Quantum coherence is critical resource for applications in quantum technology, among which quantum-enhanced sensing represents a typical example. Compared with quantum metrology with entangled states of multiple qubits, bosonic interferometers have the advantage of being hardware efficient, enabled by the infinite-dimension Hilbert space of bosonic modes. Phase estimations with precisions approaching the Heisenberg limit have been demonstrated with superpositions of Fock states of single bosonic modes. We here present a scheme for realizing quantum metrology based on a fundamentally distinct kind of superposition states, Schrödinger cat states, defined as quantum superpositions of two quasiclassial states of a bosonic mode. The interferometer aims to estimate the frequency shift of a photonic mode with respect to a reference frequency, which produces a phase-space rotation. We find that such a phase-space rotation can be extracted with a qubit, without mapping the state of the photonic mode to the qubit. This phase-space rotation is directly related to the qubit's population through the combination of a phase-space displacement, a dispersive qubit-photon interaction, and two qubit rotations. The signal-to-noise ratio exhibits a scaling approaching the Heisenberg limit, like interferometers based on Fock state superpositions, but without requiring a step-by-step procedure to prepare the resource state.

We present a gradient-based method to construct high-fidelity, two-qubit quantum gates in a system consisting of two transmon qubits coupled via a tunable coupler. In particular, we focus on single-flux-quantum pulses as a promising and scalable alternative to traditional control schemes that use microwave electronics. We develop a continuous embedding scheme to optimize these discrete pulses, taking advantage of autodifferentiation of our model. This approach allows us to achieve fSim-type gates with average gate fidelities on the order of 99.99% and controlled-$Z$ and controlled-not gates with fidelities above 99.9%. Furthermore, we provide an alternative semianalytical construction of these gates via an exact decomposition using a pair of fSim gates, which leads to the reduction in memory required to store the associated pulse sequences.

We propose a Lyapunov control strategy to accelerate dissipative quantum state preparation by designing the control fields with respect to intermediate excited states instead of the target steady state. This approach, termed intermediate-state-based Lyapunov control (ISB-LC), allows for greater flexibility in designing control Hamiltonians and facilitates faster convergence without compromising robustness. We demonstrate the effectiveness of ISB-LC using several open quantum systems, including three-, four-, and six-level configurations, as well as entangled steady-state generation in interacting Rydberg atoms. Numerical simulations show that ISB-LC can reduce convergence times by more than an order of magnitude, offering a promising route for enhancing dissipation-based quantum control protocols.

The superadditivity of quantum channel capacity is an important feature of quantum information theory, different from classical theory, which has been attracting attention. Recently a special channel called “platypus channel” exhibits superadditive quantum capacity when combined with qudit erasure channels. Here we consider the generalized platypus channel, prove that it has computable channel capacities, such as both private and classical capacity equal to 1, and in particular, the generalized platypus channel still displays the superadditivity of quantum capacity when combined with qudit erasure channels and multilevel amplitude damping channels, respectively.

Autocorrelation measurement is theoretically demonstrated to characterize attosecond pulses by studying the two-photon double ionization (TPDI) process. An interferometric autocorrelation curve is presented in the change of TPDI probability with the time delay between two identical attosecond pulses, and its full width at half maximum (FWHM) ${\tau }_e$ has a relationship ${\tau }_e=1.77\tau +15$ with the FWHM $\tau$ of the attosecond pulse. The curve is also decoded to obtain the center frequency and FWHM of the attosecond pulse by fitting.

As resonantly interacting trimers of the type AAB are progressively squeezed from $D=3$ to $D=2$, unatomic states emerge. We calculated the contacts from the high-momentum tail of the single-particle densities. The sharp increase in the contacts serves as a signature of the transition between the Efimov and unatomic regimes, characterized by the emergence of continuous scale invariance when the system reaches a critical dimension $D_c$. This continuous scale invariance starts to dominate the behavior of the system at the dimension $\overline{D}<D_c$, below which the trimers' momentum-distribution tails exhibit a power-law behavior signaling the unatomic regime. To illustrate our findings, we studied compounds with the forms ${}^7\mathrm{Li}\text{−}{}^{23}\mathrm{Na}_2$, ${}^7\mathrm{Li}\text{−}{}^{87}\mathrm{Rb}_2$, and ${}^7\mathrm{Li}\text{−}{}^{133}\mathrm{Cs}_2$. The increase in the trimers' mass imbalance reduces the interval between $D_c$ and $\overline{D}$. The emergence of unatomic states can be experimentally verified by observing the two-body contact parameter, which is a quantity directly related to the thermodynamic properties of the gas.

We present a quantitative analytical framework for calculating two-point velocity correlations in compressible quantum fluids, focusing on two key classes of superfluid excitations: vortices and rarefaction pulses. We employ two complementary approaches. First, we introduce a new Ansatz for vortex cores in planar quantum fluids that enhances analytic integrability. This Ansatz yields closed-form expressions for power spectra and velocity correlations for general vortex distributions. Using it, we identify distinct signatures of short- and long-range velocity correlations corresponding to vortex dipoles and vortex pairs, respectively. Second, we analyze the fast rarefaction pulse regime of the Jones-Roberts soliton. By applying the asymptotic high-velocity wave function, we derive analytic expressions for the velocity power spectrum and correlation function, capturing the soliton characteristic length scale. We validate our analytical results for the homogeneous system against numerical treatment of a large trapped system, finding close quantitative agreement. Our findings provide the first analytic treatment of velocity correlations for Jones-Roberts solitons in quantum fluids of light [M. Baker-Rasooli et al., Phys. Rev. Lett. 134, 233401 (2025)] and establish a foundation for characterizing vortices and solitons in compressible quantum fluids.

We consider the ground-state properties of a lattice Bose polaron, a quasiparticle arising from the interaction between an impurity confined to an optical lattice and a surrounding homogeneous Bose-Einstein condensate hosting phononic modes. We present an extension of Wegner's and Wilson's flow equation approach, the operator-valued-flow-equation approach, which allows us to calculate the renormalized dispersion of the polaron and assess the role of two-phonon scattering processes on the dispersion. The results obtained in this way are compared to a variational mean-field approach, which will allow us to demonstrate that the flow equations behave correctly at weak coupling. We find that in certain impurity phonon interaction regimes the shape of the dispersion is significantly altered by the inclusion of two-phonon scattering events as opposed to only single-phonon scattering events. Moreover, our results predict that a polaronic bound state may emerge, which is not present in Fröhlich-type models that only consider single-phonon scattering events.

A supersonic flow past an obstacle can generate a rich variety of wave excitations. This paper investigates, both analytically and numerically, two types of excitations generated by the flow of a Lee-Huang-Yang quantum fluid past an obstacle: linear radiation and oblique dark solitons. We show that wave crests of linear radiation can be accurately described by the proper modification of the Kelvin original theory, while the oblique dark soliton solution is obtained analytically by transformation of the one-dimensional soliton solution to the obstacle's reference frame. A comparison between analytical predictions and numerical simulations demonstrates good agreement, validating our theoretical approach.

Optical micromotors, key components in microrobotics and biomedical applications, have attracted widespread attention for their ability to manipulate and rotate microscopic particles using light. In this study, we present a tunable bilayer optical matter motor controlled by distance-modulated electromagnetic interlayer coupling. Using the generalized Lorentz-Mie scattering theory for multispheres and the Langevin dynamics method, we investigate the dynamics of the bilayer motor during light-driven self-organization and rotation. Through the modulation of the interlayer distance to tune coupling effects, we demonstrate the capacity to selectively manipulate the rotation modes of the motor—both layers can rotate clockwise (receiving negative optical torque), rotate anticlockwise, or counterrotate. These modes arise from changes in the relative contributions of excited electric and magnetic multipoles, offering an intuitive understanding of interlayer coupling from the perspective of multipoles. The bilayer motor exhibits stable rotation across a wide range of coupling distances due to strong long-range electromagnetic coupling, and demonstrates excellent robustness to both the polarization of the incident light and structural offset from the beam center, facilitating experimental implementation. Our structural stability analysis indicates that bilayer optical matter motors constructed from equilateral triangular units maintain remarkable stability during light-driven self-organization and rotation, even with varying interlayer coupling. This research enhances our understanding of light-matter interactions in the dynamics of bilayer optical matter systems while providing theoretical foundations for novel micromotor design.

Bound states in the continuum (BICs) are electromagnetic eigenstates that theoretically exhibit infinite quality factors. However, scattering losses induced by manufacturing defects inevitably affect the confinement performance of BICs. Studies have shown that combining multiple BICs can enhance their robustness against such defects. Currently, approaches to merging BICs at off-$\mathrm{\Gamma }$ points typically rely on altering the geometry of the central structure, which not only increases the complexity of device design but also potentially raises costs. Based on this, we propose a scheme that maintains the central structure while introducing specific defect holes to break the in-plane symmetry of the system, thereby inducing the splitting of the higher-order BIC and forming multiple off-$\mathrm{\Gamma }$ BICs. Subsequently, these split BICs, along with the accidental BICs, will merge at the off-$\mathrm{\Gamma }$ point as the structural parameters are varied. The proposed concept not only enhances optical confinement but also enables flexible control over the splitting direction of the higher-order BIC by manipulating the spatial positions of the defect apertures. More importantly, by rationally designing the positions of defects, we can construct merging BICs at arbitrary wave vectors through manipulations of split BICs and accidental BICs. Furthermore, even in the presence of manufacturing defects, the merging BICs demonstrate exceptional robustness. This provides new insights for the flexible control of BICs in diversified applications while also expanding their potential in a broader momentum space.

In this study, we propose and analyze a highly efficient scheme for realizing sensitive mass sensors in a quadratically coupled optomechanical system, assisted by a gain cavity via a nonlinear second-order sideband process. Moving beyond conventional linearized approximations, we derive analytical expressions for both the second-order sideband efficiency and mass-sensing sensitivity using a perturbation approach. Our scheme enables mass detection by monitoring shifts in the second-order sideband generation efficiency when a target mass is deposited on the dielectric membrane. Using experimentally feasible parameters, we identify the maximum achievable efficiency of the second-order upper sideband and its corresponding optimal sensitivity. Remarkably, with the aid of a gain cavity, both the sideband efficiency and mass sensitivity are significantly enhanced—particularly near the exceptional point, where improvements exceeding three orders of magnitude are demonstrated. This work provides a promising approach to high-precision mass measurement, advancing and complementing existing methodologies.

We investigate the focusing characteristics of scalar and vector beams within an atomic medium. An active-Raman-gain configuration is employed to achieve significant Kerr nonlinearity in a four-state atomic system. The probe beams can attain focusing within the medium through careful selection of input beam intensities and the spatial profile of the control field. We analytically derive the linear and third-order nonlinear susceptibilities for both scalar and vector probe beams. Our observations indicate that, in addition to the energy transfer from the control beam to the probe beam, the giant cross-Kerr nonlinearity facilitates the focusing of the scalar probe beam into a significantly smaller spot size. Conversely, the vector probe beams exhibit gain-induced narrowing. Furthermore, we evaluate the state of polarization for the vector beam at the minimum beam waist, observing a polarization rotation and a change in ellipticity during propagation. Through the mechanism of focusing, we achieve a reduced spot size for the probe beam, which may have substantial implications for resolution enhancement in microscopy applications.

We theoretically investigate light-matter interference phenomena in a hybrid optical cavity–Bose-Einstein condensate (BEC) system incorporating both an intracavity optical parametric amplifier (OPA) and atom-atom collisions. Starting from the linearized quantum Langevin equations, we derive the drift matrix and confirm system stability by verifying that all eigenvalue real parts remain negative. We then analyze the probe transmission spectrum under two complementary control mechanisms. First, with collisional interactions suppressed, increasing the OPA gain within the stability region transforms the standard Lorentzian cavity response into a tunable Fano-like profile, accompanied by asymmetric dispersive line shapes and pronounced group delays. Second, with the OPA turned off, including atomic collisions yields an intrinsically narrow Bogoliubov excitation that interferes with the cavity continuum, reproducing Fano resonances, “S-shaped” phase dispersion, slow- and fast-light windows, and positive Goos-Hänchen shifts. By examining both absorptive (real) and dispersive (imaginary) components of the transmitted field, we establish direct links between destructive interference pathways, steep phase slopes, and enhanced optical delays. Our results demonstrate that parametric amplification and collisional nonlinearities are equally versatile tools for engineering quantum interference in cavity QED with ultracold atoms, opening prospects for reconfigurable slow-light buffers, Fano-switchable photonic routers, and atom-integrated delay lines.

How environments affect the dynamics of quantum systems remains a central question in understanding transitions between quantum and classical phenomena and optimizing quantum technologies. A paradigm model to address the above question is the generalized Jaynes-Cummings model, in which a two-level particle is coupled to its environment modeled by a continuum of boson modes. Previous analytic solutions show that, starting from the initial condition that the particle is in its excited state and the boson modes are in their vacuum state, the time evolution of the probability that the particle occupies the excited state exhibits a dynamical transition as the system-environment coupling varies; when the coupling is weak, the probability decays to zero monotonically, while a finite weight of the particle is localized in the excited state when the coupling is sufficiently strong. Here we study the dynamical transition for the case that $N$ particles are initially excited with the boson modes in their vacuum state. The boson modes are assumed to follow a spectral function $J\left(\omega \right)\sim {\omega }^s$ up to a cutoff frequency ${\omega }_c$ with $s>0$. In particular, we access the effects of an all-to-all Ising type interaction we introduce between the particles. Our calculation is carried out by the nonperturbative numerical renormalization group method. We find that the value of the critical coupling for the transition exhibits a maximum at a finite $N$ and decreases with sufficiently large $N$, and is suppressed (enlarged) by a ferromagnetic (antiferromagnetic) Ising interaction. Our results enrich understanding of environmental effects on interacting quantum systems.

The photon blockade is a promising mechanism for single-photon sources. However, in most systems with photon blockade, the brightness of the quantum source (being proportional to the average photon number in cavities) is limited. A recent experiment [Lei et al., Phys. Rev. Lett. 130, 093801 (2023)] proves that Friedrich-Wintgen bound states in the continuum (BICs) arise when a microresonator containing a pair of near-resonant modes is side-coupled to an adjacent bus waveguide where the gap between them is narrow. To our knowledge, the impacts of BICs on the single-photon purity and brightness in the blockade process remain less explored. Here, we put forward a scheme to simultaneously enhance both the purity and brightness of single-photon emission in one cavity mode of such an integrated microresonator-waveguide system, while the other cavity mode is considered to be an active mode, modeled as incoherent-pumping (gain)-like laser production. Physically, this enhancement is achieved through BICs produced by quantum destructive interference between different dissipative pathways. We demonstrate that BICs enable the improvement of emission brightness by 1 to 2 orders of magnitude, accompanied by high single-photon purity of 0.98 (or strong photon blockade of 0.02) compared with the BIC-free configuration. As a check, the full numerical results for the second-order correlation function and the average photon number of the emission cavity mode, which respectively determine the purity and brightness of single-photon emission, are in excellent agreement with the semianalytical solutions. Our BICs-based work offers a promising avenue for constructing single-photon sources with high purity and high brightness.

This paper deals with the spectral densities of a dispersive dielectric object in the framework of macroscopic quantum electrodynamics based on the modified Langevin noise formalism. In this formalism, the electromagnetic field in the presence of a dielectric object has two contributions, one taking into account the polarization current fluctuations of the object and the other taking into account the vacuum field fluctuations scattered by the object. The combined effect of these fields on the dynamics of a quantum emitter is described via two independent continuous bosonic reservoirs, a medium-assisted reservoir and a scattering-assisted reservoir, each characterized by its own spectral density and initial quantum state. For initial thermal states of the two reservoirs at different temperatures, the standard approach based on the knowledge of the dyadic Green's function of the dielectric object at the quantum emitter position cannot be employed. We map the two reservoirs to a single equivalent reservoir with a temperature-dependent effective spectral density and initially in its vacuum state, focusing on the case of a homogeneous dielectric sphere. We derive analytical expressions for the medium-assisted, scattering-assisted, and effective spectral densities in this setting. We then study the dynamics of the quantum emitter for initial thermal states of the two reservoirs, adopting a nonperturbative approach.