The efficient generation of entanglement is an essential requirement for quantum communications; however, long distances can only be achieved by utilizing entangled states that can be efficiently mapped into matter. Hence, sources generating states with bandwidths naturally compatible with the linewidths of atomic transitions are crucial. We harness the indistinguishability between two spontaneous four-wave mixing processes to achieve the heralded generation of single-photon frequency-bin entangled states. State manipulation admits entanglement and generation probability optimizations yet with negligible absorption. The scheme could also be adapted to photonic and solid interfaces.

Users of quantum computers must be able to confirm they are functioning as intended, even when they are remotely accessed. In particular, if the Hilbert space dimension of the quantum computer components is not as advertised—for instance, if they suffer leakage—errors can ensue and protocols may be rendered insecure. We refine the method of delayed vectors, adapted from classical chaos theory to quantum systems, and apply it remotely on the IBMQ platform—a quantum computer composed of transmon qubits. The method witnesses, in a model-independent fashion, dynamical signatures of higher-dimensional processes. We present evidence, under mild assumptions, that the IBMQ transmons suffer state leakage, with a $p$ value no larger than $5\times {10}^{-4}$ under a single-qubit operation. We also estimate the number of shots necessary for revealing leakage in a two-qubit system.

A population transfer based on adiabatic evolutions in a three-state system undergoing an avoided crossing is considered. The efficiency of the process is analyzed in connection with the relevant parameters, bringing to light an important role of the phases of the coupling constants. The role of dissipation is also taken into account, focusing on external decays that can be described by effective non-Hermitian Hamiltonians. Though the population transfer turns out to be quite sensitive to the decay processes, for very large decay rates the occurrence of a Zeno phenomenon allows for restoring a very high efficiency.

The tunability of the helium-helium interaction through an external electric field is investigated. For a static external field, electric-field-induced resonances and associated electric-field-induced bound states are calculated for the ${}^4\mathrm{He}\text{−}{}^4\mathrm{He},{}^3\mathrm{He}\text{−}{}^4\mathrm{He}$, and ${}^3\mathrm{He}\text{−}{}^3\mathrm{He}$ systems. Qualitative agreement is found with the literature for the ${}^3\mathrm{He}\text{−}{}^4\mathrm{He}$ and ${}^3\mathrm{He}\text{−}{}^3\mathrm{He}$ systems [E. Nielsen, D. V. Fedorov, and A. S. Jensen, Phys. Rev. Lett. 82, 2844 (1999)]. The implications of the predicted electric-field-induced resonances for ${}^4\mathrm{He}\text{−}{}^4\mathrm{He}$ on the wave packet dynamics, initiated by intense laser pulses, are investigated. Our results are expected to guide next generation experiments.

We report on the Zeeman deceleration of ground-state NH radicals, using a decelerator that consists of 100 pulsed solenoids and 100 permanent hexapoles. Packets of state-selected NH $\left(X{}^3\mathrm{\Sigma }^{-},N=0,J=1\right)$ radicals are produced with final velocities ranging between 510 and 150 m/s. The velocity distributions of the packets of NH exiting the Zeeman decelerator are probed using velocity map imaging detection. We present a $1+2^{\prime }$ resonance-enhanced multiphoton ionization scheme for NH that allows for velocity map imaging detection under ion recoil-free conditions. The packets of Zeeman-decelerated NH radicals, in combination with the detection scheme, offer interesting prospects for the use of this important radical in high-resolution crossed-beam-scattering experiments.

We study the propagation of speckle patterns in a tunable nonlinear medium. Starting from a spatial pattern of photons with a normal distribution (speckle field), we characterize the evolution of the radiation to either subthermal or superthermal distributions and demonstrate the control of this evolution through a single parameter. Our experimental control of the local degree of coherence of the output photons is well described by numerical simulations that consider both dispersive and absorptive nonlinear effects. Particularly, we analyze the role of the nonlinear absorption that either enhances or competes with the dispersive effects in the evolution of speckle fields. In some realizations, an asymptotic power law is observed in the photon spatial distribution.

Temporal coupled-mode theory has been widely used to describe the physics of resonant optical systems. In general, an optical system can be constrained by energy conservation, time-reversal symmetry, and reciprocity. Most previous developments of temporal coupled-mode theory made use of all three constraints. In this paper, we consider separately the implication of each of these constraints on the parameters of temporal coupled-mode theory. For this purpose we made extensive use of the connection between a physical system and its time-reversal conjugate. This connection also indicates some of the nontrivial implications of the relation between the resonant properties of a physical system and its time-reversal conjugate. We validate these implications numerically by direct electromagnetic simulations of a guided resonance system. This work should enable the application of temporal coupled-mode theory to a wider range of resonant systems.

We show that if a nonzero $E$-field intensity is found in the focus of a circularly polarized optical vortex with phase singularity or a higher-order azimuthally polarized beam with polarization singularity, an energy backflow will occur at the center of the focal plane. Using the Richards-Wolf formulas, which describe the electromagnetic field near the tight focus of an aplanatic system containing an ideal spherical lens, we prove that the negative on-axis propagation of light occurs when focusing (i) a left-hand circularly polarized optical vortex with the topological charge $m=2$ or (ii) a conventional second-order azimuthally polarized radially symmetric laser beam. Notably, in the former case, the negative energy flow rotates on a spiral about the optical axis, while in the latter case, the Poynting vector is perpendicular to the focal plane at any point. The theoretical conclusions are confirmed by numerical simulation.

The attoclock technique which maps the emission time of a photoelectron to its detection angle is an important tool in strong-field physics. Previously, it was implemented only with circularly or elliptically polarized laser fields. Here, we show how counter-rotating bicircular laser fields can be used as an attoclock to investigate the ionization dynamics in quasilinear polarization. This is achieved by choosing the ratio of the two field strengths in a way such that the vector potential has aspects of the attoclock and time is mapped directly to the photoelectron momentum, but the shape of the electric field corresponds to approximately linear polarization during three intervals per optical cycle. We report momentum distributions calculated by solving the time-dependent Schrödinger equation for a model helium atom and obtain the mapping from photoelectron momentum to ionization time using a trajectory-free method. Unlike circular polarization where the time of maximal ionization rate typically deviates less than 5 attoseconds from the maximium of the electric field, we find positive ionization times of more than 10 attoseconds in the quasilinear case.

A general approach to conserved physical quantities originating from dynamical symmetries is presented for quantum-mechanical systems. It is illustrated that a general ansatz for the Hamiltonian leads to a differential equation for the central potential that can be solved analytically. This indicates that additional integrals of motion are closely connected to the functional form of the potential. In nonrelativistic three-dimensional quantum mechanics, we show that, besides the trivial case of a constant potential, the Coulomb and harmonic potentials are the only two examples that give rise to additional integrals of motion known as the Runge-Lenz-Laplace vector and the Demkov-Fradkin second-rank tensor, which is in agreement with earlier results. Tensors of rank higher than 2 are only conserved quantities for a constant potential and basically denote a generalization of momentum conservation for higher ranks. We also consider the relativistic case by studying the Hamiltonian form of the Dirac equation. Here we show that only a constant and the Coulomb potential lead to conserved quantities. In the case of a Coulomb potential, this is the pseudoscalar Johnson-Lippmann-Biedenharn operator, which reduces to a spin projection of the Runge-Lenz-Laplace vector in the nonrelativistic limit.

The probability densities of work that can be exerted on a quantum system initially staying in thermal equilibrium are constrained by the fluctuation relations of Jarzynski and Crooks, when the work is determined by two projective energy measurements. We investigate the question whether these fluctuation relations may still hold if one employs generalized energy measurements rather than projective ones. Restricting ourselves to a class of universal measurements which are independent of several details of the system on which the work is done, we find sets of necessary and sufficient conditions for the Jarzynski equality and the Crooks relation. The Jarzynski equality requires perfect accuracy for the initial measurement, while the final one can be erroneous. On the other hand, the Crooks relation can only tolerate a depolarizing channel as a deviation from the projective measurement for systems with a finite-dimensional Hilbert space. For a separable infinite-dimensional space only projective measurements are compatible with the Crooks relation. The results we have obtained significantly extend those of Venkatesh, Watanabe, and Talkner [New J. Phys. 16, 015032 (2014)] as well as avoid some errors present there.

Quantum simulation is a promising field where a controllable system is used to mimic another system of interest, whose properties one wants to investigate. One of the key issues for such simulation is the ability to control the environment the system couples to, be it to isolate the system or to engineer a tailored environment of interest. One strategy recently put forward for environment engineering is the use of metamaterials with negative indices of refraction. Here we build on this concept and propose a circuit-QED simulation of many-body Hamiltonians using superlattice metamaterials. We give a detailed description of a superlattice transmission line coupled to an embedded qubit, and show how this system can be used to simulate the spin-boson model in regimes where analytical and numerical methods usually fail, e.g., the strong coupling regime.

Atmospheric effects are the chief threats to the quantum properties of propagating quantum signals and may degrade the performance of quantum key distribution seriously. As one of the most important parts of continuous-variable quantum key distribution (CVQKD), a parameter estimation method has not been specially proposed in an atmospheric channel, and usually the security analysis is based on the assumption that the relevant parameters, especially the excess noise, have been previously obtained. Here we propose a parameter estimation method for Gaussian modulated coherent state continuous-variable quantum key distribution over the atmospheric link, and we investigate the impact of the atmospheric channel on the estimated values of the parameters. Based on this method, we study theoretically the effect of link fluctuations on the achievable secret key rate of CVQKD under different practical transmitted conditions. The results show that this method is unified with the physical model and can effectively resist entanglement-distillation attack. The proposed method fills in the blank of parameter estimation for implementation of practical atmospheric CVQKD.

The non-Gaussian operation can be used not only to enhance and distill the entanglement between Gaussian entangled states, but also to improve the performance of quantum communications. In this paper, we propose a non-Gaussian continuous-variable quantum key distribution (CVQKD) by using quantum catalysis (QC), which is an intriguing non-Gaussian operation in essence that can be implemented with current technologies. We perform quantum catalysis on both ends of the Einstein-Podolsky-Rosen pair prepared by a sender, Alice, and find that for the single-photon QC-CVQKD, the bilateral symmetrical quantum catalysis performs better than the single-side quantum catalysis. Attributing to characteristics of an integral within an ordered product of operators, we find that the quantum-catalysis operation can improve the entanglement property of Gaussian entangled states by enhancing the success probability of non-Gaussian operation, leading to the improvement of the QC-CVQKD system. As a comparison, the QC-CVQKD system involving zero-photon and single-photon quantum catalysis outperforms the previous non-Gaussian CVQKD scheme via photon subtraction in terms of a secret key rate, maximal transmission distance, and tolerable excess noise.

Accurate transition probabilities of La ii ion are calculated using configuration-interaction many-body perturbation theory with 10 adjustable parameters, seven of which are evaluated from energies of the La iii single-valence electron ion. Comparison is given for transitions probabilities and lifetimes with experiments and theories. Close agreement with experiment is observed for most transitions. The theoretical approach can be extended to other divalent atoms and ions with strong valence-core interactions and to more complex atoms. The theory will be useful for opacity evaluation and astrophysical applications.

Experiments on free-free electron scattering by an Ar target are reported. The angular distribution of 350 eV electrons, elastically scattered at angles between $4^{\circ }$ and ${80}^{\circ }$ in the presence of 1.17 eV photons from a Nd:YAG laser, shows a pronounced maximum close to ${47}^{\circ }$ and is symmetric about this angle, as predicted by a Kroll-Watson approximation calculation for the experimental kinematics. The experiments test a term in the approximation that is a strong function of the magnitude and direction of the momentum transfer of the elastically scattered electrons.

Owing to increasing applications of time-resolved coherent x-ray scattering for the investigation of molecular reaction dynamics, we develop a theoretical model for time-dependent x-ray diffraction from molecular and/or electronic motion in molecules. Our model shows that the violation of centrosymmetry (VOC) is a general phenomenon in time-resolved diffraction patterns. We employ our theoretical model to illustrate the VOC in time-resolved coherent x-ray diffraction from two oriented diatomic molecules undergoing rovibrational motion: lithium hydride (LiD) and hydrogen (HD). Our simulations show asymmetric x-ray diffraction images that reflect the directions of the molecular motions.

Cold molecules are desired in a wide range of research areas, like precision measurement of physical constants, high-resolution spectroscopy, and cold collisions. Scientific interest in the generation and manipulation of cold molecules has been strong for decades. Here we theoretically investigate the dynamics of an optoelectrostatic storage ring for polar molecules, which is composed of an orbiting laser beam and an electrostatic quadrupole ring. The potentials of water molecules experienced in the combined optical and electrical fields are first calculated and a three-dimensional (3D) hybrid potential well for molecules of interest is formed in the focus spot of the red-detuned laser beam. The behavior of motional water molecules under the influence of the decelerating hybrid potential well is then investigated. Later, the deceleration and trapping processes of water molecules from supersonic expansion in the hybrid potential well are numerically simulated and corresponding results are presented. Our study indicates that this optoelectrostatic storage ring, featuring a 3D potential well of controllable orbiting speed, can serve as a good platform for production and manipulation of cold polar molecules and should have important applications in the related research areas.