arXiv subjects

Xiaoshui Lin

Publications and source records attributed to Xiaoshui Lin.

17 recordsLinked to original sources

arXiv2026

Analytic Theory of Phase Transitions in Optical Metamaterials

Optical metamaterials provide a versatile platform for engineering homogeneous electromagnetic media whose distinct phases are characterized by phase diagrams in constitutive-parameter space. However, existing studies of hyperbolicity, topological properties, and exceptional-point formation often rely on highly symmetric models or case-by-case numerical parameter scans, leaving a unified analytic framework that identifies phases and phase transitions directly from the constitutive tensors lacking. Here, we develop a general theory that yields exact analytic criteria for topological transitions, exceptional-point transitions, pinch-off Lifshitz transitions, and optical Lifshitz transitions in homogeneous media. Applying this framework to a tractable example of a gyroelectric medium with anisotropic chirality, we uncover exceptional rings and negative refraction induced by gyroelectric-chiral coupling. By enabling the exact determination of phase boundaries, our theory provides a predictive framework for discovering previously unexplored electromagnetic phases and offers new principles for the systematic design of optical metamaterials.

physics.optics↗

arXiv2026

Single-Ensemble Multiparameter Squeezing with Qudits

Quantum-enhanced multiparameter sensing is often associated with distributed architectures or 2-anticoherent states, whereas squeezing in a single collective ensemble is typically limited to single-parameter metrology. Here, we show that a single ensemble can support simultaneous multiparameter squeezing when each sensor is promoted from a qubit to a qudit (i.e., spin with $d$ energy levels). We develop a general framework in which the optimal product probe state, the corresponding global readout observables, and the associated squeezing parameters are all determined from the single-site quantum Fisher information matrix. We then present a minimal qudit construction for two-parameter vector magnetic field sensing with local dimension $d=3$. We further identify a collective twisting-like interacting Hamiltonian that generates such multiparameter-squeezed states and numerically demonstrate scalable metrological gain. In particular, for a trapped-ion qutrit chain with power-law interactions, we obtain up to 12 dB enhancement in two-parameter sensing for $N=256$ sensors. Our results establish qudit-enabled multiparameter squeezing in a single ensemble as a distinct route to multiparameter quantum metrology with global readout, and highlight its potential advantage over distributed multi-ensemble strategies in the fixed-sensor-budget regime.

quant-ph↗

arXiv2026

A Deep-Learning-Boosted Framework for Quantum Sensing with Nitrogen-Vacancy Centers in Diamond

Nitrogen-vacancy (NV) centers in diamond are a versatile quantum sensing platform for high sensitivity measurements of magnetic fields, temperature and strain with nanoscale spatial resolution. A common bottleneck is the analysis of optically detected magnetic resonance (ODMR) spectra, where target quantities are encoded in resonance features. Conventional nonlinear fitting is often computationally expensive, sensitive to initialization, and prone to failure at low signal-to-noise ratio (SNR). Here we introduce a robust, efficient machine learning (ML) framework for real-time ODMR analysis based on a one-dimensional convolutional neural network (1D-CNN). The model performs direct parameter inference without initial guesses or iterative optimization, and is naturally parallelizable on graphics processing units (GPU) for high-throughput processing. We validate the approach on both synthetic and experimental datasets, showing improved throughput, accuracy and robustness than standard nonlinear fitting, with the largest gains in the low-SNR regime. We further validate our methods in two representative sensing applications: diagnosing intracellular temperature changes using nanodiamond probes and widefield magnetic imaging of superconducting vortices in a high-temperature superconductor. This deep-learning inference framework enables fast and reliable extraction of physical parameters from complex ODMR data and provides a scalable route to real-time quantum sensing and imaging.

quant-ph↗

arXiv2025

Unilateral Criticality and Phase Transition in the Cavity-Ising Model

Superradiant phase transitions from cavity light-matter coupling have been widely explored across platforms. Here, we report a unilateral critical endpoint (UCEP) and a tricritical point (TCP) in the phase diagram of the cavity-coupled transverse Ising model with $\mathbb{Z}_2$ symmetry. At zero temperature, we demonstrate that this model hosts three phases separated by two second-order and one first-order transitions. These lines intersect at a TCP and a UCEP, the latter not captured by existing phase-transition paradigms. The UCEP displays one-sided criticality: approaching the point from one side, the system behaves as a second-order transition, while from the other side it is first-order. Correspondingly, two order parameters, respectively, undergo the first- and the second-order phase transitions at the same point. We construct a minimal description of UCEP with the density of the free energy $f = c_{1}(\tilde{\alpha}^{2}+c_{2})+(\tilde{\alpha}^{2}+c_{2})^{2}\ln{\vert\tilde{\alpha}^{2}+c_{2}\vert}$, with the UCEP at $(c_{1},c_{2})=(1/e,0)$ and $\tilde{\alpha}$ being the order parameter. We further map the finite-temperature phase diagram and perform a symmetry analysis. By unifying first- and second-order signatures in a single, direction-dependent endpoint, the UCEP introduces a qualitatively new class of phase transition and may have applications in fields such as quantum measurement and quantum sensing. This work also provides an intriguing platform for exploring novel critical phenomena in cavity-coupled many-body systems with or without dissipation.

quant-ph↗

arXiv2025

Cold atomic ensembles as quantum antennas for distributed networks of single-atom arrays

Single neutral atoms in optical tweezer arrays offer a promising platform for high-fidelity quantum computing at local nodes. Nonetheless, creating entanglement between remote nodes in a distributed quantum network remains challenging due to inherently weak atom-light coupling. Here, we design a distributed quantum network architecture in which cold atomic ensembles with strong atom-light interactions act as quantum antennas, interfacing single-atom qubits with flying photons to enable high-efficiency atom-photon entanglement generation -- analogous to the role of antennas in classical communication. Using realistic experimental parameters, we estimate an efficiency of $\eta \simeq 0.548$ for generating atom-photon entanglement, a probability of $P_{E} \simeq 6 \%$ for generating atom-atom entanglement, and a remote entanglement generation rate of $16.6 $ kHz. This performance not only surpasses that of state-of-the-art cavity-based or high-numerical-aperture-lens-based architectures but also offers notable advantages in simplicity, tunability, and experimental accessibility. Our scheme also integrates a long-lived quantum memory, providing a storage advantage for quantum repeater design. By leveraging the complementary strengths of single-atom qubits for local operations and cold atomic ensembles for networking, this approach paves the way for scalable distributed quantum computing and sensing.

quant-ph↗

arXiv2025

Soliton and traveling wave solutions in coupled one-dimensional condensates

Ultracold condensates provide a unique platform for exploring soliton physics. Motivated by the recent experiments realizing the sine-Gordon model in a split one-dimensional (1D) BEC, we demonstrate that this system naturally supports various density and phase solitons. We explore the physics using the bosonization technique, in which the phase and density are conjugate pairs, and determine its effective Language equation and the associated equation of motion. We show that in the presence of asymmetry between the two condensates, new solutions beyond those in the sine-Gordon model emerge. We calculate the traveling wave solutions and soliton solutions in this model and determine their corresponding energy densities analytically. Finally, we discuss the relevance of these solutions to the experiments and discuss their observations. This theory does not rely on the mechanism of quasi-particle excitation, which yields the Lee-Huang-Yang correction in higher dimensions, and is thus much more suitable to describe the physics in 1D systems. Since the physical models have already been realized in experiments, this work opens a new frontier for the realization of various soliton and periodic solutions using two coupled condensates.

cond-mat.quant-gas↗

arXiv2025

Exact mobility edges in quasiperiodic network models with slowly varying potentials

Quasiperiodic models are important physical platforms to explore Anderson transitions in low dimensional systems, yet the exact mobility edges (MEs) are generally hard to be determined analytically. To date, the MEs in only a few models can be determined exactly. In this manuscript, we propose a new class of network models characterized by quasiperiodic slowly varying potentials and the absence of hidden self-duality, and exactly determine their MEs. We take the mosaic models with slowly varying potentials as examples to illustrate this result and derive its MEs from the effective Hamiltonian. In this method, we can integrate out the periodic sites to obtain an effective Hamiltonian with energy-dependent potentials $g(E)V$ and effective eigenenergy $f(E)$, which directly yields the MEs at $f(E) = \pm(2t^\kappa \pm g(E)V)$, where $\kappa \in \mathbb{Z}^+$. With this idea in hand, we then generalize our method to more quasiperiodic network models, including those with much more complicated geometries and non-Hermitian features. Finally, we propose the realization of these models using optical waveguides and show that the Anderson transition can be observed even in small physical systems (with lattice sites about $L = 50 - 100$). Our results provide some key insights into the understanding and realization of exact MEs in experiments.

cond-mat.dis-nn↗

arXiv2024

Hidden self-duality and exact mobility edges in quasiperiodic network models

In one-dimensional quasiperiodic systems, only a few models with exact mobility edges (MEs) have been constructed using generalized self-duality theory, Avila's global theory, or the renormalization group method. This raises an intriguing question that whether we can realize more physical models with exact solvable MEs. In this work, we uncover the hidden self-duality within a class of quasiperiodic network models constituted by periodic and quasiperiodic sites. Although the original Hamiltonians appear to lack self-duality, their effective Hamiltonians obtained by integrating out the periodic sites exhibit self-duality, which yield MEs. The well-studied mosaic model, which is the simplest case of quasiperiodic network models, was previously thought to exhibit MEs due to the absence of self-duality, but we show that they actually arise from the hidden self-duality. Using the effective Hamiltonian, we further introduce the concept of resonant states to understand the shape of MEs. Finally, we present in detail how to determine the MEs in various network models, including some non-Hermitian models, based on the hidden self-duality. These predictions can be experimentally realized using optical and acoustic waveguide arrays. Our work can greatly advance our understanding of MEs in Anderson transition.

cond-mat.dis-nn↗

arXiv2024

Landau-Zener-St\"uckelberg interference in edge state pumping

The adiabatic edge state pumping (ESP) in one dimensional model, which has important applications in topological phase transition and quantum simulation, has been widely performed in both theories and experiments. This phenomenon has been observed in some systems with sizes $L = 9 - 100$, and it seems that due to the topological protection, the ESP can be survived even in the presence of weak random potential. Yet the fundamental issues of adiabaticity for this process have not been clarified. In this paper, we revisit this problem and show that this process involves two non-adiabatic points during the transition between the edge state and bulk state, yielding non-abadiatic physics. As a result, the ESP can be described by the Landau-Zener-St\"{u}ckelberg (LZS) interference process, in which the relative phase between the edge state and the bulk state determine the fate of the edge state during pumping. Furthermore, in a relatively long chain with weak disorder, the ESP can break down due to the anti-crossing of the edge state and the bulk edge states. We unveil these physics in terms of non-adiabaticity. The new mechanisms for ESP unveiled in this work is readily accessible in experiment, and shall therefore offer a down-to-earth platform for the intriguing LZS dynamics in terms of edge states.

quant-ph↗

arXiv2024

Interacting Mathieu equation, synchronization dynamics and collision-induced velocity exchange in trapped ions

Recently, large-scale trapped ion systems have been realized in experiments for quantum simulation and quantum computation. They are the simplest systems for dynamical stability and parametric resonance. In this model, the Mathieu equation plays the most fundamental role for us to understand the stability and instability of a single ion. In this work, we investigate the dynamics of trapped ions with the Coulomb interaction based on the Hamiltonian equation. We show that the many-body interaction will not influence the phase diagram for instability. Then, the dynamics of this model in the large damping limit will also be analytically calculated using few trapped ions. Furthermore, we find that in the presence of modulation, synchronization dynamics can be observed, showing an exchange of velocities between distant ions on the left side and on the right side of the trap. These dynamics resemble to that of the exchange of velocities in Newton's cradle for the collision of balls at the same time. These dynamics are independent of their initial conditions and the number of ions. As a unique feature of the interacting Mathieu equation, we hope this behavior, which leads to a quasi-periodic solution, can be measured in current experimental systems. Finally, we have also discussed the effect of anharmonic trapping potential, showing the desynchronization during the collision process. It is hopped that the dynamics in this many-body Mathieu equation with damping may find applications in quantum simulations. This model may also find interesting applications in dynamics systems as a pure mathematical problem, which may be beyond the results in the Floquet theorem.

physics.class-ph↗

arXiv2024

Simulation of open quantum systems on universal quantum computers

The rapid development of quantum computers has enabled demonstrations of quantum advantages on various tasks. However, real quantum systems are always dissipative due to their inevitable interaction with the environment, and the resulting non-unitary dynamics make quantum simulation challenging with only unitary quantum gates. In this work, we present an innovative and scalable method to simulate open quantum systems using quantum computers. We define an adjoint density matrix as a counterpart of the true density matrix, which reduces to a mixed-unitary quantum channel and thus can be effectively sampled using quantum computers. This method has several benefits, including no need for auxiliary qubits and noteworthy scalability. Moreover, some long-time properties like steady states and the thermal equilibrium can also be investigated as the adjoint density matrix and the true dissipated one converge to the same state. Finally, we present deployments of this theory in the dissipative quantum $XY$ model for the evolution of correlation and entropy with short-time dynamics and the disordered Heisenberg model for many-body localization with long-time dynamics. This work promotes the study of real-world many-body dynamics with quantum computers, highlighting the potential to demonstrate practical quantum advantages.

quant-ph↗

arXiv2023

On the different Floquet Hamiltonians in a periodic-driven Bose-Josephson junction

The bosonic Josephson junction, one of the maximally simple models for periodic-driven many-body systems, has been intensively studied in the past two decades. Here, we revisit this problem with five different methods, all of which have solid theoretical reasoning. We find that to the order of $\omega^{-2}$ ($\omega$ is the modulating frequency), these approaches will yield slightly different Floquet Hamiltonians. In particular, the parameters in the Floquet Hamiltonians may be unchanged, increased, or decreased, depending on the approximations used. Especially, some of the methods generate new interactions, which still preserve the total number of particles; and the others do not. The validity of these five effective models is verified using dynamics of population imbalance and self-trapping phase transition. In all results, we find the method by first performing a unitary rotation to the Hamiltonian will have the highest accuracy. The difference between them will become significate when the modulating frequency is comparable with the driving amplitude. The results presented in this work indicate that the analysis of the Floquet Hamiltonian has some kind of subjectivity, which will become an important issue in future experiments with the increasing of precision. We demonstrate this physics using a Bose-Josephson junction, and it is to be hoped that the validity of these methods and their tiny differences put forward in this work can be verified in realistic experiments in future using quantum simulating platforms, including but not limited to ultracold atoms.

cond-mat.quant-gas↗

arXiv2023

Theory of mobility edge and non-ergodic extended phase in coupled random matrices

The mobility edge, as a central concept in disordered models for localization-delocalization transitions, has rarely been discussed in the context of random matrix theory (RMT). Here we report a new class of random matrix model by direct coupling between two random matrices, showing that their overlapped spectra and un-overlapped spectra exhibit totally different scaling behaviors, which can be used to construct tunable mobility edges. This model is a direct generalization of the Rosenzweig-Porter model, which hosts ergodic, localized, and non-ergodic extended (NEE) phases. A generic theory for these phase transitions is presented, which applies equally well to dense, sparse, and even corrected random matrices in different ensembles. We show that the phase diagram is fully characterized by two scaling exponents, and they are mapped out in various conditions. Our model provides a general framework to realize the mobility edges and non-ergodic phases in a controllable way in RMT, which pave avenue for many intriguing applications both from the pure mathematics of RMT and the possible implementations of ME in many-body models, chiral symmetry breaking in QCD and the stability of the large ecosystems.

cond-mat.dis-nn↗

arXiv2023

Fate of localization in coupled free chain and disordered chain

It has been widely believed that almost all states in one-dimensional (1d) disordered systems with short-range hopping and uncorrelated random potential are localized. Here, we consider the fate of these localized states by coupling between a disordered chain (with localized states) and a free chain (with extended states), showing that states in the overlapped and un-overlapped regimes exhibit totally different localization behaviors, which is not a phase transition process. In particular, while states in the overlapped regime are localized by resonant coupling, in the un-overlapped regime of the free chain, significant suppression of the localization with a prefactor of $\xi^{-1} \propto t_v^4/\Delta^4$ appeared, where $t_v$ is the inter-chain coupling strength and $\Delta$ is the energy shift between them. This system may exhibit localization lengths that are comparable with the system size even when the potential in the disordered chain is strong. We confirm these results using the transfer matrix method and sparse matrix method for systems $L \sim 10^6 - 10^9$. These findings extend our understanding of localization in low-dimensional disordered systems and provide a concrete example, which may call for much more advanced numerical methods in high-dimensional models.

cond-mat.dis-nn↗

arXiv2023

From single-particle to many-body mobility edges and the fate of overlapped spectra in coupled disorder models

Mobility edge (ME) has played an essential role in disordered models. However, while this concept has been well established in disordered single-particle models, its existence in disordered many-body models is still under controversy. Here, a general approach based on coupling between extended and localized states in their overlapped spectra for ME is presented. We show that in the one-dimensional (1d) disordered single-particle models, all states are localized by direct coupling between them. However, in $d \ge 2$ disordered single-particle and 1d disordered many-body models, the resonant hybridization between these states in their overlapped spectra makes all states be extended, while these in the un-overlapped spectra are unchanged, leading to tunable MEs. We propose several models, including two disordered many-body spin models, to verify this mechanism. Our results establish a unified mechanism for MEs and demonstrate its universality in single-particle and many-body models, which opens an intriguing avenue for the realization and verification of MEs in many-body localization.

cond-mat.dis-nn↗

arXiv2022

Numerical issues of the two-dimensional Dirac equation

The two-dimensional Dirac equation has been widely used in graphene physics, the surface of topological insulators, and especially quantum scarring. Although a numerical approach to tackling an arbitrary confining problem was proposed several years ago, several fundamental issues must be thoroughly understood and solved. In this work, we conceal and address these challenges and finally develop a complete method, validated by comparison with analytical results.

physics.comp-ph↗

arXiv2022

The general approach to the critical phase with coupled quasiperiodic chains

In disordered systems, wave functions in the Schr\"{o}dinger equation may exhibit a transition from the extended phase to the localized phase, in which the states at the boundaries or mobility edges may exhibit multifractality. Meanwhile, the Critical Phase (CP), where all states exhibit multifractal structures, has also attracted much attention in the past decades. However, a generic way to construct the CP on demand still remains elusive. Here, a general approach for this phase is presented using two coupled quasiperiodic chains, where the chains are chosen so that before coupling one of them has extended states while the other one has localized states. We demonstrate the existence of CP in the overlapped spectra in the presence of inter-chain coupling using fractal dimension and minimal scaling index based on multifractal analysis. Then we examine the generality of this physics by changing the forms of inter-chain coupling and quasiperiodic potential, where the CP also emerges in the overlapped spectra. We account for the emergence of this phase as a result of effective unbounded potential, which yields singular continuous spectra and excludes the extended states in the overlapped regimes. Finally, the realization of this CP in the continuous model using ultracold atoms with bichromatic incommensurate optical lattice is also discussed. Due to the tunability of the two chains, this work provides a general approach to realizing the CP in a tunable way. This approach may have wide applications in the experimental detection of CP and can be generalized to much more intriguing physics in the presence of interaction for the many-body CP.

quant-ph↗