Self-bound quantum droplets form when the mean-field tendency of the gas to collapse is stabilized by the effectively repulsive beyond-mean-field fluctuations. The beyond-mean-field effects depend on Rabi frequency ${\omega }_R$ and quadratic Zeeman effect $q$ for the Rabi-coupled Bose mixtures and the spinor gases, respectively. For a quantum droplet, the effects of varying ${\omega }_R$ and $q$ have recently been examined only for unpolarized Rabi-coupled Bose mixtures and unpolarized spinor gases. In this paper, we theoretically explore the stability of the droplet phase for polarized Rabi-coupled Bose mixtures and polarized spinor gases. We calculate the beyond-mean-field corrections for both gases with polarized order parameters and obtain the phase diagram of the droplets on the parameter space of Rabi frequency ${\omega }_R$ and detuning $\delta$ for Rabi-coupled mixtures and quadratic Zeeman energy $q$ and linear Zeeman energy $p$ for spinor gases. Finally, we highlight the similarities and differences between the two systems and discuss their experimental feasibility.

We investigate theoretically the nonlinear effects in a Floquet-driven optomechanical system with quadratic coupling. When the periodically modulated driving field is strong, we establish an effective time-dependent model to describe this system. Using the Keldysh approach, we calculate the retarded Green's function to study the response behavior of the system to external probing. We find that when the $n\mathrm{th}$ sideband of a probe field satisfies the condition of two-phonon resonance, the transparency window occurs. In addition, we find some abnormal behaviors of the output field, which have no counterpart in the standard optomechanical models. These results provide a promising platform for controlling light propagation and light manipulation operations which have potential applications in quantum precision measurement techniques.

We study propagation of optical vortices (OVs) in coupled parity-time ($\mathcal{P}\mathcal{T}$) symmetric multimode fibers with equal gain and loss. We show that this system has two second-order exceptional points (EPs) of spectral branches confluence where the degenerate modes are odd and even Laguerre-Gaussian (LG) modes. We study the evolution of an arbitrary incoming superposition of LG modes with the same orbital index in such a coupler. We prove that if the loss parameter exceeds its value in the lowest-lying EP, in the limit of the large coupler's length the outcoming field tends to the same attractor state depicted on the orbital Poincaré sphere (PS) by the orbital “black hole.” We show that localization of such a “black hole” on the PS does not change with variations of loss parameter beyond its value at the lowest EP, which can be called the pinning of the orbital “black hole.” We also demonstrate that the fiber ring resonator based on such a coupler can generate an optical vortex frequency comb (FC) far from the EP. The mechanism of such generation is not connected with any nonlinearity in the system. We determine the coupler's parameters under which such generation is feasible. This property can be useful for optimizing fiber-based orbital angular momentum communications.

By numerically solving the semiconductor Bloch equation, we study the high-order harmonic emission spectrum of a MgO crystal under the interaction of a linearly polarized laser pulse. Based on two-band and three-band models, by comparing the variation of the harmonic spectrum with the laser peak intensity, it is found that when the laser intensity is lower, the harmonic spectra from both cases are generally consistent above the minimum band gap. However, some harmonic intensities in the three-band model are significantly enhanced or weakened compared to the two-band case when the laser intensity is higher. By analysis of the electron population and the harmonic phase, it is demonstrated that the harmonic enhancement or reduction is caused by the interference between two interband polarization sources, one of which originates from the current between the first conduction band and the valence band and the other of which is from the polarization between the second and the first conduction band. The harmonic interference between different interband currents contains rich dynamic information and has a significant impact on the harmonic structure and yield, which is helpful for further understanding the generation mechanism of crystal harmonics.

Classical ghost imaging is a new paradigm in imaging where the image of an object is not measured directly with a pixelated detector. Rather, the object is subject to a set of illumination patterns and the total interaction of the object, e.g., reflected or transmitted photons or particles, is measured for each pattern with a single-pixel or bucket detector. An image of the object is then computed through the correlation of each pattern and the corresponding bucket value. Assuming no prior knowledge of the object, the set of patterns used to compute the ghost image dictates the image quality. In the visible-light regime, programmable spatial light modulators (SLMs) can generate the illumination patterns. In many other regimes, such as x rays, electrons, and neutrons, no such dynamically configurable modulators exist, and patterns are commonly produced by employing a transversely translated mask. Moreover, in visible-light regimes, mask switching rates exceeding the fastest SLMs may often be achieved through transverse translation (e.g., rotation) of a nonconfigurable mask. In this simulations-based paper we explore some of the properties of masks or speckle that should be considered to maximize ghost-image quality, given a certain experimental classical ghost-imaging setup employing a transversely displaced but otherwise nonconfigurable mask.

We study a system of two distant quantum emitters coupled via a one-dimensional waveguide where the electromagnetic field has a direction-dependent velocity. As a consequence, the onset of collective emission is nonsimultaneous, and, for appropriate parameters, radiation could be enhanced for one of the emitters while inhibited for the other. Interference effects enable the system to radiate in a preferential direction depending on the atomic state and the field propagation phases. We characterize such directional emission as a function of various parameters, delineating the conditions for optimal directionality.

Ghost imaging research has demonstrated that it is possible to reproduce an image of an object that has not interacted with the imaging light. In this paper we describe theoretically and demonstrate experimentally a coherent displacement imposed nonlocally on one mode of a two-mode state, replicating the ghost imaging effect in the coherent-state basis. We use it to show the possibility of a form of covert information sharing via a ghost displacement operation which enables two distant users to retrieve amplitude and phase information modulated onto a phase-independent thermal state, based only on correlated detection statistics. The displacement operation also provides a secondary probabilistic amplification effect on the mean photon number of the displaced thermal state, which could be exploited for covert quantum illumination experiments.

The Markov, as well as the secular, approximations are key assumptions that have been widely used to model decoherence in a large variety of open quantum systems, but, as far as intermolecular collisions are considered, very little has been done in the time domain. In order to probe the limits of both approximations, we here study the influence of pressure on the alignment revivals (echoes) created in properly chosen gas mixtures (HCl and ${\mathrm{CO}}_2$, pure and diluted in He) by one (two) intense and short laser pulse(s). Experiments and direct predictions using molecular-dynamics simulations consistently demonstrate, through analyses at very short times $(<15 \mathrm{ps})$ after the laser kick(s), the breakdown of these approximations in some of the selected systems. We show that the nonadiabatic laser-induced molecular alignment technique and model used in this paper directly provide detailed information on the physical mechanisms involved in the collisional dissipation. Besides this “fundamental” interest, our findings also have potential practical applications for radiative heat transfer in planetary atmospheres and climate studies. Indeed, short time delays in the dipole autocorrelation function monitoring the light absorption spectrum correspond to large detunings from the optical resonances in the frequency domain, thus influencing the atmospheric transparency windows. Furthermore, the fact that the approach tested here for linear rotors can potentially be applied to almost any gas mixture (including, for instance, nonlinear and/or reacting molecules) further strengthens and broadens the perspectives that it opens.

We studied Fresnel diffraction of a Laguerre-Gaussian beam ${\text{LG}}_{p,l}$ with arbitrary azimuthal $l$ and radial $p$ indices on a grating induced during its Raman interaction with a spatially periodic pump field in an atomic medium. The diffraction pattern turned out to be more complex than the classical Talbot effect observed when a plane wave illuminates a two-dimensional grating. The simulation results show that, under certain conditions, at distances corresponding to the classical Talbot planes (integer and fractional), periodic amplitude-phase distributions appear. The diffraction patterns are not a probe-field image in the induced grating plane, but a regular array of vortex annular-shaped microbeams with an inhomogeneous intensity distribution depending on the $l$ and $p$ indices and with a topological charge equal to that of the initial probe beam. The intensity and spatial distribution of diffraction patterns can be controlled by Raman amplification in the induced grating by varying the pump-field intensity or the Raman detuning.

Spin-mechanical hybrid systems have been widely used in quantum information processing. However, the spin-mechanical interaction is generally weak, making it a critical challenge to enhance the spin-mechanical interaction into the strong coupling or even ultrastrong coupling regime. Here, we propose a protocol that can significantly enhance the spin-mechanical coupling strength with a diamond spin vacancy and a levitated micromagnet. A driving electrical current is used to modulate the mechanical motion of the levitated micromagnet, which induces a two-phonon drive and can exponentially enhance the spin-phonon and phonon-medicated spin-spin coupling strengths. Furthermore, a high-fidelity Schrödinger cat state and an unconventional two-qubit geometric phase gate with high fidelity and faster gate speed can be achieved using this hybrid system. This protocol provides a promising platform for quantum information processing with nitrogen-vacancy spins coupled to levitated micromagnets.

We propose a scheme to implement the quantum walk for SU(1,1) in the phase space, which generalizes those associated with the Heisenberg-Weyl group. The movement of the walker described by the SU(1,1) coherent states can be visualized on the hyperboloid or the Poincaré disk. In both one-mode and two-mode realizations, we introduce the corresponding coin-flip and conditional-shift operators for the SU(1,1) group, whose relations with those for the Heisenberg-Weyl group are analyzed. The probability distribution, the standard deviation, and the von Neumann entropy are employed to describe the walking process. The nonorthogonality of the SU(1,1) coherent states precludes the quantum walk for SU(1,1) from the idealized one. However, the overlap between different SU(1,1) coherent states can be reduced by increasing the Bargmann index $k$, which indicates that the two-mode realization provides more possibilities to simulate the idealized quantum walk.

In this work, we show how the resource theory of nonstabilizerness quantifies the hardness of direct fidelity estimation protocols. In particular, the resources needed for a direct fidelity estimation conducted on generic states, such as Pauli fidelity estimation and shadow fidelity estimation protocols, grow exponentially with the stabilizer Rényi entropy. Remarkably, these protocols are shown to be feasible only for those states that are useless to attain any quantum speedup or advantage. This result suggests the impossibility of estimating efficiently fidelity for generic states and, at the same time, leaves the window open to those protocols specialized at directly estimating the fidelity of particular states. We then extend our results to quantum evolutions, showing that the resources needed to certify the quality of the implementation of a given unitary $U$ are governed by the nonstabilizerness in the Choi state associated with $U$, which is shown to possess a profound connection with out-of-time order correlators.

We use the radiative potential method to perform a detailed study of quantum electrodynamics (QED) radiative corrections to electric dipole (E1) transition amplitudes in heavy alkali-metal atoms Rb, Cs, Fr, and alkali-metal-like ions ${\mathrm{Sr}}^{+}$, ${\mathrm{Ba}}^{+}$, and ${\mathrm{Ra}}^{+}$. The validity of the method is checked by comparing with the results of rigorous QED in simple atomic potentials. We study the effects of core relaxation, polarization of the core by the E1 field, and valence-core correlations on QED, which are shown to be important in some cases. We identify several transitions for which the QED contribution exceeds the deviation between atomic theory and experiment.

We report on the observation of photoassociation (PA) near the $\text{[Rb]}5s{}^{2}S_{\frac{1}{2}}+\text{[Yb]}6s6p{}^{3}P_1$ asymptote in a mixture of ${}^{87}\mathrm{Rb}$ and ${}^{170}\mathrm{Yb}$. In a search spanning binding energies between 0.1 and 11 GHz, a single pair of interspecies PA resonances is detected around 3.1 GHz. These resonances are characterized by extracting PA rates, binding energies, and Zeeman shift coefficients. Using one of these resonances, two-photon photoassociation is performed, improving on previous measurements of the binding energies of the two least bound states in the electronic ground state and demonstrating intercombination-line photoassociation as a powerful spectroscopic tool. We discuss implications for pathways towards RbYb molecules in the absolute ground state.

We study the nature of the ergodicity-breaking “quantum many-body scar” states that appear in the PXP model describing constrained Rabi oscillations. For a wide class of bipartite lattices of Rydberg atoms, we reveal that the nearly energy-equidistant tower of these states arises from the Hamiltonian's close proximity to a generalized projector-embedding form in Phys. Rev. Lett. 119, 030601 (2017), a structure common to many models hosting quantum many-body scars. We construct a non-Hermitian but strictly local extension of the PXP model hosting exact quantum scars and show how various Hermitian scar-stabilizing extensions from the literature can be naturally understood within this framework. The exact scar states are obtained analytically as large spin states of explicitly constructed pseudospins. The quasiperiodic motion ensuing from the Néel state is finally shown to be the projection onto the Rydberg-constrained subspace of the precession of the large pseudospin.