arXiv subjects

Yong-Chang Zhang

Publications and source records attributed to Yong-Chang Zhang.

At least 19 recordsLinked to original sources

arXiv2026

Self-organized quasicrystals and their excitations in dipolar Bose-Einstein condensates via optical feedback

Quasicrystals emerge from competing interactions with incommensurate characteristic length scales that inhibit translational periodicity. Here, we show that such multiscale interactions can be realized in a dipolar Bose-Einstein condensate through the interplay between intrinsic dipole-dipole interactions and photon-mediated interactions generated by coupling to an excited-state manifold together with a suitably engineered optical feedback. This interplay gives rise to two pronounced roton instabilities in the Bogoliubov excitation spectrum, leading to a rich ground-state phase diagram including dodecagonal quasicrystals with twelvefold rotational symmetry for experimentally realistic parameters. We further propose a protocol to access these quasicrystals dynamically and develop a general numerical framework for calculating their collective excitation spectra.

cond-mat.quant-gas↗

arXiv2026

Emergent aperiodicity in Bose-Bose mixtures induced by spin-dependent periodic potentials

We study the ground-state and low-lying metastable phases of repulsive binary Bose-Einstein condensates confined in twisted, spin-dependent periodic optical lattices. For balanced mixtures, weak intercomponent interactions yield a fourfold momentum-space symmetry dictated by the lattice geometry. Increasing the coupling strength leads to the emergence of additional momentum peaks that combine with the lattice-induced structure to produce an eightfold rotationally symmetric pattern, signaling quasicrystalline order. At intermediate interactions, global phase separation suppresses this quasicrystalline state; however, at stronger coupling, local phase separation gives rise to a long-lived metastable phase in which the eightfold symmetry is restored. In this regime, a secondary ring of dominant momentum peaks appears at smaller wave vectors, indicating longer-wavelength density modulations and a crossover from lattice-dominated to interaction-driven quasicrystalline order. In contrast, imbalanced mixtures form partially miscible density clusters with eightfold-symmetric aperiodic patterns only at intermediate coupling, while stronger interactions drive global phase separation and permanently destroy quasicrystalline order. Real-time simulations demonstrate that these aperiodic structures are dynamically stable and experimentally accessible. Our results show that quasicrystalline order can emerge in binary condensates without explicitly aperiodic lattices and reveal population balance as a key ingredient for stabilizing quantum quasicrystals.

cond-mat.quant-gas↗

arXiv2025

Non-Equilibrium Probing of Topological Supersolids in Spin-Orbit-Coupled Dipolar Condensates

A chiral supersolid is a quantum phase that simultaneously exhibits crystalline order, superfluidity, and topological spin texture, with spontaneously broken translational, U(1) gauge, and chiral symmetries. Here, we demonstrate a chiral supersolid with tunable non-equilibrium dynamics in a spin-orbit coupled dipolar Bose-Einstein condensate. By adjusting dipolar interaction and spin-orbit coupling, we uncover two distinct quantum phase transitions: (i) a first-order transition from a single skyrmion superfluid to a triangular meron supersolid, and (ii) a second-order transition from this superfluid to a square skyrmion supersolid. These phases are characterized by their lattice symmetries, nonclassical rotational inertia, and spin textures. Under parity-time symmetric dissipation, we predict phase-dependent damping of the current oscillations, directly linked to the superfluid fraction. The predicted chiral supersolid phase can be experimentally observed in ultracold magnetic atoms with spin-orbit coupling. Our results establish dipolar quantum gases as a platform for designing topological matter with spintronic functionality.

cond-mat.quant-gas↗

arXiv2025

The Solid-state Physics of Rydberg-dressed Bosonic Mixtures

We explore phases of two-component Rydberg-dressed Bose-Einstein condensates in three spatial dimensions. The competition between the effective ranges of inter- and intra-component soft-core interactions leads to a rich variety of ground states. These include states resembling ionic compounds with face-centered cubic or simple cubic lattice structure. Upon increasing the scattering length, the dimensionality of the symmetry-breaking is lower due to the suppression of large densities, leading to segregated planar or tubular density profiles. We also show that these states are not only stable ground states, but can also emerge dynamically upon time evolution.

cond-mat.quant-gas↗

arXiv2025

Nonreciprocal and long-range three-body interactions in Bose-Einstein condensates induced by optical feedback

We propose generating long-range and nonreciprocal three-body interactions in quantum gases via optical feedback. By placing a quasi-two-dimensional Bose-Einstein condensate (BEC) in front of two reflecting mirrors and illuminating it with dichromatic laser beams, these driving optical fields traverse the BEC twice, thereby inducing a feedback effect on the atoms. We demonstrate that this optical feedback gives rise to an effective three-body atom-atom interaction with remarkable long-range and nonreciprocal properties. Due to its long-range nature, this three-body interaction can cause unique spatial symmetry-breaking behaviors in the BEC, resulting in various stable stationary states as well as unexpected diffusive collapse. Notably, a distinct ring state emerges through a purely self-organizing process. Furthermore, by analyzing the real-time dynamics of the BEC, we show that the nonreciprocal nature of this interaction can lead to intriguing self-acceleration of the condensate, seemingly violating Newton's law of motion. Additionally, our scheme offers a highly controllable setting, where pairwise two-body interactions can be tuned to vanish. This flexibility provides a promising route for exploring exotic physics associated with multi-body interactions.

cond-mat.quant-gas↗

arXiv2025

Super-Heisenberg scaling in a triple point criticality

We investigate quantum-enhanced metrology in a triple point criticality and discover that quantum criticality can not always enhance measuring precision. We have developed suitable adiabatic evolution protocols approaching a final point around the triple point to effectively restrain excitations, which could accelerate the adiabatic evolutions and lead to an exponential super-Heisenberg scaling. This scaling behavior is quite valuable in practical parameter estimating experiments with limited coherence time. The super-Heisenberg scaling will degrade into a sub-Heisenberg scaling if the adiabatic parameter modulations adopted can not reduce excitations and weaken the slowing down effect. Additionally, a feasible experimental scheme is also suggested to achieve the anticipated exponential super-Heisenberg scaling. Our findings strongly indicate that criticality-enhanced metrology can indeed significantly enhance measuring precision to a super-Heisenberg scaling when combining a triple point and beneficial parameter modulations in the adiabatic evolution, which will be conducive to the exploration of other super-Heisenberg scaling and their applications.

quant-ph↗

arXiv2024

Accessing elusive two-dimensional phases of dipolar Bose-Einstein condensates by finite temperature

It has been shown that dipolar Bose-Einstein condensates that are tightly trapped along the polarization direction can feature a rich phase diagram. In this paper we show that finite temperature can assist in accessing parts of the phase diagram that otherwise appear hard to realise due to excessively large densities and number of atoms being required. These include honeycomb and stripe phases both in the thermodynamic limit as well as with a finite extent using both variational and numerical calculations. We account for the effect of thermal fluctuations by means of Bogoliubov theory employing the local density approximation. Furthermore, we exhibit real-time evolution simulations leading to such states. We find that finite temperatures can lead to a significant decrease in the necessary particle number and density that might ultimately pave a route for future experimental realisations.

cond-mat.quant-gas↗

arXiv2024

Nonlinearity-induced dynamical self-organized twisted-bilayer lattices in Bose-Einstein condensates

Creating crystal bilayers twisted with respect to each other would lead to large periodic supercell structures, which can support a wide range of novel electron correlated phenomena, where the full understanding is still under debate. Here, we propose a new scheme to realize a nonlinearity-induced dynamical self-organized twisted-bilayer lattice in an atomic Bose-Einstein condensate (BEC). The key idea here is to utilize the nonlinear effect from the intrinsic atomic interactions to couple different layers and induce a dynamical self-organized supercell structure, dramatically distinct from the conventional wisdom to achieve the static twisted-bilayer lattices. To illustrate that, we study the dynamics of a two-component BEC and show that the nonlinear interaction effect naturally emerged in the Gross-Pitaevskii equation of interacting bosonic ultracold atoms can dynamically induce both periodic (commensurable) and aperiodic (incommensurable) moiré structures. One of the interesting moiré phenomena, i.e., the flat-band physics, is shown through investigating the dynamics of the wave packet of BEC. Our proposal can be implemented using available state-of-the-art experimental techniques and reveal a profound connection between the nonlinearity and twistronics in cold atom quantum simulators.

cond-mat.quant-gas↗

arXiv2024

On the infrared cutoff for dipolar droplets

The beyond mean-field physics due to quantum fluctuations is often described by the Lee-Huang-Yang (LHY) correction, which can be approximately written as a simple analytical expression in terms of the mean-field employing local density approximation. This model has proven to be very successful in predicting the dynamics in dipolar Bose-Einstein condensates both qualitatively and quantitatively. Yet, a small deviation between experimental results and the theoretical prediction has been observed when comparing experiment and theory of the phase boundary of a free-space quantum droplet. For this reason, we revisit the theoretical description of quantum fluctuations in dipolar quantum gases. We study alternative cutoffs, compare them to experimental results and discuss limitations.

cond-mat.quant-gas↗

arXiv2024

Quantum droplets in dipolar condensate mixtures with arbitrary dipole orientations

Through considering a two-component dipolar Bose-Einstein condensate, we investigate the influence of the angle between the polarization orientations of the two species on the ground states, and show that the miscibility between the two components can be adjusted not only by the inter-component contact interaction but also by the polarization angle. Particularly, in the presence of an external confinement, the two species exhibit a preference for splitting into multiple droplets separated from each other in the immiscible regime, featuring a zig-zag like profile. Furthermore, the number of separated droplets depends solely on the polarization angle. This introduces a promising degree of freedom for exploring emergent states of matter in dipolar quantum gases.

cond-mat.quant-gas↗

arXiv2023

Metastable Patterns in one- and two-component dipolar Bose-Einstein Condensates

In this paper we study metastable states in single- and two-component dipolar Bose-Einstein condensates. We show that this system supports a rich spectrum of symmetries that are remarkably stable despite not being ground states. In a parameter region where striped phases are ground states, we find such metastable states that are energetically favourable compared to triangular and honeycomb lattices. Among these metastable states we report a peculiar ring-lattice state, which is led by the competition between triangular and honeycomb symmetries and rarely seen in other systems. In the case of dipolar mixtures we show that via tuning the miscibility these states can be stabilized in a broader domain by utilising inter-species interactions.

cond-mat.quant-gas↗

arXiv2023

Localization-delocalization Transition in an electromagnetically induced photonic lattice

We investigate the localization-delocalization transition (LDT) in an electromagnetically induced photonic lattice. A four-level tripod-type scheme in atomic ensembles is proposed to generate an effective photonic moiré lattice through the electromagnetically induced transparency (EIT) mechanism. By taking advantage of the tunable atomic coherence, we show that both periodic (commensurable) and aperiodic (incommensurable) structure can be created in such a photonic moiré lattice via adjusting the twist angle between two superimposed periodic patterns with square primitive. Furthermore, we also find that by tuning the amplitudes of these two superimposed periodic patterns, the localization-delocalization transition occurs for the light propagating in the aperiodic moiré lattice. Such localization is shown to link the fact that the flat bands of moiré lattice support quasi-nondiffracting localized modes and thus induce the localization of the initially localized beam. It would provide a promising approach to control the light propagation via the electromagnetically induced photonic lattice.

cond-mat.quant-gas↗

arXiv2022

Nonequilibrium steady state in a large magneto-optical trap

Considering light-mediated long-range interactions between cold atoms in a magneto-optical trap (MOT), we present numerical evidence of a nonequilibrium steady state (NESS) for sufficiently large number of atoms (> 10^8). This state manifests itself as the appearance of an anisotropic distribution of velocity when a MOT approaches the threshold beyond which self-oscillating instabilities occur. Our three-dimensional (3D) spatiotemporal model with nonlocal spatial dependencies stemming from the interatomic interactions has recently been compared successfully to predict different instability thresholds and regimes in experiments with rubidium atoms. The behavior of the NESS is studied as a function of the main MOT parameters, including its spatiotemporal characteristics.

physics.atom-ph↗

arXiv2021

Phases of supersolids in confined dipolar Bose-Einstein condensates

Dipolar Bose-Einstein condensates represent a powerful platform for the exploration of quantum many-body phenomena arising from long-range interactions. A series of recent experiments has demonstrated the formation of supersolid states of matter. Subsequent theoretical works have shown that quantum fluctuations can affect the underlying phase transition and may lead to the emergence of supersolids with various lattice structures in dipolar condensates. In this work we explore the signatures of such different geometries in confined finite condensates. In addition to previously found triangular lattices, our analysis reveals a rich spectrum of states, from honeycomb patterns and ring structures to striped supersolids. By optimizing relevant parameters we show that transitions between distinct supersolids should be observable in current experiments.

cond-mat.quant-gas↗

arXiv2020

Self-bound droplet clusters in laser-driven Bose-Einstein condensates

We investigate a two-dimensional Bose-Einstein condensate that is optically driven via a retro-reflecting mirror, forming a single optical feedback loop. This induces a peculiar type of long-range atomic interaction with highly oscillatory behavior, and we show here how the sign of the underlying interaction potential can be controlled by additional optical elements and external fields. This additional tunability enriches the behavior of the system substantially, and gives rise to a surprising range of new ground states of the condensate. In particular, we find the emergence of self-bound crystals of quantum droplets with various lattice structures, from simple and familiar triangular arrays to complex superlattice structures and crystals with entirely broken rotational symmetry. This includes mesoscopic clusters composed of small numbers of quantum droplets as well as extended crystalline structures. Importantly, such ordered states are entirely self-bound and stable without any external in-plane confinement, having no counterpart to other quantum-gas settings with long-range atomic interactions.

cond-mat.quant-gas↗

arXiv2019

Blockade-induced resonant enhancement of the optical nonlinearity in a Rydberg medium

We predict a resonant enhancement of the nonlinear optical response of an interacting Rydberg gas under conditions of electromagnetically induced transparency. The enhancement originates from a two-photon process which resonantly couples electronic states of a pair of atoms dressed by a strong control field. We calculate the optical response for the three-level system by explicitly including the dynamics of the intermediate state. We find an analytical expression for the third order susceptibility for a weak classical probe field. The nonlinear absorption displays the strongest resonant behavior on two-photon resonance where the detuning of the probe field equals the Rabi frequency of the control field. The nonlinear dispersion of the medium exhibits various spatial shapes depending on the interaction strength. Based on the developed model, we propose a realistic experimental scenario to observe the resonance by performing transmission measurements.

physics.atom-ph↗

arXiv2019

Supersolidity around a critical point in dipolar Bose Einstein condensates

We explore spatial symmetry breaking of a dipolar Bose Einstein condensate in the thermodynamic limit and reveal a critical point in the phase diagram at which crystallization occurs via a second order phase transition. This behavior is traced back to the significant effects of quantum fluctuations in dipolar condensates, which moreover stabilize a new supersolid phase, namely a regular honeycomb pattern with maximal modulational contrast and near-perfect superfluidity.

cond-mat.quant-gas↗

arXiv2018

Long-range interactions and symmetry-breaking in quantum gases through optical feedback

We consider a quasi two-dimensional atomic Bose Einstein condensate interacting with a near-resonant laser field that is back-reflected onto the condensate by a planar mirror. We show that this single-mirror optical feedback leads to an unusual type of effective interaction between the ultracold atoms giving rise to a rich spectrum of ground states. In particular, we find that it can cause the spontaneous contraction of the quasi two-dimensional condensate to form a self-bound one-dimensional chain of mesoscopic quantum droplets, and demonstrate that the observation of this exotic effect is within reach of current experiments.

physics.atom-ph↗