arXiv subjects

Yu-Yu Zhang

Publications and source records attributed to Yu-Yu Zhang.

At least 19 recordsLinked to original sources

arXiv2026

Non-Hermiticity of an anomalous superradiant phase

We present a Hermitian squeezing-Dicke model as a minimal closed-system setting for non-Hermitian many-body physics. Squeezing generates an effective parity-time ($\mathcal{PT}$) symmetry and exchanges gainy and lossy eigenmodes without dissipation. We identify an anomalous dynamical superradiant phase (DSRP) that exhibits by a mean-field superradiant order in the $\mathcal{PT}$ symmetry-breaking regime, accompanied by a $\mathbb{Z}_2$-symmetry-breaking ground-state quantum phase transition. Additionally, it undergoes a $\mathcal{PT}$-breaking dynamical phase transition at an exceptional point (EP). The excitation energy vanishes near both boundaries, giving critical slowing down. The vacuum photon number is divergent at the ground-state transition but is finite and nonanalytic at the EP, distinguishing the DSRP from the conventional superradiant phase and from a standard dynamical instability. Squeezing causes the coupling phase to act as an artificial magnetic flux, which yields flux-controlled nonreciprocal quantum amplification in the DSRP. Our results offer an alternative pathway to non-Hermitian many-body physics and nonreciprocal quantum amplification in closed Hermitian systems.

quant-ph↗

arXiv2026

Anomalous superradiant phases in a quantum Rabi zigzag chain under a synthetic gauge field

We present the phase diagram of a one-dimensional quantum Rabi zigzag chain under an artificial gauge field, which is implemented via phase-modulated next-nearest-neighbor (NNN) hopping. For a system size of $N=8$, we analytically determine the order parameters and critical thresholds, identifying five distinct superradiant phases. For strong nearest-neighbor (NN) coupling, the system exhibits either a ferro-superradiant (FSR) or an antiferro-superradiant (AFSR) phase depending on its sign. Increasing NNN coupling induces a stripe antiferro-superradiant (sAFSR) phase with antiparallel NNN and parallel NN displacements, which is distinct from the conventional AFSR phase. Tuning the magnetic flux drives the system into chiral or antichiral superradiant phases with opposite ground-state photon currents, which are different from the conventional superradiant phases. Remarkably, geometric frustration within each triangular plaquette stabilizes chiral phases even at small magnetic flux while suppressing the sAFSR phase, which vanishes for $N=6$. Mapping the system onto an effective magnetic Hamiltonian yields an analogous phase diagram featuring distinct magnetic phases, offering insight into the anomalous superradiant phases. This equivalence establishes a quantum optical platform for simulating chiral magnetism.

quant-ph↗

arXiv2026

Anomalous topological superradiant phases

We present a novel set of light-matter topology realized by implementing a finite-component quantum Rabi array with a photonic analog of the Su-Schrieffer-Heeger (SSH) configuration. We demonstrate how complex light-matter couplings with species-dependent phases lead to the closure of superradiance-induced band gap in a manner that differs from that in the SSH model. We uncover an topological superradiant phase transition from a normal phase to a topological superradiant electromagnet phase, which is characterized both by a local order parameter and a global topological invariant. Novel superradiance-enhanced edge states emerge with significantly amplified excitations superior to those in topological normal phase. Strikingly, tuning light-atom coupling induces novel topological superradiant electric and magnetic phases, exhibiting chiral edge-mode excitation at opposite boundaries. Our proposed setup offers a tunable platform for topological quantum optics, advancing applications in topological superradiant lasers.

quant-ph↗

arXiv2025

Meissner-Like Currents of Photons in Anomalous Superradiant Phases

We present Meissner-like photon currents in a quantum Rabi zigzag chain under staggered synthetic magnetic fields. The ground state of the Meissner superradiant phase hosts persistent chiral edge currents in a sequence of cancellation of antiparallel vortex pairs, akin to surface currents of the Meissner effect in superconductors. The Meissner phase displays distinct vortex structures and anomalous scaling exponents, arising from geometric frustration effects. Modifying the staggered flux triggers transitions to even- or odd-vortex superradiant phases, where the chiral edge currents flow exclusively in even or odd cavities with localized vortices, respectively. Enhanced interspecies interactions induce the vanishing of currents in a ferromagnetic superradiant phase. Our results enable observation of stabilized photon vortices and edge currents with analogy to quantum Hall-like robustness in light-matter coupling systems.

quant-ph↗

arXiv2025

Higher-order exceptional lines in a non-Hermitian JaynesCummings triangle

Higher-order exceptional points (EPs) in non-Hermitian systems showcase diverse physical phenomena but require more parameter space freedom or symmetries. It leads to a challenge for the exploration of high-order EP geometries in low-dimensional systems. Here we observe both a third-order exceptional surface and line in a Jaynes-Cummings triangle consisting of three cavities arranged in a ring. A fine-tuning artificial magnetic field dramatically enriches the emergence of the third-order exceptional lines ($3$ELs), which require only three tuning parameters in the presence of chiral symmetry and parity-time (PT) symmetry. Third-order EPs amplify the effect of perturbations through a cube-root response mechanism, displaying a greater sensitivity than second-order EPs. We develop novel fidelity and Loschmidt echo using the associated-state biorthogonal approach, which successfully characterizes EPs and quench dynamics even in PT breaking regime. Our work advances the use of higher-order EPs in quantum technology applications.

quant-ph↗

arXiv2024

Phase Transitions in the Anisotropic Dicke-Stark Model with $A$-square terms

The superradiant phase transition (SRPT) is forbidden in the standard isotropic Dicke model due to the so-called no-go theorem induced by A-square term. In the framework of the Dicke model, we demonstrate that SRPTs can occur at both zero and finite temperatures if we intrinsically tune the rotating wave and count-rotating atom-cavity coupling independently, and/or introduce the nonlinear Stark coupling terms, thus overcoming the no-go theorem. The phase transitions in this so-called anisotropic Dicke-Stark model share the same universality class with the original Dicke model. The critical coupling strength of this model decreases with the isotropic constant gradually, but can be driven to zero quickly with the strong nonlinear Stark coupling. We believe that we have proposed a feasible scheme to observe the SRPT in the future solid-state experiments.

quant-ph↗

arXiv2024

Biorthogonal Dynamical Quantum Phase Transitions in Non-Hermitian Systems

By utilizing biorthogonal bases, we develop a comprehensive framework for studying biorthogonal dynamical quantum phase transitions in non-Hermitian systems. With the help of the previously overlooked associated state, we define the automatically normalized biorthogonal Loschmidt echo. This approach is capable of handling arbitrary non-Hermitian systems with complex eigenvalues and naturally eliminates the negative value of Loschmidt rate obtained without the biorthogonal bases. Taking the non-Hermitian Su-Schrieffer-Heeger model as a concrete example, a $1/2$ change of dynamical topological order parameter in biorthogonal bases is observed which is not shown in self-normal bases. Furthermore, we discover that the periodicity of biorthogonal dynamical quantum phase transitions depends on whether the two-level subsystem at the critical momentum oscillates or reaches a steady state.

quant-ph↗

arXiv2024

Quantum fluctuations and unusual critical exponents in a quantum Rabi Triangle

Quantum fluctuations of a quantum Rabi triangle are studied using an analytical approach beyond the mean-field theory. By applying an artificial magnetic field among three cavities, time-reversal symmetry breaking is manifested through a directional transfer dynamics of photons. In contrast to previous studies, we focus on the scaling exponents of the fluctuations of the local photon number and the position variance near the critical point. By accurate calculation using Bogoliubov transformation we show that two scaling laws emerge respectively for the frustrated cavity and the remaining cavities, which are associated with the geometric frustrations. Especially, for the frustrated cavity, the scaling exponent in the chiral superradiant phase is different from that in the frustrated antiferromagnetic superradiant phase without an artificial magnetic field. The unusual scaling exponents predict distinct universality classes from the single-cavity Rabi universality. We suggest that the accurate critical exponents in few-body system is useful for identifying exotic quantum phase transition in light-matter coupling system.

quant-ph↗

arXiv2024

Quantum tricriticality and universal scaling in a tricritical quantum Rabi system

Quantum tricriticality, a unique form of high-order criticality, is expected to exhibit fascinating features including unconventional critical exponents and universal scaling laws. However, a quantum tricritical point (QTCP) is much harder to access, and the corresponding phenomena at tricriticality have rarely been investigated. In this study, we explore a tricritical quantum Rabi model, which incorporates a nontrivial parameter for adjusting the coupling ratio between a cavity and a three-level atom. The QTCP emerges at the intersection of a first- and second-order superradiant phase transitions according to Landau theory. By using finite-frequency scaling analyses for quantum fluctuations and the mean photon number, universal critical exponents differentiate the QTCP from the second-order critical point. We find that the phase transition at the tricritical point goes beyond the conventional second-order phase transition. Our work explores an interesting direction in the generalization of the well-known Rabi model for the study of higher-order critical points due to its high control and tunability.

quant-ph↗

arXiv2023

Quantum Rabi hexagonal ring in an artificial magnetic field

We present exotic quantum phases in a quantum Rabi hexagonal ring, which is derived by an analytical solution. We find that an artificial magnetic field applied in the ring induces an effect magnetic flux in the even and odd subring. It gives rise to two chiral quantum phases besides a ferro-superradiant and an antiferro-superradiant phases. With analogy to the magnetic system, two chiral phases are distinguished by the magnetization orientation in the $xy$ plane in two subrings, which correspond to skyrmion structures with different vorticity. In such chiral phases, photons in the subrings triangle flow in the same or opposite directions by comparing to the current in the hexagonal ring, which depend on the signs of the induced magnetic flux in the subrings. Interestingly, the critical exponents of the excitation energy in two chiral phases are the same as that of the subring triangle, exhibiting subring-size dependent critical exponents. Our analysis can be straightforwardly extended to a larger lattice size with subrings of a triangular or hexagonal structure, predicting a novel universality class of superradiant phase transitions. An implementation of the system considered is an exciting prospect in quantum many-body simulations of light-matter interactions in future.

quant-ph↗

arXiv2022

Chiral Quantum Phases and Tricriticality in a Dicke Triangle

The existence of quantum tricriticality and exotic phases are found in a Dicke triangle (TDT) where three cavities, each one containing an ensemble of three-level atoms, are connected to each other through the action of an artificial magnetic field. The conventional superradiant phase (SR) is connected to the normal phase through first- and second-order boundaries, with tricritical points located at the intersection of such boundaries. Apart from the SR phase, a chiral superradiant (CSR) phase is found by tuning the artificial magnetic field. This phase is characterized by a nonzero photon current and its boundary presents chiral tricritical points (CTCPs). Through the study of different critical exponents, we are able to differentiate the universality class of the CTCP and TCP from that of second-order critical points, as well as find distinctive critical behavior among the two different superradiant phases. The TDT can be implemented in various systems, including atoms in optical cavities as well as the circuit QED system, allowing the exploration of a great variety of critical manifolds.

quant-ph↗

arXiv2022

Understanding the quantum Rabi ring using analogies to quantum magnetism

We map a quantum Rabi ring, consisting of $N$ cavities arranged in a ring geometry, into an effective magnetic model containing the XY exchange and the Dzyaloshinskii Moriya (DM) interactions. The analogue of the latter is induced by an artificial magnetic field, which modulates photon hopping between nearest-neighbor cavities with a phase. The mean-field behavior of both systems is almost identical, facilitating the description of the different phases in the quantum optical model through simple arguments of competing magnetic interactions. For the square geometry ($N=4$) the rich phase diagram exhibits three superradiant phases denoted as ferro-superradiant, antiferro-superradiant and chiral superradiant. In particular, the DM interaction is responsible for the chiral phase in which the energetically degenerate configurations of the order parameters are similar to the in-plane magnetizations of skyrmions with different helicities. The antiferro-superradiant phase is suppressed in the triangle geometry ($N=3$) as geometric frustration contributes to stabilize the chiral phase even for small values of the DM interaction. The chiral phases for odd and even $N$ show a different scaling behavior close to the phase transition. The equivalent behavior on both systems opens the possibility of simulating chiral magnetism in a few-body quantum optical platform, as well as understanding one system using the insights gained from the other.

quant-ph↗

arXiv2021

Possibility of superradiant phase transitions in coupled two-level atoms

Although the oscillator strength sum rule forbids the phase transition in ideal non-interacting two-level atoms systems, we present the possibility of the quantum phase transition in the coupled two-level atoms in a cavity. The system undergoes the superradiant phase transition in the thermodynamics limit and this transition is account for the atom-atom attractive interaction, exhibiting a violation of the sum rule. The bosonic coherent state technique has been adopted to locate the quantum critical point accurately in the finite-size system. We predict the existence of the superadiant phase transition as the number of atoms increases, satisfying all the constraints imposed by the sum rule.

quant-ph↗

arXiv2021

Quantum tricriticality of chiral-coherent phase in quantum Rabi triangle

The interplay of interactions, symmetries and gauge fields usually leads to intriguing quantum many-body phases. To explore the nature of emerging phases, we study a quantum Rabi triangle system as an elementary building block for synthesizing an artificial magnetic field. We develop an analytical approach to study the rich phase diagram and the associated quantum criticality. Of particular interest is the emergence of a chiral-coherent phase, which breaks both the $\mathbb{Z}_2$ and the chiral symmetry. In this chiral phase, photons flow unidirectionally and the chirality can be tuned by the artificial gauge field, exhibiting a signature of broken time-reversal symmetry. The finite-frequency scaling analysis further confirms the associated phase transition to be in the universality class of the Dicke model. This model can simulate a broad range of physical phenomena of light-matter coupling systems, and may have an application in future developments of various quantum information technologies.

quant-ph↗

arXiv2021

Thermoelectric Hall conductivity of the fractional quantum Hall systems on a disk

For the fractional quantum Hall states on a finite disc, we study the thermoelectric transport properties under the influence of an edge and its reconstruction. In a recent study on a torus [Phys. Rev. B 101, 241101 (2020)], Sheng and Fu found a universal non-Fermi liquid power-law scaling of the thermoelectric conductivity $α_{xy} \propto T^η$ for the gapless composite Fermi-liquid state. The exponent $η\sim 0.5$ appears an independence of the filling factors and the details of the interactions. In the presence of an edge, we find the properties of the edge spectrum dominants the low-temperature behaviors and breaks the universal scaling law of the thermoelectric conductivity. In order to consider individually the effects of the edge states, the entanglement spectrum in real space is employed and tuned by varying the area of subsystem. In non-Abelian Moore-Read state, the Majorana neutral edge mode is found to have more significant effect than that of the charge mode in the low temperature.

cond-mat.str-el↗

arXiv2021

Analytically solvable model to the spin Hall effect with Rashba and Dresselhaus spin-orbit couplings

When the Rashba and Dresslhaus spin-orbit coupling are both presented for a two-dimensional electron in a perpendicular magnetic field, a striking resemblance to anisotropic quantum Rabi model in quantum optics is found. We perform a generalized Rashba coupling approximation to obtain a solvable Hamiltonian by keeping the nearest-mixing terms of Laudau states, which is reformulated in the similar form to that with only Rashba coupling. Each Landau state becomes a new displaced-Fock state with a displacement shift instead of the original Harmonic oscillator Fock state, yielding eigenstates in closed form. Analytical energies are consistent with numerical ones in a wide range of coupling strength even for a strong Zeeman splitting. In the presence of an electric field, the spin conductance and the charge conductance obtained analytically are in good agreements with the numerical results. As the component of the Dresselhaus coupling increases, we find that the spin Hall conductance exhibits a pronounced resonant peak at a larger value of the inverse of the magnetic field. Meanwhile, the charge conductance exhibits a series of plateaus as well as a jump at the resonant magnetic field. Our method provides an easy-to-implement analytical treatment to two-dimensional electron gas systems with both types of spin-orbit couplings.

cond-mat.quant-gas↗

arXiv2020

Optimal building block of multipartite quantum battery

To take quantum advantage of collective effects in many-body system, we design an elementary block for building multipartite quantum battery, which enables charging an atomic ensemble with optimal numbers in a common thermal bath. One achieves maximum free energy as the stored energy in the steady state, which is prior to each atom parallel charging independently. It ascribes to quantum collective effects in the ensemble of atoms induced by the competition between the coherent driving and decoherent dissipation. The corresponding thermodynamic efficiency of the energy storage is analyzed. The existence of the optimal elementary units of multipartite quantum battery provide a guideline for designing a realizable charging scheme.

quant-ph↗

arXiv2020

Robust photon transmission in nonlinear parity-time-symmetric cavities

We explore the photon transfer in the nonlinear parity-time-symmetry system of two coupled cavities, which contains nonlinear gain and loss dependent on the intracavity photons. Analytical solution to the steady state gives a saturated gain, which satisfy the parity-time symmetry automatically. The eigen-frequency self-adapts the nonlinear saturated gain to reach the maximum efficiency in the steady state. We find that the saturated gain in the weak coupling regime does not match the loss in the steady state, exhibiting an appearance of a spontaneous symmetry-breaking. The photon transmission efficiency in the parity-time-symmetric regime is robust against the variation of the coupling strength, which improves the results of the conventional methods by tuning the frequency or the coupling strength to maintain optimal efficiency. Our scheme provides an experimental platform for realizing the robust photon transfer in cavities with nonlinear parity-time symmetry.

quant-ph↗