Orthogonal circularly polarized light is essential for multiplexing tunable metasurfaces. Mainstream spin-decoupled metasurfaces, consisting of numerous meta-atoms with mirror symmetry, rely on the cooperative modulation of the Pancharatnam-Berry (PB) phase and the propagation phase. This paper demonstrates spin-decoupled functionality through the synergistic utilization of planar chiral meta-atom phase response and the PB phase. Based on the Jones calculus, it has been found that meta-atoms with chiral $C_2$ symmetry possess a larger geometric parameter range with high cross-polarization ratio compared to those with mirror symmetry or higher symmetries at the same aspect ratio. This characteristic is advantageous in terms of enabling high-efficiency manipulation and enhancing the signal-to-noise ratio. As an example, 10 kinds of $C_2$-symmetric chiral meta-atoms with a H-like shape are selected by the self-adaptive genetic algorithm to attain a full $2\pi$ phase span with an interval of $\pi /5$. To mitigate the additional propagation phase change of the guided modes originating from the arrangement alternation upon the rotation of the meta-atoms, the enantiomer of chiral meta-atoms and its PB phase delay are adopted to minimize the difference between the actual and desired target phases. A polarization-insensitive metalens and a chiral virtual-moving metalens array are designed to demonstrate the spin-decoupled function with both high efficiency and signal-to-noise ratio. The work in this paper may trigger more exciting and interesting spin-decoupled multiplexing metasurfaces and broaden the prospect of chiroptical applications.

We investigate the pump wavelength dependence of population inversion in a nitrogen ion when nitrogen gas is irradiated by a femtosecond laser pulse in the wavelength range from ultraviolet to near-infrared. It is found that the population inversions corresponding to the transitions at 391 and 428 nm are very sensitive to the laser wavelength, with extreme values near 400 and 1000 nm. For the ultraviolet pump, the population inversions for 391 and 428 nm transitions occur at different wavelengths: a maximum inversion for the first corresponds to a minimum for the second. This is attributed to the near-resonance single-photon transition combined with the Raman process. A significantly stronger inversion is observed for the near-infrared laser, with the maximum depending on the laser intensity, ascribed to the ac Stark effect. The Floquet theory confirms this conclusion and reveals that the three-photon transition assisted by the strong laser field leads to this enhanced population inversion. This study provides the optimal conditions for air lasing based on ionic transitions of nitrogen molecules.

We demonstrate the existence of a prethermal dynamical phase transition (DPT) for fluctuating optical beams propagating in nonlinear dispersive media. The DPT can be probed by suddenly changing in time the dispersion and nonlinearity parameters of the medium (thus realizing a temporal interface), a procedure that emulates a quench in a massive ${\phi }^4$ model. Above a critical value of the quench identifying the transition, the fluctuating beam after the temporal interface is characterized by a correlation length that diverges algebraically at the transition. Below the critical quench, the beam exhibits an algebraic relaxation and a self-similar scaling. Our analysis also reveals a dimensional crossover of the critical exponent, a characteristic feature of the optical DPT.

We theoretically investigate the quantum coherence and its nonreciprocity in a cavity magnomechanical system composed of a spinning microwave resonant cavity and an yttrium iron garnet sphere. The optical Fizeau shift related to the driving direction of the microwave cavity is responsible for an important source of nonreciprocity in the stability of the system, which leads to the nonreciprocal quantum coherence. Remarkably, the quantum coherence of the mechanical motion mode can be greater than that of the microwave photon and the magnon modes, which describes a significant enhancement of the correlations of the system. We also analyze in detail the influence of the strength of the magnomechanical coupling, the Kerr nonlinearity, and the effective detunings on the quantum coherence. Our paper paves the way for optimizing the stability range of coupled systems and maintaining more reliable quantum coherence by designing and modulating the system parameters in cavity magnetomechanical systems.

In this work we focus on two classes of games: xor nonlocal games and xor* sequential games with monopartite resources. xor games have been widely studied in the literature of nonlocal games, and we introduce xor* games as their natural counterpart within the class of games where a resource system is subjected to a sequence of controlled operations and a final measurement. Examples of xor* games are $2\to 1$ quantum random access codes (QRAC) and the chsh* game introduced by Henaut et al. [Phys. Rev. A 98, 060302 (2018)]. We prove, using the diagrammatic language of process theories, that under certain assumptions these two classes of games can be related via an explicit theorem that connects their optimal strategies, and so their classical (Bell) and quantum (Tsirelson) bounds. We also show that two of such assumptions, the reversibility of transformations and the bidimensionality of the resource system in the xor* games, are strictly necessary for the theorem to hold by providing explicit counterexamples. We conclude with several examples of pairs of xor/xor* games and by discussing in detail the possible resources that power the quantum computational advantages in xor* games.

Nowadays, the quantum swap gate has become an integral part of quantum computing, so the investigation of methods for its realization seems to be an important practical problem for various quantum-optical and information applications. In the present paper, we propose a scheme for performing a swap logic operation in discrete variables in the framework of quantum nondemolition interaction between an atomic ensemble and a multimode light with orbital angular momentum. We discuss in detail the procedure for revealing two-qubit closed subsystems on a set of atomic and field states for different values of the driving field orbital momentum. We also demonstrate the possibility of implementing a parallel multi-two-qubit quantum swap gate.

The approach of photoemission orbital tomography, i.e., the orbital density reconstruction from photoemission of planar molecular layers by using a formalism equivalent to a Fourier transformation, is transferred to free atoms. Absolute radial orbital densities of neon $1s$, $2s$, and $2p$ orbitals are reconstructed with a central-field one-electron model, using well-known atomic photoionization data. The model parameters are optimized by a Markov chain Monte Carlo method with Bayesian inference from which uncertainties for the reconstructed orbital densities are derived. The presented model opens the path for photoemission orbital tomography as a powerful tool, as well as for a quantitative analysis.

Considering bosonic atoms immersed in a dilute Fermi gas, we theoretically investigate medium-induced bosonic clusters associated with fermion-mediated two- and three-body interactions. Using the variational approach combined with the fermion-mediated interactions, we numerically calculate the binding energies of two- and three-body bosonic clusters in a one-dimensional system. It is found that the bosonic clusters can be formed even with a repulsive boson-boson interaction due to the fermion-mediated interactions. Our results would be relevant for ultracold-atomic systems as well as analog quantum simulations of alpha clusters in neutron-rich matter.

Metasurfaces offer a flexible framework for the manipulation of light properties in the realm of thin-film optics. Specifically, the polarization of light can be effectively controlled through the use of thin phase plates. This study aims to introduce a surrogate optimization framework for these devices. The framework is applied to develop two kinds of vortex phase masks (VPMs) tailored for application in astronomical high-contrast imaging. Computational intelligence techniques are exploited to optimize the geometric features of these devices. The large design space and computational limitations necessitate the use of surrogate models like partial least-squares kriging, radial basis functions, or neural networks. However, we demonstrate the inadequacy of these methods in modeling the performance of VPMs. To address the shortcomings of these methods, a data-efficient evolutionary optimization setup using a deep neural network as a highly accurate and efficient surrogate model is proposed. The optimization process in this study employs a robust particle swarm evolutionary optimization scheme, which operates on explicit geometric parameters of the photonic device. Through this approach, optimal designs are developed for two design candidates. In the most complex case, evolutionary optimization enables optimization of the design that would otherwise be impractical (requiring too many simulations). In both cases, the surrogate model improves the reliability and efficiency of the procedure, effectively reducing the required number of simulations by up to 75% compared to conventional optimization techniques.

Majorana stars, the $2S$ spin coherent states that are orthogonal to a spin-$S$ state, offer an elegant method to visualize quantum states, disclosing their intrinsic symmetries. These states are naturally described by the corresponding multipoles. These quantities can be experimentally determined and allow for an SU(2)-invariant analysis. We investigate the relationship between Majorana constellations and state multipoles, thus providing insights into the underlying symmetries of the system. We illustrate our approach with some relevant and informative examples.

The Kirkwood-Dirac (KD) quasiprobability is a quantum analog expression of the phase-space probability in classical statistical mechanics, yet it can behave nonclassically by taking negative or nonreal values. The nonclassicality of the KD quasiprobability distribution has lately been shown to provide advantages in some quantum information tasks and shed light on a number of aspects of quantum physics. Characterization and quantification of KD nonclassicality are crucial both theoretically and experimentally. Most of the existing measures of KD nonclassicality concentrate merely on the negative or/and imaginary part(s) of the KD distribution. Yet it is not clear whether these measures can account for all nonclassicality in the distribution. In this paper, we suggest a KD nonclassicality measure based on the two quantum modification terms in the quasiprobability distribution. The present paper is mainly motivated by the following two considerations: First, the two quantum modification terms are actually the sources of the negative and imaginary parts of the KD quasiprobability respectively. Secondly, these quantum modification terms deviate the KD quasiprobability from the classical joint probability distribution function of two successive projective measurements (termed as the Wigner formula), which can be obtained from the multiplication principle in classical probability theory. Our measure refines the quantification of KD nonclassicality in the sense that even for real and nonnegative KD distribution, its quantum modification terms can still exist, resulting in a nonvanishing KD nonclassicality. As an illustration, we analyze the KD nonclassicality of the positive KD distribution in the postselected metrology scheme [S. Das, S. Modak, and M. N. Bera, Phys. Rev. A 107, 042413 (2023)], which provides interpretation on the source of quantum advantages. We also give an upper bound on the KD nonclassicality measure and offer an easily verifiable sufficient condition for nonvanishing KD nonclassicality.

Using standard results from statistics, we show that for Gaussian quantum systems the distribution of a time measurement at a fixed position can be directly inferred from the distribution of a position measurement at a fixed time as given by the Born rule. In an application to a quantum particle of mass $m$ falling in a uniform gravitational field $g$, we use this approach to obtain an exact explicit expression for the probability density of the time of arrival (TOA). In the long time-of-flight approximation, we predict that the average positive relative shift with respect to the classical TOA in case of a zero initial mean velocity is asymptotically given by $\delta =\frac{q^2}{2}$ when the factor $q\equiv \frac{\hslash }{2m\sigma \sqrt{2gx}}\ll 1$ (semiclassical regime) and by $\delta =\sqrt{{\frac{2}{\pi }}^{\phantom{\int }}}q$ when $q\gg 1$ (quantum regime), where $\sigma$ is the width of the initial Gaussian wave packet and $x$ is the mean distance to the detector. We also discuss experimental conditions under which these predictions can be tested.

Bell diagonal states constitute a well-studied family of bipartite quantum states that arise naturally in various contexts in quantum information. In this paper we generalize the notion of Bell diagonal states to the case of unequal local dimensions and investigate their entanglement properties. We extend the family of entanglement criteria of Sarbicki et al. [Phys. Rev. A 101, 012341 (2020)] to non-Hermitian operator bases to construct entanglement witnesses for the class of generalized Bell diagonal states. We then show how to optimize the witnesses with respect to noise robustness. Finally, we construct bound entangled states that are detected by these witnesses, but not by the usual computable cross norm or realignment and de Vicente criteria.

The purpose of this paper is to discuss the existence of a nontrivial tradeoff relation for estimating two unitary-shift parameters in a relativistic spin-1/2 system. It is shown that any moving observer cannot estimate two parameters simultaneously, even though a parametric model is classical in the rest frame. This transition from the classical model to a genuine quantum model is investigated analytically using a one-parameter family of quantum Fisher information matrices. This paper proposes to use an indicator that can not only detect the existence of a tradeoff relation but can also evaluate its strength. Based on the proposed indicator, this paper investigates the nature of the tradeoff relation in detail.

Quantum information processing has witnessed significant advancements through the application of qubit-based techniques within universal gate sets. Recently, exploration beyond the qubit paradigm to $d$-dimensional quantum units or qudits has opened new avenues for improving computational efficiency. This paper delves into the qudit-based approach, particularly addressing the challenges presented in the high-fidelity implementation of qudit-based circuits due to increased complexity. As an innovative approach towards enhancing qudit circuit fidelity, we explore algebraic relations, such as the Yang-Baxter-like turnover equation, which may enable circuit compression and optimization. The paper introduces the turnover relation for the three-qutrit time propagator and its potential use in reducing circuit depth. We further investigate whether this relation can be generalized for higher-dimensional quantum circuits, including a focused study on the one-dimensional spin-1 Heisenberg model. Our paper outlines both rigorous and numerically efficient approaches to potentially achieve this generalization, providing a foundation for further explorations in the field of qudit-based quantum computing.

We exploit fluctuational electrodynamics to present trace expressions for the torque experienced by arbitrary objects in a passive, nonabsorbing, rotationally invariant background environment. We present trace expressions for equilibrium Casimir torque which complement recently derived nonequilibrium torque expressions and explicate their relation to the Casimir free energy. We then use the derived trace expressions to calculate, via Lagrange duality, semianalytic structure-agnostic bounds on the Casimir torque between an anisotropic (reciprocal or nonreciprocal) dipolar particle and a macroscopic body composed of a local isotropic electric susceptibility, separated by vacuum.

The photoionization and subsequent Auger decay of the $2s$ subshell in vapor phase potassium are investigated using multielectron coincidence spectroscopy with synchrotron radiation. This method, capable of detecting multiple particles simultaneously, enables a comprehensive description of all cascade decay paths of the K $2s$ vacancy. It establishes, at each step of the $2s$ cascade decay, whether the outer $4s$ electron is a spectator or participates in the decay. The study determines the populations of the cascade final state and the different ionization rates. Experimental findings are compared with theoretical predictions derived from the multiconfiguration Dirac-Fock approach and previous observations of cascade processes in argon, emphasizing differences attributed to the presence of an additional $4s$ electron. The research contributes to our understanding of the electronic structure of potassium and quantum mechanical transition processes.