We investigate the collective excitations of anisotropic dipolar supersolids in planar confinement, focusing on triangular and stripe phases in situations where the dipoles are tilted to have a component in the plane. Using Bogoliubov-de Gennes calculations and hydrodynamic theory, we identify the elastic parameters that govern the long-wavelength dynamics, including two orientational coefficients that capture the broken rotational symmetry induced by dipole tilt. Analytical expressions for the speeds of sound are obtained along the principal axes for triangular supersolids and along any propagation direction for the stripe supersolid. Our results provide a unified framework for understanding sound propagation in anisotropic dipolar supersolids and establish connections to recent experiments on sound propagation in striped Bose-Einstein condensates.

Quantum vortices separated by distances much larger than their core size interact via their long-range velocity field. At smaller separations, however, the influence of the core's compressibility strongly influences the vortex dynamics. Using the example of a compact necklace of five vortices, it is shown that close-range effects lead to a new (slower) state of orbital motion, which is not predicted by the usual long-range theory. It is demonstrated that control can be exerted over the vortex dynamics by modulating the interaction strength to induce Rabi oscillations between the fast and slow orbital states. A better understanding of these close-range states may have consequences for physical scenarios involving compact vortex clusters, such as quantum fluids with dissipative losses.

The quasi-continuous-wave background (qCWB) is a common phenomenon in the nonlinear dynamics of ultrafast fiber lasers, yet its role in soliton-state transitions remains insufficiently understood. Here, we confirm the presence of qCWB by combining soliton filtering with short-time Fourier transform measurements. Mutual information and Pearson correlation analyses reveal that qCWB plays a decisive role in nonequilibrium switching between soliton molecules and pulsating soliton molecules. We further identify a bidirectional feedback mechanism, in which qCWB induces soliton-state switching via positive feedback, while the resulting soliton states exert negative feedback on qCWB, thereby maintaining dynamic balance. This mechanism enables a qCWB-based optical encoding platform for active control of ultrafast laser states.

The anomalies in light scattering associated with singularities are of great importance in both basic physics research and engineering applications. In this work, we focus on the coherent perfect absorption (CPA) singularities in an anti-parity-time (anti-PT) symmetric system and explore the coherent absorption characteristics of the system under various damping conditions. We theoretically and experimentally demonstrated CPA in both anti-PT symmetric phase and anti-PT symmetry broken phase, where dissipation plays a crucial role in the absorption efficiency of the system. Additionally, we theoretically observe pairs of CPA singularities with opposite topological charges, indicating the topological nature of the CPA in the anti-PT symmetric phase. We depict the movement of the zeros of the scattering matrix in the complex-frequency plane and the generation and annihilation processes of topological charges under different dissipative scenarios. Our work offers a promising platform for investigating non-Hermitian physics in open systems and provides valuable insights into the manipulation of electromagnetic waves in photonic systems.

Multiphoton entangled states are pivotal resources for implementing optical quantum information protocols. Recently, energy-time-entangled triphotons have been observed in hot atomic ensembles. However, in these protocols, the complex fifth-order nonlinear susceptibility entailed by four- or five-level systems limits our understanding of triphoton generation. Here, to directly capture the generation mechanism of triphotons and their associated optical properties, we investigate the generation of energy-time-entangled triphotons in a six-level cold atomic ensemble. The fifth-order nonlinear susceptibility indicates the existence of two sets of spontaneous six-wave mixing in the system. Notably, triphoton generation in this system is subject to stringent timing constraints. Collectively, these characteristics give rise to threefold coincidence counts, which—dominated by the fifth-order nonlinear susceptibility—exhibit asymmetrically damped Rabi oscillations in the two-dimensional time domain. Furthermore, we analytically derive that the temporal correlation properties of conditional two-photon states are preserved—a unique feature of $W$-class tripartite entanglement. These results not only lay the groundwork for the experimental preparation of triphotons using six-level systems but also provide key support for understanding the generation mechanism of triphotons involving more complex fifth-order nonlinear susceptibilities.

We present a theoretical study of enhanced lateral and rotary photon drag within a four-level quantum-dot setup under the effect of dual interdot tunneling. Using the density-matrix formalism, we derive analytical expressions for the susceptibility, group index, and phase index under weak-field conditions. By introducing dual interdot tunneling in a four-level quantum-dot molecule, we demonstrate a mechanism for tunable, slow-light-enhanced photon drag, where the interaction between tunneling couplings and control fields allows precise manipulation of dispersion and light dragging. Our results reveal that rotary and lateral photon drag depend on the difference between the group and phase indices, leading to significant phase and frequency shifts even at relatively low translational and rotational velocities. The study highlights the potential of tunneling-coupled quantum-dot molecules as versatile platforms for slow-light-enhanced photon drag, with promising applications in precision metrology and nanoscale photonic devices.

We analytically evaluate the efficiency of continuous dynamical decoupling (CDD) to curb decoherence in generic qubit setups where diverse sources of noise can be present. Previous theoretical approaches to CDD have mainly focused on its potential to cope with longitudinal fluctuations. Here, the basic scenario tackled with CDD is generalized. Apart from dealing with pure dephasing induced by diagonal noise, we consider the impact of transverse fluctuations, usually present in the practical arrangements. In particular, the implications of anisotropic noisy inputs are studied. Additionally, we analyze the role of the fluctuations in the dressing of the qubit by the CDD field of control: since the driving field is usually switched on through linear ramps of its characteristic parameters, the associated dressing of the original states can be described in terms of noisy Landau-Zener transitions. In our approach, based on a sequence of unitary transformations, the noise entering the system is cast into effective stochastic terms whose spectral characteristics are dependent on the driving parameters. This description allows the design of strategies to mitigate the impact of the fluctuations using controlled changes in the effective-noise properties. Significant robustness of CDD against the generalization of the basic scenario can be achieved through an appropriate choice of the parameters of control.

Quantum coherence is an exquisitely quantum phenomenon that depends on both probability amplitudes and relative phases. Standard coherence measures quantify superposition within density matrices but cannot distinguish ensembles that produce the same mixed state through different distributions of pure states. Building on the geometric formulation of quantum mechanics, we introduce the probability-phase mutual information $I(P;\mathrm{\Phi })$. We show that it characterizes quantum coherence at the ensemble level and that ensemble coherence systematically exceeds density-matrix coherence, thus quantifying the structure lost when averaging over pure states. Eventually, its relevance for quantum thermodynamics, quantum information theory, and deep thermalization is highlighted by explicit examples: canonical ensembles reveal temperature-dependent probability-phase correlations absent from thermal density matrices. We show that the probability of converting an ensemble into another one is bound by the ratio of their $I(P;\mathrm{\Phi })$, and that a nonvanishing $I(P;\mathrm{\Phi })$ signals the breakdown of deep thermalization.

Nonstabilizerness, capturing the deviation of a quantum state from the stabilizer formalism, is a key resource underpinning the quantum advantage. The recently introduced stabilizer Rényi entropy (SRE) offers a tractable measure of nonstabilizerness, avoiding the complexity of conventional methods. We study SRE in translation-invariant matrix product states (MPS), deriving exact expressions for representative states and introducing a numerically stable algorithm, named the bond-density matrix renormalization group, to compute the SRE density in infinite systems. Applying this method, we obtain high-precision SRE densities for the ground state of the one-dimensional Ising model. We also analyze the nononsite SRE density, showing it is bounded by a universal function of entanglement entropy, and further prove that two-site mutual SRE vanishes asymptotically in injective MPS. Our work not only introduces a powerful method for extracting the SRE density in quantum many-body systems, but also numerically reveals a fundamental connection between nonstabilizerness and entanglement, thereby paving the way for deeper theoretical investigations into their interplay.

Digital-analog is a quantum computational paradigm that employs the natural interaction Hamiltonian of a system as the entangling resource, combined with single qubit gates, to implement universal quantum operations. As in the case of its digital gate-based counterpart, designing digital-analog circuits that employ optimal quantum resources often requires an exceedingly large classical computational time. In this work we find a suboptimal solution to this exponentially large problem, showing that it can be solved within polynomial computational time. In particular, we provide an exact solution for the problem of expressing arbitrary two-body Hamiltonians as the sum of local unitary transformations of an arbitrary Ising Hamiltonian, with the total number of required terms being at most quadratic in system size. This allows us to design a digital-analog simulation protocol that avoids employing numerical optimization over a large parameter space at the preprocessing stage, minimizing computational resources and allowing for further scaling.

We formulate a robust optimal control algorithm to synthesize minimum-energy pulses that can transfer a cold atom system into various momentum states. The algorithm uses adaptive linearization of the evolution operator and sequential quadratic programming to iterate the control towards a minimum-energy pulse that achieves optimal target state fidelity. Robustness to parameter variation is achieved using Legendre polynomial approximation over the domain of variation. The method is applied to optimize the Bragg beam splitting operation in ultracold atom interferometry. Even in the presence of 10–40% variability in the initial momentum dispersion of the atomic cloud and the intensity of the optical pulse, the algorithm reliably converges to a control protocol that robustly achieves unprecedented momentum levels with high fidelity for a single-frequency multiphoton Bragg diffraction scheme (e.g., $|\pm 40\hslash k\rangle$). We show the advantages of our method by comparison to stochastic optimization using sampled parameter values, provide detailed sensitivity analyses, and verify performance of the designed pulses in laboratory experiments.

Optical modes exhibiting high quality Q factors and minimal dispersion in momentum space are crucial for enhancing light-matter interactions, including nonlinear optical processes and slow-light phenomena. Bound states in the continuum (BICs) represent a promising photonic platform owing to their theoretically infinite Q factors. However, for conventional individual BICs, the Q factor diminishes exponentially upon deviation from the BIC momentum singularity, and further enhancement of optical nonlinear conversion efficiency remains actively pursued. Here, we merge symmetry-protected and Friedrich-Wintgen BICs within a silicon metasurface to create a super-BIC to boost the Q factors of the associated quasi-BIC mode with ultraflatband dispersion and topological robustness. The ultraflat merging BIC exhibits a near-zero group velocity with a record group index of $5.0\times {10}^8$ and group delay of $8.9\times {10}^5\mathrm{ps}$, surpassing conventional BICs by 2 orders of magnitude, which stems from the near-zero dispersion of the super-BIC-induced flatband. For nonlinear optics, the structure enhances the polarization-sensitive third- and fifth-harmonic generation efficiencies by 6 and 12 orders of magnitude, respectively, achieving conversions efficiencies of $9.6\times {10}^{-3}$ (third-harmonic generation) and $1.8\times {10}^{-10}$ (fifth-harmonic generation) at an ultralow pump density of $43\mathrm{mW}/\mathrm{c}{\mathrm{m}}^2$. Notably, it maintains an average electric-field enhancement of $3.28\times {10}^4$ even under fabrication imperfections. This flatband design establishes a paradigm for robust light-matter interaction control, enabling applications in ultralow-power nonlinear photonics and on-chip slow-light devices.

In conventional quadratic nonlinear interactions with femtosecond lasers, group-velocity mismatch (GVM) is typically regarded as a detrimental effect that restricts the phase-matching bandwidth. Here, we demonstrate a counterintuitive ultrafast parametric process where nonvanished GVMs actively facilitate efficient energy transfer from picosecond pumps to femtosecond signals. The mechanism of such an efficient parametric interaction is revealed: the pump-signal temporal walk off enables traveling-wave difference-frequency generation (DFG), sustaining continuous amplification of the pulse leading edge, while the signal-idler temporal walk off rapidly removes the idler from the interaction region, effectively suppressing back conversion and locking energy into the signal. This process can be implemented in standard collinear Type-II beta-barium-borate (β-BBO) crystals. Numerical simulations confirm $90\%$ pump depletion efficiency while generating amplified femtosecond signals with ∼50 nm bandwidth (6.5 times beyond the conventional phase-matching bandwidth). This work establishes a compact and efficient route to generate high-power femtosecond pulses directly from picosecond laser systems.

Chern insulator systems are realizable in numerous physical systems and can support robust nonreciprocal transmission of energy. A routing functionality constructed from two counteroriented Chern insulator regions, using coupled Haldane-type systems is proposed. By adjusting the strength of a magnetic field and the frequency of an antenna source, it is possible to steer the flow of energy either completely to the left, completely to the right, or split. Alternatively, two sources can be used to direct the flow of energy. This formulation has the potential to serve as a robust and reconfigurable component in optical transmission.

We study the localization and scattering of a single photon in a waveguide coupled to qubit arrays with quasiperiodic spacings. As the quasiperiodic strength increases, localized subradiant states with extremely long lifetime appear around the resonant frequency and form a continuum band. In stark contrast to the fully disordered waveguide QED where all states are localized, we analytically find that the fraction of localized states is up to $(3-\sqrt{5})/2$ when the modulation frequency is $(1+\sqrt{5})/2$. The localized and delocalized states can be related to excitation in flat and curved inverse energy bands under the approximation of large-period modulation. When the quasiperiodic strength is weak, an extended subradiant state can support the transmission of a photon. However, as the quasiperiodic strength increases, localized subradiant states can completely block the transmission of a single photon in resonance with the subradiant states, and enhance the overall reflection. At a fixed quasiperiodic strength, we also find mobility edge in transmission spectrum, below and above which the transmission is either turned on or off as system size increases.

Subwavelength atomic arrays offer a powerful platform for engineering cooperative light-matter interactions and enabling quantum metasurfaces. We demonstrate that a two-dimensional array of three-level atoms operating under electromagnetically induced transparency can function as a tunable dual-band atomic mirror, where two independently controllable reflection bands emerge from the collective optical responses mediated by dipole-dipole interactions. These resonances yield dual reflection bands with asymmetric linewidths, whose spectral positions and bandwidths can be tuned through the control-field parameters, dipole orientation, incident geometry, and lattice constant. We further identify the conditions under which additional diffraction orders emerge, which delineate the operational and tunable range of the atomic mirror via its collective-mode structure. This scheme provides a fully tunable dual-band atomic mirror operating across broad frequency and angular ranges, potentially supporting the development of reconfigurable photonic elements in low-energy atomic-array platforms.

Quantum dissipation is studied in the superradiant phase of the extended Dicke model. It is demonstrated analytically by quantum mechanical derivation of the Lindblad equation for the Dicke model in the superradiant state coupled to Caldeira-Leggett thermal bath that the effective viscosity appearing in the semiclassical equations of motion of polaritonic condensate survives in the zero-temperature limit $T\to 0$. The nonzero contribution to viscosity contains prefactor $n_B(\omega ,T)+1$ with the Bose-Einstein function $n_B$ of harmonic oscillators in the thermal bath, indicating that virtual excitations of harmonic oscillators in the thermal bath coupled to the polaritons of Dicke model give rise to effective viscosity in the $T\to 0$ limit. Besides, it is demonstrated analytically that correct expression for Lindbladian in the superradiant phase should be built using condensate-shifted creation and annihilation operators of the photons and pseudospin operators in Holstein-Primakoff representation of the coupled to photons two-level systems in order that the system could relax to its minimum energy in the $T\to 0$ limit due to thermal bath provided effective viscosity.

Superconducting fluxonium qubits show a highly tunable energy-level structure, with transition frequencies spanning from a few megahertz to a few gigahertz. This range is well aligned with the operational frequencies of highly coherent micro- and nanomechanical resonators, making fluxonium an attractive candidate for hybrid electromechanical systems. In this work we theoretically investigate a flux-tunable electromechanical system consisting of a fluxonium qubit coupled to a suspended mechanical resonator. The coupling arises from the motion-induced modulation of magnetic flux through the fluxonium loop, enabling both transverse and longitudinal electromechanical interactions that are tunable via external magnetic fields. By optimizing the design parameters of the fluxonium qubit, we demonstrate the feasibility of achieving strong resonant single-photon coupling near the flux-frustration point. We analyze the system dynamics across different coupling regimes, identifying signatures of electromagnetically induced transparency in the longitudinal regime and mode splitting in the transverse regime. Additionally, we show that ground-state preparation of both subsystems is possible through sideband cooling of the mechanical resonator. These results suggest that a fluxonium-based hybrid electromechanical device could be a promising platform for studying macroscopic quantum phenomena and for applications in quantum technology.