Janus dipoles have exotic features in near-field coupling since they have two distinct faces, namely, the so-called coupling and noncoupling faces. When the out-coupler (e.g., a waveguide) is oriented toward the noncoupling (coupling) face of Janus dipoles, the guided waves can be inefficiently (efficiently) excited. Moreover, the noncoupling face of Janus dipoles can be engineered on demand to fully suppress the excitation of guided waves with either $s$ polarization or $p$ polarization, giving rise to the single-polarization perfect Janus dipole. However, whether it is possible to achieve a dual-polarization perfect Janus dipole in near-field coupling remains elusive. Here we theoretically reveal the possibility to realize a dual-polarization perfect Janus dipole, which requires the judicious design of both the constituent dipole moments of Janus dipoles and the dispersion relation of waveguides. When placing the dual-polarization perfect Janus dipole in the middle of two parallel identical waveguides, we theoretically find that the $p$-polarized guided waves are exclusively excited in one waveguide, while the $s$-polarized guided waves are excited solely in the other waveguide. Our findings might provide an enticing route for the development of polarization-multiplexed photonic devices, quantum routing, and on-chip signal processing.

The topologically protected configuration embedded in skyrmions has prompted some investigations into their fundamental properties and versatile applications, sparking interest and guiding ongoing development. The topological protection associated with skyrmions was initially observed in systems with interactions. It is widely believed that skyrmions are stable, yet relevant confirmation and empirical research remain limited. A pertinent question is whether skyrmion configurations formed by a single classical beam with two coupled degrees of freedom also exhibit topological stability. In this study, we affirm this hypothesis by investigating the effects of local decoherence. We analytically and numerically demonstrate the topological resilience of skyrmions and the occurrence of transition points of skyrmion numbers in local decoherence across three typical decoherence channels. On the other hand, we show that these qualities are independent of the initial state. From the numerical results, we find that inhomogeneous but continuous decoherence channels also have the same behaviors and maintain the topological stability of skyrmions as homogeneous decoherence channels do. These properties of skyrmions contribute to further applications in various areas, including communication and imaging.

Topological interface states (TISs) and zero-index behavior are two unique phenomena in photonic systems. TISs enable robust wave propagation along a topological interface, while zero-index behavior allows waves to travel without phase variation. However, photonic structures that simultaneously exhibit both topological properties and zero-index behavior remain largely unexplored. In this work, we present two distorted Kekulé photonic crystals capable of supporting a pair of gapless pseudospin TISs. Since this degeneracy occurs at the center of the Brillouin zone, the TISs inherently combine topological protection with a zero-index response, which we refer to as zero-index TISs. This enables unidirectional wave propagation with minimal phase variation while maintaining robustness against interface corners. Our work offers an effective approach to achieving zero-index topological mode, with potential applications in signal transmission, qubit entanglement, and beyond.

Modulation instability of one-dimensional plane waves, fundamental solitons, and interactions are investigated in competing cubic-quintic nonlinear media with fourth-order diffraction (FOD). Analytical results of the growth rate of modulation instability spectra are obtained with linear-stability analysis and confirmed numerically. Plane waves can propagate stably when modulation instability can be suppressed by FOD and competing nonlinearities. In addition, bifurcated solutions of fundamental Gaussian-type solitons are obtained by using a variational approach. Numerical simulations show that solitons are unstable (stable) in the regime of normal (anomalous) FOD. Interactions of in-phase and out-of-phase solitons are also demonstrated numerically, which show that normal (anomalous) FOD weakens (strengthens) the attractions of in-phase solitons and also weakens (strengthens) the repulsions of out-of phase solitons. It is also demonstrated that competing self-defocusing quintic nonlinearities always strengthen the attractive (repulsive) force between in-phase (out-of-phase) solitons.

We investigate the transient dynamics of the quantum Stuart-Landau oscillator, a paradigmatic quantum system exhibiting a quantum limit cycle and synchronization. From the energy dynamics, we determine a condition for the classical regime of transient dynamics and the limit cycle. Additionally, we formulate a guess function that fits the classical-regime steady-state Wigner function. The equation of motion for the Wigner function is derived and compared to the Kramers-Moyal equation for stochastic processes. We then characterize the classical-like behavior as the system evolves from a coherent state, noting the slow decay of neighboring-level coherence. We also study the evolution of the Wigner negativity as an indicator of nonclassicality, showing its temporary increase for some specific cases. To quantify the evolution speed, we examined the system's Lindbladian spectra, particularly the Liouvillian gap. Finally, we record the time it takes to reach the steady state for some Fock, thermal, and coherent states. The parameter dependence of the steady-state time may differ from the Liouvillian gap, and the limit-cycle attraction is significantly slower for coherent states compared to Fock or thermal states. For the diagonal states, there are fast convergence regimes for which the steady-state time is locally minimized. This study provides a deeper insight into the transient behavior of self-sustained quantum systems.

Negative differential thermal conductance (NDTC), a hallmark of nonlinear quantum thermal transport, plays a critical role in the design of quantum thermal devices, such as thermal diodes and transistors. The Lindblad dynamics predicts that the heat current through two coupled atoms increases with the increasing temperature difference of two bosonic reservoirs. However, in this paper, we uncover the suppressive effect on the heat current in this nonequilibrium system using the Bloch-Redfield master equations, which indicate the emergence of NDTC. Our findings underscore the crucial role of beyond-Lindblad dynamics in accurately capturing nonlinear features in quantum thermodynamic systems.

We propose a quantum interferometric protocol that leverages spin-dependent spatial displacements to enable high-precision parameter estimation beyond classical limits. By inducing a unitary coupling between a particle's spin degree of freedom and its momentum, the protocol generates entanglement between spin states and spatial positions, resulting in coherent spatial superpositions. Interferometric reconstruction of the resulting phase differences enables Heisenberg-limited sensitivity for parameters encoded in the spin Hamiltonian. As a concrete application, we demonstrate the protocol's effectiveness in magnetic field sensing, where the field is transduced into spatial interference fringes. Quantum Fisher information analysis confirms sub-shot-noise scaling, and the protocol's feasibility is discussed for physical platforms including ultracold atoms and nitrogen-vacancy centers. Our framework provides a versatile approach to quantum metrology with potential extensions to multiparameter sensing and gravitational wave detection.

In the field of quantum information, robust entanglement has received widespread attention. In this work, we investigate the magnon-mediated robust entanglement between the acoustic phonon and the center-of-mass motion (CMM) phonon in a yttrium iron garnet sphere. By analyzing the system's dynamic evolution, we demonstrate that phonon-phonon entanglement, which is robust with respect to the magnon mode, can be generated at a specific moment. Furthermore, we introduce a scheme for generating stable bipartite entanglement that remains robust against varying initial states by utilizing an engineered reservoir. This approach maintains effectiveness even under high dissipation rates of the magnon mode, leveraging the magnon dissipation as a beneficial resource. The approaches we present for generating robust phonon-phonon entanglement not only offer a feasible method for monitoring internal acoustic modes within magnetic systems using easily detectable CMM phonons, but also provide a viable platform for the manipulation and processing of quantum information.

We present a geometric optimization method for implementing quantum gates by optimally controlling the Hamiltonian parameters, with the goal of approaching the Mandelstam-Tamm quantum speed limit (MT-QSL). Achieving this bound requires satisfying precise geometric conditions that govern the evolution of quantum states. We extend this geometric framework to quantum unitary operators in arbitrary dimensions and analyze the conditions necessary for saturation of the bound. Additionally, we show that the quantum brachistochrone, when generalized to operators, does not, in general, saturate the MT-QSL bound. Finally, we propose a systematic optimal control strategy based on geometric principles to approach the quantum speed limit for unitary driving. We illustrate this optimization method on a set of four well-known two-qubit quantum gates. Our procedure significantly reduces the deviation from the optimal quantum speed limit while preserving high quantum fidelity.

We explore the efficacy of optical pumping on the $5s4d{}^{1}D_2\text{−}5s8p{}^{1}P_1$ $\left(448\mathrm{nm}\right)$ transition in a magneto-optical trap (MOT) of Sr atoms. The number of trapped atoms is enhanced by a factor of $12.0\left(6\right)$ relative to the case without repumping light, which is six times as large as that obtained using the pumping transition $5s4d{}^{1}D_2\text{−}5s6p{}^{1}P_1$ $\left(717\mathrm{nm}\right)$. This enhancement is limited by decay pathways that bypass the $5s4d{}^{1}D_2$ state, namely $5s5p{}^{1}P_1\to 5s4d{}^{3}D_1\to 5s5p{}^{3}P_0$ and $5s5p{}^{1}P_1\to 5s4d{}^{3}D_2\to 5s5p{}^{3}P_2$, which account for 8% of the total loss of the trapped atoms. We determine the decay rates for the $5s5p{}^{1}P_1\to 5s4d{}^{3}D_1$ and $5s5p{}^{1}P_1\to 5s4d{}^{3}D_2$ transitions to be $66\left(6\right){\mathrm{s}}^{-1}$ and $2.4\left(2\right)\times {10}^2{\mathrm{s}}^{-1}$, respectively. Furthermore, we experimentally demonstrate, when the trap beam diameter is small, escape of atoms in the $5s4d{}^{1}D_2$ state, which has a relatively long lifetime of $400\mu \mathrm{s}$, becomes a dominant loss mechanism, and that the $448\mathrm{nm}$ pumping light effectively suppresses this escape. Our findings will contribute to improved laser cooling and fluorescence imaging in cold strontium atom platforms, such as quantum computers based on optical tweezer arrays.

High-order harmonic generation (HHG), a robust tabletop source for producing attosecond pulses, has been extensively utilized in attosecond metrology. Traditionally, HHG driven by classical laser fields involves two typical quantum paths (short and long quantum paths) contributing to harmonic emission. Here we demonstrate that these quantum paths in HHG can be selectively controlled using squeezed light, a form of nonclassical light. Our results indicate that the long (short) quantum path of HHG will be dramatically suppressed in the phase (amplitude)-squeezed fields. The time-frequency analysis reveals that this quantum path control stems from the quantum fluctuations in the squeezed light, which modify the phase matching of harmonic emission from different quantum states of the squeezed light. Such a quantum path selection can be achieved for the whole harmonic plateau, which has great potential to generate ultrashort isolated attosecond pulses with duration less than one atomic unit of time.

The analytical solution for partially coherent beams propagating in one-dimensional (1D) uniform or binary periodic photonic environments is presented. The cross-spectral density (CSD) of a partially coherent Gaussian-Schell model (GSM) beam and a partially coherent Hermite-Gaussian correlated Schell-model (HGCSM) beam in uniform periodic photonic lattices is derived in detail. These analytical expressions demonstrate that the evolution of the spectral density is determined not only by the characteristics of the light sources, such as the coherence or correlation function structure, but also by the properties of the photonic lattices, specifically the coupling coefficient. Our findings pave the way for the analytical study of the propagation of partially coherent beams in photonic lattices.

Quantum entanglement is a crucial resource in quantum technologies, enabling advancements in quantum computing, quantum communication, and quantum precision measurement. Here, we propose a method to enhance optomechanical entanglement by introducing an optical Kerr nonlinear medium and a squeezed vacuum reservoir of the optomechanical cavity. By performing the displacement and squeezing transformations, the system can be reduced to a standard linearized optomechanical system with normalized driving detuning and linearized-coupling strength, in which the optical and mechanical modes are, respectively, coupled to an optical vacuum bath and a mechanical heat bath. We focus on the entanglement generation in the single stable regime of the system. By evaluating the steady-state logarithm negativity, we find that the optomechanical entanglement can be enhanced within a wide range of the Kerr constant. In addition, the Kerr nonlinearity can extend the stable region, enabling considerable entanglement generation in the blue-sideband parameter region. We also investigate the dependence of the entanglement generation on the average thermal phonon occupation of the mechanical bath and the optical driving amplitude. It is found that the presence of the Kerr nonlinearity allows the generation of optomechanical entanglement even when the thermal phonon occupation of the mechanical bath is as high as 3000. Our findings will provide valuable insights into enhancing fragile quantum resources in quantum systems.

Structured light offers a powerful approach to tailor light–matter interactions in quantum systems with chiral properties. While chirality has been extensively studied in passive platforms, the role of optical gain in controlling chiral quantum dynamics remains largely unexplored. In this work, we develop a general theoretical framework to describe the dynamics of an atomic system interacting with structured gain environments, where amplification depends on the light's polarization or momentum. By quantizing the electromagnetic field in linear bianisotropic media with gain and extending the Lindblad formalism to these settings, we derive a master equation governing the atom's irreversible evolution. We show that chiral gain can break time-reversal symmetry and drive the system toward a symmetry-broken steady state with nonreciprocal properties. This effect is illustrated in detail for moving plasmonic substrates, which exhibit inherently chiral gain that selectively amplifies transitions of a given handedness. Our results establish chiral gain as a novel mechanism for engineering nonreciprocal quantum steady states.

A relativistic collapse model for distinguishable particles is presented. Position and time, for each particle, are the fundamental operators of the theory. The Schrödinger equation is of the continuous spontaneous localization form, with a Hermitian Hamiltonian and an anti-Hermitian, white-noise dependent, Hamiltonian. It generates state vector evolution parametrized by an “evolution parameter.” It is shown how this can be interpreted as an evolving state in spacetime with collapses satisfying Born rule probabilities, and how certain choices of collapse generating operators lead to states of definite mass and definite configuration in spacetime. The model is Poincaré covariant and conserves energy in expectation.

When the rotating frequency of a noninteracting Bose-Einstein condensate (BEC) confined in a weak anisotropic harmonic potential is suddenly quenched to its trapping frequency, the condensate evolves from its ground state to a single-mode squeezed state with exponentially growing quantum fluctuation anisotropy. Such a squeezed state is called the geometrically squeezed state. However, for interacting BECs with two-body collisions, a similar quench only results in quantum fluctuations oscillating periodically without squeezing. In this work, we identify superfluid stability as the key factor behind this nonsqueezing phenomenon, with the periodic oscillations arising from collective excitations of a stable collective excitation mode. By strategically breaking the stability criteria, we propose a dynamical approach for generating squeezing that can exponentially suppress quantum fluctuations in a relatively short time, surpassing the efficiency of existing experimental preparation schemes.

We study coherent light scattering from linear crystals with up to twelve ${}^{40}\mathrm{Ca}^{+}$ ions, acting as single photon emitters. Light scattering is induced by two-photon laser excitation, starting from the $S_{1/2}\to D_{5/2}$ quadrupole transition at 729 nm followed by the $D_{5/2}\to P_{3/2}$ dipole transition at 854 nm, from where the ions decay back to the $S_{1/2}$ ground state via emission of a photon near 393 nm. We realize spin-selective excitation from the Zeeman-split ground states $S_{1/2}, \mathrm{m}=\pm 1/2$, of the ${\text{Ca}}^{+}$ ions and observe, in the far-field, spin-dependent interference patterns displaying the spin textures of the ion crystals. We investigate their dynamics by measuring the temporal evolution of the spatial Fourier frequencies of the observed patterns.

The quantum approximate optimization algorithm (QAOA) is a variational quantum algorithm (VQA) ideal for noisy intermediate-scale quantum (NISQ) processors, and is highly successful in solving combinatorial optimization problems (COPs). It has been observed that the optimal parameters obtained from one instance of a COP can be transferred to another instance, resulting in generally good solutions for the latter. In this work, we propose a refinement scheme in which only a subset of QAOA layers is optimized following parameter transfer, with a focus on the Max-Cut problem. Our motivation is to reduce the complexity of the loss landscape when optimizing all the layers of high-depth QAOA circuits, as well as to reduce the optimization time. We investigate the potential hierarchical roles of different layers and analyze how the approximation ratio scales with increasing problem size. Our findings indicate that the selective layer optimization scheme offers a favorable trade-off between solution quality and computational time, and can be more beneficial than full optimization at a lower optimization time.

We improve Gaussian-boson-sampling circuits by integrating the unitary-averaging (UA) protocol, previously demonstrated to protect unknown Gaussian states from phase errors [Swain et al., Phys. Rev. A 110, 032622 (2024)]. Our work extends the applicability of UA to mitigate arbitrary interferometric noise, including beam-splitter and phase-shifter imperfections. Through comprehensive numerical analysis, we demonstrate that UA consistently achieves higher fidelity and success probability than unprotected circuits, establishing its robustness in noisy conditions. Remarkably, enhancement is maintained across varying numbers of modes with respect to the noise. We further derive a power-law formula predicting performance gains in large-scale systems, including 100-mode and 216-mode configurations. A detailed step-by-step algorithm for implementing the UA protocol is also provided, offering a practical road map for advancing near-term quantum technologies.