraction techniques designed for classical machine learning models encounter challenges when applied to NISQ computers due to significant noise in current devices. In this paper, we introduce QuantumLeak, an effective and accurate QNN model extraction technique from cloud-based NISQ machines. Compared to existing classical model stealing techniques, QuantumLeak improves local VQC accuracy by 4.99\%$\sim$7.35\% across diverse datasets and VQC architectures. △ Less

Submitted 15 March, 2024; originally announced March 2024.

Journal ref: published in IJCNN 2024

Cabello's nonlocality argument for multisetting high-dimensional systems and its experimental test

Authors: Ming Yang, Dongkai Zhang, Lixiang Chen

Abstract: Recent advancements have expanded Hardy's nonlocality arguments into multisetting and multidimensional systems to enhance quantum correlations. In comparison with Hardy's nonlocal argument, Cabello's nonlocal argument (CNA) emerges as a superior choice for illustrating nonlocal features. An open question persists regarding the potential extension of CNA to arbitrary (k, d) scenarios. Here, we answ… ▽ More Recent advancements have expanded Hardy's nonlocality arguments into multisetting and multidimensional systems to enhance quantum correlations. In comparison with Hardy's nonlocal argument, Cabello's nonlocal argument (CNA) emerges as a superior choice for illustrating nonlocal features. An open question persists regarding the potential extension of CNA to arbitrary (k, d) scenarios. Here, we answer this question both in theory and experiment. Theoretically, by utilizing compatibility graphs, we construct a new logical framework for multisetting and multidimensional CNA, demonstrating an increase in the maximum successful probability with setting k and dimension d. Experimentally, by employing controllable photonic orbital angular momentum entanglement, we exhibit nonlocality with an experimentally recorded probability of 20.29% in the (2, 4) scenario and 28.72% in the (6, 2) scenario. Our work showcases a sharper contradiction between quantum mechanics and classical theory, surpassing the bound limited by the original version. △ Less

Submitted 24 March, 2026; v1 submitted 12 March, 2024; originally announced March 2024.

An Effective Way to Determine the Separability of Quantum State

Authors: Ma-Cheng Yang, Cong-Feng Qiao

Abstract: We propose in this work a practical approach to address the longstanding and challenging problem of quantum separability, leveraging the correlation matrices of generic observables. General separability conditions are obtained by dint of constructing the measurement-induced Bloch space, which in essence come from the intrinsic constraints in the space of quantum state. The novel approach can not o… ▽ More We propose in this work a practical approach to address the longstanding and challenging problem of quantum separability, leveraging the correlation matrices of generic observables. General separability conditions are obtained by dint of constructing the measurement-induced Bloch space, which in essence come from the intrinsic constraints in the space of quantum state. The novel approach can not only reproduce various established entanglement criteria, it may as well brings about some new results, possessing obvious advantages for certain bound entangled states and the high dimensional Werner states. Moreover, it is found that criteria obtained in our approach can be directly transformed into entanglement witness operators. △ Less

Submitted 2 May, 2025; v1 submitted 12 March, 2024; originally announced March 2024.

Comments: 16 pages, 3 figures

Journal ref: Quantum 9, 1734 (2025)

Quantum Uncertainty Equalities and Inequalities for Unitary Operators

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

Abstract: We explore the uncertainty relation for unitary operators in a new way and find two uncertainty equalities for unitary operators, which are minimized by any pure states. Additionally, we derive two sets of uncertainty inequalities that unveil hierarchical structures within the realm of unitary operator uncertainty. Furthermore, we examine and compare our method for unitary uncertainty relations to… ▽ More We explore the uncertainty relation for unitary operators in a new way and find two uncertainty equalities for unitary operators, which are minimized by any pure states. Additionally, we derive two sets of uncertainty inequalities that unveil hierarchical structures within the realm of unitary operator uncertainty. Furthermore, we examine and compare our method for unitary uncertainty relations to other prevailing formulations. We provide explicit examples for better understanding and clarity. Results show that the hierarchical unitary uncertainty relations establish strong bounds. Moreover, we investigate the higher-dimensional limit of the unitary uncertainty equalities. △ Less

Submitted 25 October, 2024; v1 submitted 14 January, 2024; originally announced January 2024.

Comments: 16 pages, 4 figures. Close to the published version

Journal ref: Quantum Information Processing (2024) 23:333

Can Bell inequalities be tested via scattering cross-section at colliders ?

Authors: Song Li, Wei Shen, Jin Min Yang

Abstract: In current studies for testing Bell inequalities at colliders, the reconstruction of spin correlations from scattering cross-sections relies on the bilinear form of the spin correlations, but not all local hidden variable models (LHVMs) have such a property. To demonstrate that a general LHVM cannot be rule out via scattering cross-section data, we propose a specific LHVM, which can exactly duplic… ▽ More In current studies for testing Bell inequalities at colliders, the reconstruction of spin correlations from scattering cross-sections relies on the bilinear form of the spin correlations, but not all local hidden variable models (LHVMs) have such a property. To demonstrate that a general LHVM cannot be rule out via scattering cross-section data, we propose a specific LHVM, which can exactly duplicate the same scattering cross-section for particle production and decay as the standard quantum theory, making it indistinguishable at colliders in principle. Despite of this, we find that reconstructing spin correlations through scattering cross-sections can still exclude a broad class of LHVMs, e.g., those models employing classical spin correlations as a surrogate for quantum spin correlations. △ Less

Submitted 4 July, 2024; v1 submitted 2 January, 2024; originally announced January 2024.

Comments: 14 pages, 2 figures

All-optical modulation with single-photons using electron avalanche

Authors: Demid V. Sychev, Peigang Chen, Yuheng Chen, Morris Yang, Colton Fruhling, Alexei Lagutchev, Alexander V. Kildishev, Alexandra Boltasseva, Vladimir M. Shalaev

Abstract: The distinctive characteristics of light, such as high-speed and low-loss propagation, low cross-talk and low power consumption, along with photons unique quantum properties, make it most suitable for various applications in communication, high-resolution imaging, optical computing, and emerging quantum information technologies. One limiting factor, though, is the weak optical nonlinearity of conv… ▽ More The distinctive characteristics of light, such as high-speed and low-loss propagation, low cross-talk and low power consumption, along with photons unique quantum properties, make it most suitable for various applications in communication, high-resolution imaging, optical computing, and emerging quantum information technologies. One limiting factor, though, is the weak optical nonlinearity of conventional media that poses challenges for the control of light with ultra-low intensities. In this work, we demonstrate all-optical modulation enabled by electron avalanche process in silicon, using a control beam with single-photon light intensities. The observed process corresponds to a record-high nonlinear refractive index of $n_{2}$~$1.3*10^{-2} m^2/W$, which is several orders of magnitude higher than the best known nonlinear optical materials. Our approach opens the possibility of gigahertz-speed, and potentially even faster, optical switching at the single-photon level, which could enable a family of novel on-chip photonic and quantum devices operating at room temperature. △ Less

Submitted 31 March, 2025; v1 submitted 18 December, 2023; originally announced December 2023.

Extending the coherence time limit of a single-alkali-atom qubit by suppressing phonon-jumping-induced decoherence

Authors: Zhuangzhuang Tian, Haobo Chang, Xin Lv, Mengna Yang, Zhihui Wang, Pengfei Yang, Pengfei Zhang, Gang Li, Tiancai Zhang

Abstract: In the fields of quantum metrology and quantum information processing with the system of optically trapped single neutral atoms, the coherence time of qubit encoded in the electronic states is regarded as one of the most important parameters. Longer coherence time is always pursued for higher precision of measurement and quantum manipulation. The coherence time is usually assumed to be merely dete… ▽ More In the fields of quantum metrology and quantum information processing with the system of optically trapped single neutral atoms, the coherence time of qubit encoded in the electronic states is regarded as one of the most important parameters. Longer coherence time is always pursued for higher precision of measurement and quantum manipulation. The coherence time is usually assumed to be merely determined by relative stability of the energy between the electronic states, and the analysis of the decoherence was conducted by treating the atom motion classically. We proposed a complete description of the decoherence of a qubit encoded in two ground electronic states of an optically trapped alkali atom by adopting a full description of the atomic wavefunction. The motional state, i.e., the phonon state, is taken into account. In addition to decoherence due to the variance of differential light shift (DLS), a new decoherence mechanism, phonon-jumping-induced decoherence (PJID), was discovered and verified experimentally. The coherence time of a single-cesium-atom qubit can be extended to $T_2\approx 20$ s by suppressing both the variances of DLS and PJID by trapping the atom in a blue-detuned bottle beam trap (BBT) and preparing the atom in its three-dimensional motional ground states. The coherence time is the longest for a qubit encoded in an optically trapped single alkali atom. Our work provides a deep understanding of the decoherence mechanism for single atom qubits and thus provides a new way to extend the coherence time limit. The method can be applied for other atoms and molecules, opening up new prospects for high-precision control the quantum states of optically trapped atoms or molecules. △ Less

Submitted 26 February, 2025; v1 submitted 18 December, 2023; originally announced December 2023.

Comments: 6 pages, 4 figures in the main text; 7 pages, 9 figures in the supplementary material

Journal ref: Optica 11, 1391-1396 (2024)

Realization of edge states along a synthetic orbital angular momentum dimension

Authors: Yu-Wei Liao, Mu Yang, Hao-Qing Zhang, Zhi-He Hao, Jun Hu, Tian-Xiang Zhu, Zong-Quan Zhou, Xi-Wang Luo, Jin-Shi Xu, Chuan-Feng Li, Guang-Can Guo

Abstract: The synthetic dimension is a rising method to study topological physics, which enables us to implement high-dimensional physics in low-dimensional geometries. Photonic orbital angular momentum (OAM), a degree of freedom characterized by discrete yet unbounded, serves as a suitable synthetic dimension. However, a sharp boundary along a synthetic OAM dimension has not been demonstrated, dramatically… ▽ More The synthetic dimension is a rising method to study topological physics, which enables us to implement high-dimensional physics in low-dimensional geometries. Photonic orbital angular momentum (OAM), a degree of freedom characterized by discrete yet unbounded, serves as a suitable synthetic dimension. However, a sharp boundary along a synthetic OAM dimension has not been demonstrated, dramatically limiting the investigation of topological edge effects in an open boundary lattice system. In this work, we make a sharp boundary along a Floquet Su-Schrieffer-Heeger OAM lattice and form approximate semi-infinite lattices by drilling a pinhole on the optical elements in a cavity. The band structures with zero ($\pmπ$) energy boundary states are measured directly, benefiting from the spectra detection of the cavity. Moreover, we obtain the edge modes moving from the gap to the bulk by dynamically changing the boundary phase, and we reveal that interference near the surface leads to spectrum discretization. Our work provides a new perspective to observe edge effects and explore practical photonics tools. △ Less

Submitted 29 November, 2023; originally announced November 2023.