Spin-to-orbit conversion of light is a dynamical optical phenomenon in nonparaxial fields leading to various manifestations of the spin and orbital Hall effect. However, the effects of the spin-orbit interaction (SOI) have not been explored extensively for structured Gaussian beams carrying no intrinsic orbital angular momentum. Indeed, the SOI effects on such structured beams can be directly visualized due to azimuthal rotation of their transverse intensity profiles, a phenomenon we call the rotational spin-Hall effect. In this paper we show that for an input circularly polarized (right or left) Hermite-Gaussian $\left({\text{HG}}_{10}\right)$ mode, the SOI leads to a significant azimuthal rotation of the transverse intensity distribution of both the orthogonal circularly polarized (left or right) component and the longitudinal field intensity with respect to the input intensity profile. We validate our theoretical and numerically simulated results experimentally by tightly focusing a circularly polarized ${\text{HG}}_{10}$ beam in an optical tweezers configuration and projecting out the opposite circular polarization component and the transverse distribution of the longitudinal field intensity at the output of the tweezers. We also measure the rotational shift as a function of the refractive index contrast in the path of the tightly focused light and in general observe a proportional increase. The enhanced spin-orbit conversion in these cases may lead to interesting applications in inducing complex dynamics in optically trapped birefringent particles using structured Gaussian beams with no intrinsic orbital angular momentum.

We revisit the theoretical description of the ultrastrong light-matter interaction in terms of exactly solvable effective Hamiltonians. A perturbative approach based on polaronic and spin-dependent squeezing transformations provides an effective Hamiltonian for the quantum Rabi model up to the second order in the expansion parameter. The model consistently includes both rotating and counter-rotating terms, going therefore beyond the rotating-wave approximation. Analytical and numerical results show that the proposed Hamiltonian performs better than the Bloch-Siegert model when calculating operator averages (e.g., the mean photon number and number of excitations). This improvement is due to a refined calculation of the dressed states within the present model. Regarding the frequency shift induced by the qubit-photon interaction, we find a different sign from the Bloch-Siegert value. This influences the structure of the eigenstates in a nontrivial way and ensures the correct calculation of the number of excitations associated with a given dressed state. As a consistency check, we show that the exactly solvable independent boson model is reproduced as a special limit case of the perturbative Hamiltonian.

Collective excitations in regular structures of strongly interacting driven dipoles have been found to lead to strongly antibunched statistics of the emitted photons $[g^{\left(2\right)}\left(0\right)\ll 0.5]$, but precise control of the number of emitters and their spatial distribution is in general a complex experimental task. We overcome this limitation by showing quantum light emission from densely packed driven dipoles does not require their arrangement in any specific structure, pointing strongly towards possible single-photon sources based on hot thermal vapors confined within nanocells, or on irregular quantum dot ensembles.

Exceptional points (EPs) are a special kind of singularity that appear as topological defects in parameter-dependent open systems. Here we propose the concept of conjugate EPs, where a level-repulsion phenomenon between two coupled complex states can occur in the vicinity of a square-root branch point, which is analytically associated with the presence of two complex conjugate EPs. Depending on the iteration parameter, two corresponding levels are analytically connected via one of two conjugate EPs. Here, we report the hosting of two conjugate EPs in two complementary equivalent systems connected with time-reversal ($\mathcal{T}$) symmetry by using the framework of a gain-loss assisted dual-mode planar optical waveguide. We establish that if the complex potential of any system hosts an EP, then the $\mathcal{T}$-symmetric potential of the same system can host the associated conjugate EP. Owing to the EP-aided nonadiabatic population transfer based on device chirality, the reverse-chiral responses of two $\mathcal{T}$-symmetric devices have been explored in the context of an asymmetric-mode-conversion process. The proposed scheme has the potential to open up a credible platform to study the physics of EPs in $\mathcal{T}$-symmetric systems.

In this work we study the production and swapping of non-Gaussian multipartite entanglement in a setup containing a parametric amplifier which generates three photons in different modes coupled to three qubits. We prove that the entanglement generated in this setup is of non-Gaussian nature. We introduce witnesses of genuine tripartite non-Gaussian entanglement, valid for both mode and qubit entanglement. Moreover, those witnesses show that the entanglement generated among the photons can be swapped to the qubits, and indeed the qubits display non-Gaussian genuine tripartite entanglement over a wider parameter regime, suggesting that our setup could be a useful tool to extract entanglement generated in higher-order parametric amplification for quantum metrology or quantum computing applications.

Magic can be distributed nonlocally in many-body entangled states, such as the low energy states of condensed matter systems. Using the Bravyi-Kitaev magic state distillation protocol, we find that nonlocal magic is distillable and can improve the distillation outcome. We analyze a few explicit examples and show that spin squeezing can be used to convert nondistillable states into distillable ones. Our analysis also suggests that the conventional product input states assumed by magic distillation protocols are extremely atypical among general states with distillable magic. It further justifies the need for studying a diverse range of entangled inputs that yield magic states with high probability.

Variational quantum algorithms (VQAs) have been considered to be useful applications of noisy intermediate-scale quantum (NISQ) devices. Typically, in VQAs, a parametrized ansatz circuit is used to generate a trial wave function, and the parameters are optimized to minimize a cost function. On the other hand, blind quantum computing (BQC) has been studied in order to provide a quantum algorithm with security by using cloud networks. A client with a limited ability to perform quantum operations hopes to have access to a quantum computer of a server, and BQC allows the client to use the server's computer without leakage of the client's information (such as input, running quantum algorithms, and output) to the server. However, BQC is designed for fault-tolerant quantum computing, and this requires many ancillary qubits, which may not be suitable for NISQ devices. Here, we propose an efficient way to implement the NISQ computing with guaranteed security for the client. In our architecture, only $N+1$ qubits are required, under an assumption that the form of ansätze is known to the server, where $N$ denotes the necessary number of the qubits in the original NISQ algorithms. The client only performs single-qubit measurements on an ancillary qubit sent from the server, and the measurement angles can specify the parameters for the ansätze of the NISQ algorithms. The no-signaling principle guarantees that neither parameters chosen by the client nor the outputs of the algorithm are leaked to the server. This work paves the way for new applications of NISQ devices.

We study the Doppler-cooling of radial two-dimensional (2D) Coulomb crystals of trapped barium ions in a radiofrequency trap. Ions in radial 2D crystals experience micromotion of an amplitude that increases linearly with the distance from the trap center, leading to a position-dependent frequency modulation of laser light in each ion's rest frame. We use two tones of Doppler-cooling laser light separated by approximately 100 MHz to efficiently cool distinct regions in the crystals with differing amplitudes of micromotion. This technique allows us to trap and cool more than 50 ions populating four shells in a radial 2D crystal, where with a single tone of Doppler cooling light we are limited to 30 ions in three shells. We also individually characterize the micromotion of all ions within the crystals and use this information to locate the center of the trap and to determine the Matthieu parameters $q_x$ and $q_y$.

We determine in a nonperturbative way the critical velocity for superfluidity of a generic quantum fluid flowing past a localized obstacle in the one-dimensional mean-field regime. We get exact expressions in the narrow- and wide-obstacle limits and interpolate them numerically using an original relaxation algorithm for the stationary problem. The existence of a Josephson-type critical current across a very high and slowly varying obstacle is discussed. Particle losses, if present, are treated within an adiabatic approach of the dynamics giving results in excellent agreement with full numerics. Relevant for experiments with quantum fluids of matter, of mixed matter-light, and of light, our study paves the way for further nonperturbative investigations in higher dimensions and beyond mean-field theory.

Recent experiments on spontaneous emission of atomic matter waves [Krinner et al., Nature (London) 559, 589 (2018); Stewart et al., Phys. Rev. Research 2, 043307 (2020)] open a window into the behavior of quantum emitters coupled to a waveguide. Here we develop an approach based on infinite products to study this system theoretically, without the need to approximate the band dispersion relation of the waveguide. We solve the system for a one-dimensional array of one, multiple, and an infinite number of quantum emitters and compare with the experiments. This leads to a detailed characterization of the decay spectrum, with a family of in-gap bound states, mechanisms for enhanced Markovian emission different from superradiance, and the emergence of matter-wave polaritons.

The gas-phase ion-neutral reaction of ${\mathrm{CCl}}^{+}$ $+$ benzene $\left({\mathrm{C}}_6{\mathrm{H}}_6\right)$ is investigated using a linear Paul ion trap coupled to a time-of-flight mass spectrometer. Low collision energies are achieved by sympathetically cooling ${\mathrm{CCl}}^{+}$ reactant ions with cotrapped laser-cooled ${\mathrm{Ca}}^{+}$. The observed products include the astrochemically relevant carbocations ${\mathrm{C}}_7{\mathrm{H}}_5^{+}$, ${\mathrm{C}}_5{\mathrm{H}}_3^{+}$, and ${\mathrm{C}}_3{\mathrm{H}}_3^{+}$, as well as ${\mathrm{C}}_3{\mathrm{H}}_2{\mathrm{Cl}}^{+}$. Branching ratios of these products are measured, and ${\mathrm{C}}_7{\mathrm{H}}_5^{+}$, a carbon-growth species, is favored. Complementary electronic structure calculations provide thermodynamic limits for the reaction and allow for assignment of reaction products to specific structural isomers. Only one exoergic isomer is identified for each observed product with the exception of ${\mathrm{C}}_7{\mathrm{H}}_5^{+}$ where many identified structural isomers are exoergic. The results from this Letter broaden our understanding of the reactivity and possible role of ${\mathrm{CCl}}^{+}$ and ${\mathrm{C}}_6{\mathrm{H}}_6$ in interstellar chemistry. Furthermore, this Letter provides insight into a potential pathway to larger carbocations that may be precursors to more complex polycyclic aromatic hydrocarbons.

In the demonstrations of slow light in photorefractive (PR) materials, the delays are only reported for long pulse durations (ms to s) that are in discrepancy with typical ns-pulse length in data processing applications. Here we demonstrate that the slowdown of light in a PR crystal can be also achieved in the nanosecond regime at room temperature. We experimentally show that the use of a pulsed laser with a high peak intensity may indeed reduce the ${\mathrm{Sn}}_2{\mathrm{P}}_2{\mathrm{S}}_6$ crystal response time to a scale of ns and achieve a two-wave mixing (TWM) gain of $7.9{\text{cm}}^{-1}$. We show the slowdown of light pulses whose duration ranges from 10 to 100 ns with a fractional delay up to 0.3. These results are similar to those obtained in optical fibers, except that the slow light with PR TWM is achieved through smaller propagation lengths and at wide wavelength ranges. This could reveal potential applications of PR slow light systems and have a significant impact for future optical communications systems.

We investigate the longstanding problem of thermalization of quantum systems coupled to an environment by focusing on a bistable quartic oscillator interacting with a finite number of harmonic oscillators. In order to overcome the exponential wall that one usually encounters in grid-based approaches to solve the time-dependent Schrödinger equation of the extended system, methods based on the time-dependent variational principle are best suited. Here we will apply the method of coupled coherent states [D. V. Shalashilin and M. S. Child, J. Chem. Phys. 113, 10028 (2000)]. By investigating the dynamics of an initial wave function on top of the barrier of the double well, it will be shown that only a handful of oscillators with suitably chosen frequencies, starting in their ground states, is enough to drive the bistable system close to its uncoupled ground state. The long-time average of the double-well energy is found to be a monotonously decaying function of the number of environmental oscillators in the parameter range that was numerically accessible.

We analyze the efficiency of the quantum Otto cycle applied to a superconducting cavity. We consider its description in terms of a full quantum scalar field in a one-dimensional cavity with a time-dependent boundary condition that can be externally controlled to perform and extract work unitarily from the system. We study the performance of this machine when acting as a heat engine as well as a refrigerator. It is shown that, in a nonadiabatic regime, the efficiency of the quantum cycle is affected by the dynamical Casimir effect that induces a sort of quantum friction that diminishes the efficiency. We also find regions of parameters where the effect is so strong that the machine can no longer function as an engine since the work that would be produced is completely consumed by the quantum friction. However, this effect can be avoided for some particular temporal evolutions of the boundary conditions that do not change the occupation number of the modes in the cavity, leading to a highly improved efficiency.

We study supersolid-like crystalline structures emerging in the stationary states of a quasi-two-dimensional spin-orbit (SO)-coupled spin-2 condensate in the ferromagnetic, cyclic, and antiferromagnetic phases by solving a mean-field model. Interplay of different strengths of SO coupling and interatomic interactions gives rise to a variety of nontrivial density patterns in the emergent solutions. For small SO-coupling strengths $\gamma$ $(\gamma \approx 0.5)$, the ground state is an axisymmetric multiring soliton for polar, cyclic, and weakly ferromagnetic interactions, whereas for stronger ferromagnetic interactions a circularly asymmetric soliton emerges as the ground state. Depending on the values of interaction parameters, with an increase in SO-coupling strength, a stripe phase may also emerge as the ground state for polar and cyclic interactions. For intermediate values of SO-coupling strength $(\gamma \approx 1)$, in addition to these solitons, one could have a quasidegenerate triangular-lattice soliton in all magnetic phases. On further increasing the SO-coupling strength $(\gamma ⪆4)$, a square-lattice and a superstripe soliton emerge as quasidegenerate states. The emergence of all these solitons can be inferred from a study of solutions of the single-particle Hamiltonian.

Encouraged by recent experimental breakthroughs, we investigate the types of bright solitons that can exist in the presence of a Kerr nonlinearity and quadratic, quartic, and sextic dispersion by examining the nature of the soliton tails. We verify the soliton shape by full numerical calculations and find analytic solutions for some parameters. We reveal the sensitivity of the interaction dynamics to the particular choice of the dispersion parameters. These results provide a framework for experiments in this regime and for exploring parameter spaces with even higher dimensions.

In the presence of arbitrary three-dimensional linear media with material loss and amplification, we present an electromagnetic field quantization scheme for quasinormal modes (QNMs), extending previous work for lossy media [S. Franke et al., Phys. Rev. Lett. 122, 213901 (2019)]. Applying a symmetrization transformation, we show two fundamentally different ways for constructing a QNM photon Fock space, including (i) where there is a separate operator basis for both gain and loss, and (ii) where the loss and gain degrees of freedom are combined into a single basis. These QNM operator bases are subsequently used to derive the associated QNM master equations, including the interaction with a quantum emitter, modeled as a quantized two-level system (TLS). We then compare the two different quantization approaches, and also show how commonly used phenomenological methods to quantize light in gain-loss resonators are corrected by several important aspects, such as a loss-induced and gain-induced intermode coupling, which appears through the rigorous treatment of loss and amplification on a dissipative mode level. For specific resonator designs, modeled in a fully consistent way with the classical Maxwell equations with open boundary conditions, we then present numerical results for the quantum parameters and observables of a TLS weakly interacting with the medium-assisted field in a gain-loss microdisk resonator system, and discuss the validity of the different quantization approaches for several gain-loss parameter regimes.

Sensing and measurement tasks in severely adverse conditions such as loss, noise, and dephasing can be improved by illumination with quantum states of light. Previous results have shown a modest reduction in the number of measurements necessary to achieve a given precision. Here, we compare three illumination strategies for estimating the relative phase in a noisy, lossy interferometer. When including a common phase fluctuation in the noise processes, we show that using an entangled probe achieves an advantage in parameter estimation precision that scales with the number of entangled modes. This work provides a theoretical foundation for the use of highly multimode entangled states of light for practical measurement tasks in experimentally challenging conditions.

We employ the Bogoliubov approximation to study how the quantum geometry of the helicity states affects the superfluid properties of a spin-orbit-coupled Bose gas in continuum. In particular we derive the low-energy Bogoliubov spectrum for a plane-wave condensate in the lower helicity band and show that the geometric contributions to the sound velocity are distinguished by their linear dependences on the interaction strength; that is, they are in sharp contrast to the conventional contribution which has a square-root dependence. We also discuss the roton instability of the plane-wave condensate against the stripe phase and determine their phase-transition boundary. In addition we derive the superfluid density tensor by imposing a phase twist on the condensate order parameter and study the relative importance of its contribution from the interband processes that is related to the quantum geometry.