Superradiance has drawn increasing interest, motivated by the development of quantum technology. In general, a large number of atoms are favored for inducing a strong superradiance. However, our analytical and numerical calculations of light scattering from an atomic grating longitudinally pumped by a laser show that when the atom number is small, superradiant light scattering (SLS) is induced, that is, the reflected light intensity is proportional to the square of the atom number, whereas the SLS is saturated with the increase in the atom number. In our calculation, using the coupled-wave theory, we treat the atomic grating as a dynamical photonic crystal and find rich time-dependent optical properties due to collective atomic recoil motion, which is available within research under current experimental condition. Thus, our paper could also spark an investigation of the optical features of tunable photonic crystals in the time domain.

Optomechanical systems have attracted intensive attention in various physical experiments. With an optomechanical system, the displacement of or the force acting on a mechanical oscillator can be precisely measured by utilizing optical interferometry. As a mechanical oscillator, a suspended mirror is often used in over a milligram scale optomechanical systems. However, the tiny suspended mirror in a linear cavity can be unstable in its yaw rotational degree of freedom due to optical radiation pressure. This instability curbs the optical power that the cavity can accumulate in it and imposes a limitation on the sensitivity. Here, we show that the optical radiation pressure can be used to trap the rotational motion of the suspended mirror without additional active feedback control when the $g$ factors of the cavity are negative and one mirror is much heavier than the other one. Furthermore, we demonstrate experimentally the validity of the trapping. We measured the rotational stiffness of a suspended tiny mirror with various intracavity power. The result indicates that the radiation pressure of the laser beam inside the cavity actually works as a positive restoring torque. Moreover, we discuss the feasibility of observing quantum radiation pressure fluctuation with our experimental setup as an application of our trapping configuration.

We implement a shape optimization algorithm for body-assisted light-matter interactions described by the formalism of macroscopic quantum electrodynamics. The approach uses the level-set method to represent and incrementally evolve dielectric environments. Utilizing finite-difference time-domain techniques we demonstrate the ability of the algorithm by optimizing the rate of resonance energy transfer in two dimensions. The resulting geometries enhance the transfer rate by several orders of magnitude.

It is well known that light can possess spin angular momentum (SAM) and orbital angular momentum (OAM) in the longitudinal direction. Interesting novel light with transverse SAM or OAM has also been discovered recently, with the latter being called spatiotemporal optical vortices. Here we present a route to generate spatiotemporal optical vortices by means of high harmonic generation from relativistic laser plasma interactions. Three-dimensional particle-in-cell simulations show the evidence of edge dislocations in wave front structures and circulating energy flow in space-time. Further calculations of the SAM, OAM and total angular momentum confirm the harmonics carrying pure transverse OAM with noninteger values. The results open up a possibility to generate intense spatiotemporal optical vortices in the extreme ultraviolet region with attosecond duration.

We study the transfer of angular momentum of guided photons to a two-level atom with an electric quadrupole transition near an optical nanofiber. We show that the generation of the axial orbital torque of the driving guided field on the atom is governed by the internal-state selection rules for the quadrupole transition and by the angular momentum conservation law with the photon angular momentum given in the Minkowski formulation. We find that the torque depends on the photon angular momentum, the change in the angular momentum of the atomic internal state, and the quadrupole-transition Rabi frequency. We calculate numerically the torques for the quadrupole transitions between the sublevel $M=2$ of the hyperfine-structure level $5S_{1/2}F=2$ and the sublevels $M^{\prime }=0$, 1, 2, 3, and 4 of the hyperfine-structure level $4D_{5/2}{F}^{\prime }=4$ of a ${}^{87}\mathrm{Rb}$ atom. We show that the absolute value of the torque for the higher-order mode ${\mathrm{HE}}_{21}$ is larger than that of the torque for the fundamental mode ${\mathrm{HE}}_{11}$ except for the case $M^{\prime }-M=2$, where the torque for the mode ${\mathrm{HE}}_{21}$ is vanishing.

Each mode $j$ of the electromagnetic field is mathematically equivalent to a harmonic oscillator with a wave function ${\psi }_j\left(x_j\right)$ in the quadrature representation. An approach was recently introduced in which ${\psi }_j\left(x_j\right)$ was further quantized to produce a field operator ${\widehat{\psi }}_j\left(x_j\right)$ [J. D. Franson, Phys. Rev. A 104, 063702 (2021)]. This approach allows a generalization of quantum optics and quantum electrodynamics based on an unknown mixing angle $\gamma$ that is somewhat analogous to the Cabibbo angle or the Weinberg angle. The theory is equivalent to conventional quantum electrodynamics for $\gamma =0$, while it predicts an inelastic photon scattering process for $\gamma \ne 0$. Here we report the results of an optical scattering experiment that set an upper bound of $\left|\gamma \right|\le 1.93\times {10}^{-4}$ rad at the 99% confidence level, provided that the particles created by the operator ${\widehat{\psi }}_j\left(x_j\right)$ have negligible mass. High-energy experiments would be required to test the theory if the mass of these particles is very large.

We study the nonlinear spectra and multiphoton correlation functions of a waveguide system coupled to a two-level system (including realistic dissipation channels) with a time-delayed coherent feedback. We compute these observables by extending a recent quantum trajectory discretized-waveguide (QTDW) approach which exploits quantum trajectory simulations and a collisional model for the waveguide to efficiently simulate the quantum dynamics. Following a description of the general technique, we show how to calculate the first- and second-order quantum correlation functions, in the presence of a coherent pumping field. With a short delay time, we show how feedback can be used to filter out the central peak of the Mollow triplet or switch the output between bunched and antibunched photons by proper choice of round trip phase. We further show how the loop length and round trip phase affect the zero-time second-order quantum correlation function, an indicator of bunching or antibunching. New resonances introduced through the feedback loop are also shown through their appearance in the incoherent output spectrum from the waveguide. We explain these results in the context of the waiting time distributions of the system output and individual trajectories, uniquely stochastic observables that are easily accessible with the QTDW model.

In order to understand the capabilities and limitations of quantum computers, it is necessary to develop methods that efficiently characterize and benchmark error channels present on these devices. In this paper, we present a method that faithfully reconstructs a marginal (local) approximation of the effective noise (MATEN) channel, that acts as a single layer at the end of the circuit. We first introduce a dual-map framework that allows us to analytically derive expectation values of observables with respect to noisy circuits. These findings are supported by numerical simulations of the quantum approximate optimization algorithm (QAOA) that also justify the MATEN, even in the presence of nonlocal errors that occur during a circuit. Finally, we demonstrate the performance of the method on Rigetti's Aspen-11 quantum computer for QAOA circuits up to six qubits, successfully predicting the observed measurements on a majority of the qubits.

We study the radiative decay of the $2s$ state of one-electron and one-muon ions, where the two-photon mechanism plays an important role. Due to the nuclear size corrections, the radiative decay of the $2s$ state in the electron and muon ions is qualitatively different. Based on the accurate relativistic calculation, we introduce a two-parameter approximation, which makes it possible to describe the two-photon angular-differential transition probability for the polarized emitted photons with high accuracy. The emission of photons with linear and circular polarizations is studied separately. We also investigate the transition probabilities for the polarized initial and final states. The investigation is performed for ions with atomic numbers $1\le Z\le 120$.

An accurate determination of various parameters of ${\mathrm{Ar}}^{13+}$ is presented in this work. The wave functions and energy levels of the low-lying states within the configurations of $2s^{2}2p$ and $2s2p^2$ are calculated using the fully relativistic multiconfiguration Dirac-Hartree-Fock method, while the wave functions of higher excited states with the configurations $2s^{2}nl(n\ge 3$, $l=s,p,d)$ are obtained using the relativistic configuration interaction plus core polarization method. Then, the absorption oscillator strengths, transition probabilities, polarizabilities, and $g$ factors are determined. The contributions of electron correlation effects, Breit interaction, and quantum electrodynamics effects are also investigated. The present results agree well with the available theoretical and experimental results. The $g$ factors of the $2s^{2}2p{}^{2}P_{1/2,3/2}$ states agree with the experimental measurements on a level of ${10}^{-6}$.

In this work, we study the phenomenon of quantum friction in a system consisting on an atom moving at a constant speed parallel to a metallic plate. We use a hydrodynamic model to describe the degrees of freedom of a clean metal without internal dissipation. We model the polarizable atom as a two-level system with a unique ($l=0$) ground state and a threefold degenerate ($l=1$) excited state. We show that a quantum frictional force is present even in the absence of intrinsic damping in the metal, but that there is a threshold in the relative velocity that gives rise to such a force. In particular, we find that for friction to occur, the atom must move at a velocity larger than the effective speed of sound in the material, a condition that can be reached near empty or filled bands, where the Fermi velocity (which is proportional to the sound speed) becomes low. We provide analytical arguments to show that this result holds at all orders in perturbation theory.

Quantum theory of high-order above-threshold ionization (HATI) of atoms, prepared in a coherent superposition of bound states, by a strong laser field is formulated. Numerical results are obtained using the solutions of the time-dependent Schrödinger equation and the improved strong-field approximation. As an example, coherent superposition of the excited $2s$ and $2p$ states of the He atom is used. Applying a two-level model, it is shown how the relative amplitude and phase of the states in this coherent superposition can be controlled with a weak resonant laser pulse. Numerical results presented for HATI by a strong nonresonant probe laser pulse confirm that the photoelectron spectra and the momentum distributions strongly depend on the relative phase of the states in the prepared coherent superposition. For the case of HATI by a strong resonant laser pulse, we modify our strong-field-approximation theory by applying the two-level model to describe the time evolution of the atomic bound state before the instant of ionization. Contrary to the HATI from a single state by a long laser pulse, the corresponding photoelectron spectra of HATI from the coherent superposition of states depend on the carrier-envelope phase of the resonant ionizing pulse. Therefore, the strong-field ionization from a coherent superposition of states by a resonant laser pulse can be used to determine the carrier-envelope phase for long pulses for which the standard methods such as stereo–above-threshold ionization technique fail. The results strongly depend on the relative amplitude and phase of states in the coherent superposition of states, which can be controlled with a weak resonant laser pulse.

We demonstrate neuronlike spiking dynamics in a dissipative Bose-Hubbard dimer model, describing two coupled nonlinear Kerr cavities with a finite photon lifetime and under coherent pumping of just one cavity. Spiking dynamics appear due to the excitable nature of the system. In this context, excitable excursions in the phase space correspond to spikes in the temporal evolution of the cavity intensities, i.e., of the number of photons. In our case, excitability is mediated by the destruction of an oscillatory state in a global homoclinic bifurcation. In this type of excitability (known as type I) the period of the oscillatory state diverges when approaching the bifurcation. Beyond this point, the system exhibits excitable dynamics under the application of suitable perturbations. We have also characterized the effect that additive Gaussian noise has on the spiking dynamics, showing that the system undergoes a coherence resonance for a given value of the noise strength.

A model for calculating the quasinormal modes (QNMs) of homogeneous and core-shell spheroidal resonators is formulated in the null-field framework. Relying on a multipole expansion of the electromagnetic fields, the Mittag-Leffler theorem is employed to extract the QNMs from the poles of a response matrix. Physical quantities such as the extinction and scattering efficiency and the total internal energy of a mode are expressed directly in terms of the multipole expansion coefficients. We apply the model to study hybrid anapoles, which are radiationless states characterized by an enhancement of the total internal energy. Interference between excited QNMs and the background comprising of all other QNMs leads to a suppression of the extinction efficiency. Further, we show this suppression is due to Fano-like interference between resonant and background multipoles.

We consider open quantum systems whose dynamics is governed by a time-independent Markovian Lindblad master equation. Such systems approach their stationary state on a timescale that is determined by the spectral gap of the generator of the master equation dynamics. In a recent paper [Carollo et al., Phys. Rev. Lett. 127, 060401 (2021)] it was shown that under certain circumstances it was possible to exponentially accelerate the approach to stationarity by performing a unitary transformation of the initial state. This phenomenon can be regarded as the quantum version of the so-called Mpemba effect. The transformation of the initial state removes its overlap with the dynamical mode of the open system dynamics that possesses the slowest decay rate and, thus, determines the spectral gap. Whereas this transformation can be exactly constructed in some cases, it is, in practice, challenging to implement. Here we show that even far simpler transformations constructed by a global unitary spin rotation allow to exponentially speed up relaxation. We demonstrate this using simple dissipative quantum spin systems, which are relevant for current quantum simulation and computation platforms based on trapped atoms and ions.

Non-Hermitian quantum systems with explicit time dependence are of ever increasing importance. There are only a handful of models that have been analytically studied in this context. Here, a $\mathcal{PT}$-symmetric non-Hermitian $N$-level Landau-Zener type problem with two exceptional points of $N\mathrm{th}$ order is introduced. The system is Hermitian for asymptotically large times, far away from the exceptional points, and has purely imaginary eigenvalues between the exceptional points. The full Landau-Zener transition probabilities are derived, and found to show a characteristic binomial behavior. In the adiabatic limit the final populations are given by the ratios of binomial coefficients. It is demonstrated how this behavior can be understood on the basis of adiabatic analysis, despite the breakdown of adiabaticity that is often associated with non-Hermitian systems.

Providing an optimal path to a quantum annealing algorithm is key to finding good approximate solutions to computationally hard optimization problems. Reinforcement is one of the strategies that can be used to circumvent the exponentially small energy gaps of the system in the annealing process. Here a time-dependent reinforcement term is added to the Hamiltonian in order to give lower energies to the most probable states of the evolving system. In this study, we take local entropy in the configuration space for the reinforcement and apply the algorithm to a number of easy and hard optimization problems. The reinforced algorithm performs better than the standard quantum annealing algorithm in the quantum search problem, where the optimal parameters behave very differently depending on the number of solutions. Moreover, the reinforcements can change the discontinuous phase transitions of the mean-field $p$-spin model ($p>2$) to a continuous transition. The algorithm's performance in the binary perceptron problem is also superior to that of the standard quantum annealing algorithm, which already works better than a classical simulated annealing algorithm.

For solid-state spin systems, the collective spin motion in a single crystal embodies multiple magnetostatic modes. Recently, it was found that the cross-Kerr interaction between the higher-order magnetostatic mode and the Kittel mode introduces a new operable degree of freedom. In this work we propose a scheme to entangle two magnon modes via the cross-Kerr nonlinearity when the bias field is inhomogeneous and the system is driven. Quantum entanglement persists at the steady state, as demonstrated by numerical results using experimentally feasible parameters. Furthermore, we also demonstrate that entangled states can survive better in the system where self-Kerr and cross-Kerr nonlinearities coexist. Our work provides insights and guidance for designing experiments to observe entanglement between different degrees of freedom within a single ferrimagnetic crystal. Additionally, it may stimulate potential applications in quantum information processing using spintronic devices.

Recent experiments show the existence of collective decoherence in quantum systems. We study the possibility of quantum computation in a decoherence-free subspace which is robust against such kinds of decoherence processes. This passive protection protocol can be especially advantageous for continuous quantum computation such as quantum annealers. As an example we propose to use decoherence-protected adiabatic quantum computation for the Grover search problem.

In this work, total and differential ionization cross sections of uracil ionized by ${\mathrm{C}}^{6+}$ projectiles are calculated by extending an adaptative classical trajectory Monte Carlo model recently introduced in the ion-${\mathrm{H}}_2\mathrm{O}$ collisions context. Calculated cross sections are contrasted and benchmarked against available experimental and theoretical data at impact energies in the MeV/u range. Present results are found in good agreement with the reported experimental data at different levels of differentiability. In addition, the electron emission contributions arising from single and multiple electron ionization are explicitly determined and their partial dominance analyzed in terms of the specific angular and/or energetic ranges.