arXiv subjects

Yong-Chun Liu

Publications and source records attributed to Yong-Chun Liu.

At least 19 recordsLinked to original sources

arXiv2026

Nuclear Spin Squeezing Based on Spin-Exchange Collisions

Isolation from environment leads to the days-long lifetime of noble-gas nuclear spins, but also brings great challenges to the preparation, manipulation, and measurement of nuclear-spin quantum states. Here we find that nuclear spin squeezing, with ultra-long lifetime and huge atomic number, can be efficiently obtained and manipulated based on the coherent spin-exchange interaction between alkali-metal and noble-gas ensembles. Thanks to the considerable advantage of our proposal in preparing the spin squeezing of the macroscopic atomic ensemble with a huge atomic number, even the nuclear spin-squeezed state containing 10^20 atoms or more is obtainable with preexisting techniques. Further, the days-long storage and measurement of nuclear spin squeezing can be performed by the coherent manipulation of a magnetic field. This proposal can be implemented in hot atomic ensembles, whose ease of access and high adaptability to various environments will significantly facilitate the research and application of nuclear-spin nonclassical states in precision measurement, quantum information, and fundamental physics.

quant-ph↗

arXiv2026

Non-Hermitian reshaping of high-order Landau modes

When charged particles are subjected to strong magnetic fields, they form discrete energy levels known as Landau levels. The Landau levels consist of a series of degenerate states of Landau modes, making them a promising platform for large-capacity information processing. However, to date, exploiting the high-order Landau modes and control their spatial distributions has remained elusive. Here, we propose to construct magnetic fields, electric fields, and imaginary momentum simultaneously to reshape high-order Landau modes in non-Hermitian systems. By building a non-Hermitian electric circuit platform, we experimentally realize pseudomagnetic fields via inhomogeneous coupling and pseudoelectric fields via a gradient on-site potential, while simultaneously introducing an imaginary momentum via non-reciprocal coupling. We directly observe multi-frequency single-peak localization of high-order Landau modes. Our work provides a universal method for manipulating high-order Landau modes and exploring applications in nonHermitian systems, such as frequency multiplexing and wave packet reshaping.

physics.optics↗

arXiv2026

Metastable confinement in Rydberg lattice gauge theories

Confinement and string breaking are two fundamental phenomena in gauge theories. Signatures of both are currently pursued in quantum-simulator experiments, opening a new angle on strongly interacting dynamics of gauge fields out of equilibrium, complementary to traditional particle-physics settings. In this work, we report the emergence of metastable confinement dynamics in a U(1) lattice gauge theory, originating from the competition between string tension and four-Fermi coupling - a competition that naturally arises in Rydberg atom arrays. We show that the initial string state can be resonantly melted through controlled energy matching, a phenomenon we identify as resonant string breaking. We demonstrate this mechanism for both static and Floquet-driven systems, where periodic modulation generates a spectrum of tunable sideband resonances. Our work provides new insights into the mechanisms of confinement and string breaking driven by long-range interactions and time-dependent fields, which are available in current quantum simulators on a variety of platforms.

cond-mat.quant-gas↗

arXiv2026

Generating large-scale Greenberger-Horne-Zeilinger-like states in lattice spin systems

Greenberger-Horne-Zeilinger (GHZ) state is a typical maximally entangled state which is pursued in both fundamental research and emerging quantum technologies. Preparing large-scale GHZ states in lattice spin systems is particularly appealing for quantum advantages, but conventional schemes face great challenges in scalability. Here we propose a universal and scalable scheme to generate large-scale GHZ-like states, which share similar entanglement and metrological properties with standard GHZ states, in lattice spin systems through global Floquet engineering. Our scheme requires only global operations and shows great advantage for large particle number. It is applicable to systems with arbitrary interaction ranges, offering a practical pathway for large-scale implementation of many-body entangled states in various systems.

quant-ph↗

arXiv2025

General theory for geometry-dependent non-Hermitian bands

In two- and higher-dimensional non-Hermitian lattices, systems can exhibit geometry-dependent bands, where the spectrum and eigenstates under open boundary conditions depend on the bulk geometry even in the thermodynamic limit. Although geometry-dependent bands are widely observed, the underlying mechanism for this phenomenon remains unclear. In this work, we address this problem by establishing a higher-dimensional non-Bloch band theory based on the concept of "strip generalized Brillouin zones" (SGBZs), which describe the asymptotic behavior of non-Hermitian bands when a lattice is extended sequentially along its linearly independent axes. Within this framework, we demonstrate that geometry-dependent bands arise from the incompatibility of SGBZs and, for the first time, derive a general criterion for the geometry dependence of non-Hermitian bands: non-zero area of the complex energy spectrum or the imaginary momentum spectrum. Our work opens an avenue for future studies on the interplay between geometric effects and non-Hermitian physics, such as non-Hermitian band topology.

cond-mat.mes-hall↗

arXiv2025

A Solvable Semi-infinite Fock-state-lattice SSH Model: the Stable Topological Zero Mode and the Non-Hermitian Bound Effect

Fock-state lattice (FSL) offers a powerful quantum simulator for topological phenomena due to the unbounded scalability and ease of implementation. Nevertheless, the unique topological properties induced by its site-dependent coupling have remained elusive, mainly due to the challenge of handling an infinite state space without translational symmetry. Here, we rigorously analyze the topological features of a semi-infinite FSL-based Su-Schrieffer-Heeger (SSH) model, in both Hermitian and non-Hermitian realms, by mapping it to the solvable Jaynes-Cummings (JC) model via a unitary displacement transformation. We find a more stable topological zero mode than the conventional SSH model, originating from the bound state at the inherent domain wall under anisotropic conditions. With gain and loss introduced, we predict a non-Hermitian bound effect (NHBE), i. e., any state overlapping with the bound state will quickly stabilize to the domain wall, with the minimal stabilization time occurring in the vicinity of exceptional point (EP). The paritytime (PT ) phase transition can be observed by the oscillating-to-steady crossover of dynamics in the subspace orthogonal to the bound state. Furthermore, a concrete experimental proposal based on the trapped-ion setup is provided.

quant-ph↗

arXiv2025

Open-system analogy of Berry conjecture

Berry conjecture is central to understanding quantum chaos in isolated systems and foundational for the eigenstate thermalization hypothesis. Here we establish an open-system analogy of the Berry conjecture, connecting quantum steady states to classical dissipative attractors in the semiclassical limit. We demonstrate that the Wigner distribution of quantum steady states delocalizes over classical chaotic attractors in the semiclassical limit. We validate this correspondence using a Floquet Kerr oscillator. In the chaotic phase, the quasi-steady state is dominated by the chaotic delocalization instead of the quantum fluctuations, resulting in entropy divergence in the semiclassical limit. This entropy divergence provides a robust chaos signature beyond non-Hermitian random matrix approaches. We further identify dissipative phase transitions via Liouvillian gap closures, revealing a discrete time crystal phase and its breakdown into chaos at strong driving. Our framework thus establishes a universal paradigm for quantum chaos in open systems.

quant-ph↗

arXiv2025

Exact steady state of quantum van der Pol oscillator: critical phenomena and enhanced metrology

Quantum criticality of open many-body systems has attracted lots of interest for emergent phenomena and universality. Here we present the exact steady state of the quantum van der Pol oscillator using the complex $P$-representation. We show the threshold corresponds to a dissipative phase transition with abrupt changes of steady-state properties and enhanced metrology. The critical behaviors and finite-size effects are investigated through the analytical steady state. Moreover, we obtain divergent quantum Fisher information (QFI) in the thermodynamic limit both at the critical point and in the time crystal phase, but only the QFI at the critical point approaches the Heisenberg limit. We further prove that the steady-state photon number is the optimized estimated observable with the largest signal-to-noise ratio. We show that the Heisenberg-limited metrology originates from the larger enhancement of the susceptibility than the standard deviation of the photon number. Our work reveals the underlying relation between the time crystal, dissipative phase transition, and enhanced metrology.

quant-ph↗

arXiv2025

Geometric origin of self-intersection points in non-Hermitian energy spectra

Unlike Hermitian systems, non-Hermitian energy spectra under periodic boundary conditions can form closed loops in the complex energy plane, a phenomenon known as point gap topology. In this paper, we investigate the self-intersection points of such non-Hermitian energy spectra and reveal their geometric origins. We rigorously demonstrate that these self-intersection points result from the intersection of the auxiliary generalized Brillouin zone and the Brillouin zone in one-band systems, as confirmed by an extended Hatano-Nelson model. This finding is further generalized to multi-band systems, illustrated through a non-Hermitian Su-Schrieffer-Heeger model. Moreover, we address multiple self-intersection points and derive the geometric conditions for general n-fold self-intersection points. Our results enhance the fundamental understanding of generic non-Hermitian quantum systems and provide theoretical support for further experimental investigations of energy self-intersection points.

quant-ph↗

arXiv2025

Nuclear Spin Induced Transparency

Electromagnetically induced transparency (EIT) is an important quantum optical phenomenon which provides a crucial tool for light manipulation. However, typically the transparency window is broad, limited by the coherence time of the metastable state. Here we show that extremely narrow transparency window can be realized using nuclear spin induced transparency (NSIT), which is achieved by combining optical field, magnetic field and the spin-exchange interaction between noble-gas nuclear spins and alkali-metal electronic spins. The width of the NSIT window can be several orders of magnitude smaller than that of conventional EIT, and even reaches sub-mHz range due to the long coherence time of nuclear spins. The scheme holds great potential for applications in slow light and magnetic field sensing.

quant-ph↗

arXiv2024

Ultrastrong photon superbunching from electron shelving and time integral

Photon correlation is at the heart of quantum optics and has important applications in quantum technologies. Here we propose a universally applicable mechanism that can generate the superbunching light with ultrastrong second-order and higher-order correlations hitherto unreachable. This mechanism arises from the combined effect of electron shelving and time integral of fluorescence based on a cascaded quantum system comprising an emitter and a filter or a cavity QED system, and has high experimental feasibility according to current experimental techniques. Besides, both the correlation degrees and the frequency of the light can be flexibly varied over broad ranges. Both the research and technological applications on strong correlations can be extensively facilitated due to this readily accessible and manipulated mechanism for generating photon correlation.

quant-ph↗

arXiv2024

Revealing spontaneous symmetry breaking in continuous time crystals

Spontaneous symmetry breaking plays a pivotal role in physics ranging from the emergence of elementary particles to the phase transitions of matter. The spontaneous breaking of continuous time translation symmetry leads to a novel state of matter named continuous time crystal (CTC). It exhibits periodic oscillation without the need for periodic driving, and the relative phases for repetitively realized oscillations are random. However, the mechanism behind the spontaneous symmetry breaking in CTCs, particularly the random phases, remains elusive. Here we propose and experimentally realize two types of CTCs based on distinct mechanisms: manifold topology and near-chaotic motion. We observe both types of CTCs in thermal atomic ensembles by artificially synthesizing spin-spin nonlinear interactions through a measurement-feedback scheme. Our work provides general recipes for the realization of CTCs, and paves the way for exploring CTCs in various systems.

quant-ph↗

arXiv2024

Subnatural-linewidth fluorescent single photons

Subnatural-linewidth single photons are ofvital importance in quantum optics and quantum information science. According to previous research, it appears difficult to utilize resonance fluorescence to generate single photons with subnatural linewidth. Here we propose a universally applicable approach to generate fluorescent single photons with subnatural linewidth, which can be implemented based on Λ-shape and similar energy structures. Further, the general condition to obtain fluorescent single photons with subnatural linewidth is revealed. The single-photon linewidth can be easily manipulated over a broad range by external fields, which can be several orders ofmagnitude smaller than the natural linewidth. Our study can be easily implemented in various physical platforms with current experimental techniques and will significantly facilitate the research on the quantum nature of resonance fluorescence and the technologies in quantum information science.

quant-ph↗

arXiv2024

Fast Generation of GHZ-like States Using Collective-Spin XYZ Model

The Greenberger-Horne-Zeilinger (GHZ) state is a key resource for quantum information processing and quantum metrology. The atomic GHZ state can be generated by one-axis twisting (OAT) interaction $H_{\mathrm{OAT}}=χJ_{z}^{2}$ with $χ$ the interaction strength, but it requires a long evolution time $χt=π/2$ and is thus seriously influenced by decoherence and losses. Here we propose a three-body collective-spin XYZ model which creates a GHZ-like state in a very short timescale $χt\sim \ln {N}/N$ for $N$ particles. We show that this model can be effectively produced by applying Floquet driving to an original OAT Hamiltonian. Compared with the ideal GHZ state, the GHZ-like state generated using our model can maintain similar metrological properties reaching the Heisenberg-limited scaling, and it shows better robustness to decoherence and particle losses. This work opens the avenue for generating GHZ-like states with a large particle number, which holds great potential for the study of macroscopic quantum effects and for applications in quantum metrology and quantum information.

quant-ph↗

arXiv2024

Non-Hermitian skin effect and nonreciprocity induced by dissipative couplings

We study the mechanism for realizing non-Hermitian skin effect (NHSE) via dissipative couplings, in which the left-right couplings have equal strengths but the phases do not satisfy the complex conjugation. Previous realizations of NHSE typically require unequal left-right couplings or on-site gain and loss. In this work we find that when combined with the multichannel interference provided by a periodic dissipative-coherent coupling structure, the dissipative couplings can lead to unequal left-right couplings, inducing NHSE. Moreover, we show that the non-Hermiticity induced by dissipative couplings can be fully transformed into nonreciprocity-type non-Hermiticity without bringing extra gain-loss-type non-Hermiticity. Thus, this mechanism enables unidirectional energy transmission without introducing additional insertion loss. Our work opens a new avenue for the study of non-Hermitian topological effects and the design of directional optical networks.

quant-ph↗

arXiv2024

Time Crystal in a Single-mode Nonlinear Cavity

Time crystal is a class of non-equilibrium phases with broken time-translational symmetry. Here we demonstrate the time crystal in a single-mode nonlinear cavity. The time crystal originates from the self-oscillation induced by a linear gain and is stabilized by a nonlinear damping. We show in the time crystal phase there are sharp dissipative gap closing and pure imaginary eigenvalues of the Liouvillian spectrum in the thermodynamic limit. Dynamically, we observe a metastable regime with the emergence of quantum oscillation, followed by a dissipative evolution with a time scale much smaller than the oscillating period. Moreover, we show there is a dissipative phase transition at the Hopf bifurcation of the model, which can be characterized by the photon number fluctuation in the steady state. These results pave a new promising way for further experiments and deepen our understanding of time crystals.

quant-ph↗

arXiv2023

Nonlinear time-reversal interferometry with arbitrary quadratic collective-spin interaction

Atomic nonlinear interferometry has wide applications in quantum metrology and quantum information science. Here we propose a nonlinear time-reversal interferometry scheme with high robustness and metrological gain based on the spin squeezing generated by arbitrary quadratic collective-spin interaction, which could be described by the Lipkin-Meshkov-Glick (LMG) model. We optimize the squeezing process, encoding process, and anti-squeezing process, finding that the two particular cases of the LMG model, one-axis twisting and two-axis twisting outperform in robustness and precision, respectively. Moreover, we propose a Floquet driving method to realize equivalent time reverse in the atomic system, which leads to high performance in precision, robustness, and operability. Our study sets a benchmark in achieving high precision and robustness in atomic nonlinear interferometry.

quant-ph↗

arXiv2023

Spin Squeezing through Collective Spin-Spin Interactions

Spin squeezing provides crucial quantum resource for quantum metrology and quantum information science. Here we propose that one axis-twisted (OAT) spin squeezing can be generated from free evolution under a general coupled-spin model with collective spin-spin interactions. We further propose pulse schemes to recover squeezing from parameter imperfections, and reach the extreme squeezing with Heisenberg-limited measurement precision scaling as $1/N$ for $N$ particles. This work provides a feasible method for generating extreme spin squeezing.

quant-ph↗