We study the screening of a homogeneous oscillating external electric field $E_0$ in noble-gas atoms using atomic many-body calculations. At zero frequency of the oscillations ($\omega =0$) the screened field $E\left(r\right)$ vanishes at the nucleus, $E\left(0\right)=0$. However, the profile of the field $E\left(r\right)$ is complicated, with the magnitude of the field exceeding the external field $E_0$ at certain points. For $\omega >0$ the field $E(r,\omega )$ strongly depends on $\omega$ and at some points may exceed the external field $E_0$ many times. The field at the nucleus is not totally screened and grows with $\omega$ faster than ${\omega }^2$. It can even be enhanced when $\omega$ comes close to resonance with a frequency of an atomic transition. This field interacts with CP-violating nuclear electric dipole moments creating new opportunities for studying them. The screening of the external field by atomic electrons may strongly suppress (or enhance near an atomic resonance) the low energy nuclear electric dipole transitions.

We show that indirect spin-spin interactions between effective spin-1/2 systems can be realized in two parallel one-dimensional optical lattices loaded with polar molecules and/or Rydberg atoms. The effective spin can be encoded into low-energy rotational states of polar molecules or long-lived states of Rydberg atoms, tightly trapped in a deep optical lattice. The spin-spin interactions can be mediated by Rydberg atoms, placed in a parallel shallow optical lattice, interacting with the effective spins by charge-dipole (for polar molecules) or dipole-dipole (for Rydberg atoms) interaction. Indirect $XX$, Ising, and $XXZ$ interactions with interaction coefficients $J^{\perp }$ and $J^{zz}$ sign varying with interspin distance can be realized, in particular, the $J_1-J_2 XXZ$ model with frustrated ferro-(antiferro-)magnetic nearest (next-nearest) neighbor interactions.

We consider polarization properties of the unpolarized emission of an ensemble of classical emitters with randomly varying polarization. The light is supposed to be unpolarized in the sense that all three polarization-related components of its Stokes vector are zero. At the same time, the mean-square values of these components should not be necessarily zero, may differ from each other, and, therefore, may provide additional information about properties of individual emitters. Experimentally, this information is revealed as dependence of the polarization noise on the azimuth of the quarter-wave plate placed before the polarization-sensitive detector. This dependence appears to be different for the emitters randomly polarized over the equator of the Poincaré sphere, or preferentially located on its poles, or uniformly covering the whole sphere. We show that full quantitative analysis of the polarization-noise anisotropy allows one, in the framework of the proposed model, to get information about polarization characteristics of individual emitters hidden in the emission of the ensemble. Vitality of the method is illustrated by its application to polarization analysis of the polariton laser emission, which is shown to predominantly arise from linearly polarized emitters.

An open question in experimental physics is the characterization of gravitational effects in quantum regimes. We propose an experimental setup that uses well-tested techniques in cavity optomechanics to observe the effects of the gravitational interaction between two micromechanical oscillators on the interference of the cavity photons through the shifts in the visibility of interfering photons. The gravitational coupling leads to a shift in the period and magnitude of the visibility whose observability is within reach of current technology. We discuss the feasibility of the setup as well as the effects on entanglement due to gravitational interaction.

In this paper, we propose a theoretical scheme to detect the emitter-resonator coupling strength in the ultra-strong-coupling regime in the quantum Rabi model via introducing an auxiliary resonator. We demonstrate the total system as a two-mode Rabi model and obtain the ground state by the transformed rotating-wave approximation, which is shown to be superior to the usually applied rotating-wave approximation. Here, the coupling strength is detected by monitoring the average excitation number in the auxiliary resonator, and the sensitivity of the detection scheme is discussed analytically.

Many nonlinear optical technologies require the two-mode spectral amplitude function that describes them—the joint spectral amplitude (JSA)—to be separable. We prove that the JSA factorizes only when the incident pump field and phase-matching function are Gaussian functions. We show this by mapping our problem to a known result, in continuous variable quantum information, that only squeezed states remain unentangled when combined on a beam splitter. We then conjecture that only a squeezed state minimizes entanglement when sent through a beam splitter with another prespecified ket. This implies that to maximize JSA separability when one of the (pump or nonlinear medium) functions is non-Gaussian, the other function must be Gaussian. This answers an outstanding question about optimal design of certain nonlinear processes and is of practical interest to researchers using waveguide nonlinear optics to generate and manipulate quantum light.

In the framework of the nonsecular perturbation theory based on the Bogoliubov averaging method, the coherent dynamics of multiphoton Raman transitions in a two-level spin system driven by an amplitude-modulated microwave field is studied. Closed-form expressions for the Rabi frequencies of these transitions are obtained beyond the rotating-wave approximation for the low-frequency driving component. It is shown that spin states dressed by the high-frequency component of the driving field are shifted due to the Bloch-Siegert-like effect caused by antiresonant interactions with the strong low-frequency driving. We predict that with increasing the order of the Raman transition, the Rabi frequency decreases and the contribution of the Bloch-Siegert shift to this frequency becomes dominant. It is found that the amplitude and phase of the Rabi oscillations strongly depend on the initial phase of the low-frequency field as well as on detuning from multiphoton resonance. The recent experimental data for the second- and third-order Raman transitions observed for the nitrogen-vacancy center in diamond (Z. Shu et al., arXiv:1804.10492) are well described in the frame of our approach. Our results may provide new possibilities for coherent control of quantum systems.

The usual multipolar Hamiltonian for atom-light interaction features a nonrelativistic moving atom interacting with electromagnetic fields which inherently follow Lorentzian symmetry. This combination can lead to situations where atoms appear to experience a friction force, when in fact they only change their internal mass-energy due to the emission or absorption of a photon. Unfortunately, the simple Galilean description of the atom's motion is not sufficient to distinguish between a change in momentum due to acceleration and a change in momentum due to a change in internal mass-energy. In this work we show how a low-order relativistic correction can be included in the multipolar atom-light Hamiltonian. We also give examples how this affects the most basic mechanical interactions between atoms and photons.

We introduce a variant of quantum and classical electrodynamics formulated on the grounds of a hypothesis of existence of a preferred frame of reference—a formalism complementary to that regarding the structure of the space of photonic states, presented by us recently [Phys. Rev. A 97, 062106 (2018)]. The present approach makes a unique test theory to search for a preferred frame for photons in experiments, one of which is suggested.

Modern laser systems are able to generate short and intense laser pulses ionizing matter in the poorly explored barrier-suppression regime. Field ionization in this regime is studied analytically and numerically. For analytical studies, both the classical and the quantum approaches are used. Two approximations to solve the time-dependent Schrödinger equation are proposed: the free electron approximation, in which the atomic potential is neglected, and the motionless approximation, in which only the external field term is considered. In the motionless approximation, the ionization rate in extremely strong fields is derived. The approximations are applied to several model potentials and are verified using numeric simulations of the Schrödinger equation. A simple formula of the ionization rate both for the tunnel and the barrier-suppression regimes is proposed. The formula can be used, for example, in particle-in-cell codes for simulations of the interaction of extremely intense laser fields with matter.

We present a reformulation of the nonrelativistic many-body quantum dynamics of Bose-Fermi mixtures via stochastic wave functions. We show that, within a wide range of two-particle interactions and a certain class of greater-than-two-particle interactions, the quantum dynamics can be mapped exactly onto a set of single-particle wave functions, with their evolution governed by a stochastic process.

We present a vectorial analysis of the behavior of the electromagnetic field in the presence of boundaries with parabolic geometry. The relevance of the use of symmetries to find explicit closed expressions for the electromagnetic fields is emphasized. Polarization and phase related angular momenta of light have an essential role in the proper definition of the generator ${\mathfrak{A}}_3$ of a symmetry transformation that distinguishes the parabolic geometry. Quantization of the electromagnetic field in terms of the resulting elementary modes is performed. The important case of a boundary defined by an ideal parabolic mirror is explicitly worked out. The presence of the mirror restricts the eigenvalues of ${\mathfrak{A}}_3$ available to the electric and magnetic fields of a given mode via compact expressions. Modes previously reported in the literature are particular cases of those described in this work.

By using electromagnetically induced transparency phenomena (EIT) in three-level atoms, we propose an interferometry technique for measuring constant classical force. The classical force is estimated by measuring the momentum, which is quantum nondemolition observable in this scheme, of the atoms. Using EIT phenomena, under appropriate conditions, we show that the thermal noise and the backaction noise in the interferometer can be suppressed. With no backaction noise and no thermal noise, the sensitivity of this technique is limited by shot noise only.

In this work, we study the problem of single-shot discrimination of von Neumann measurements, which we associate with measure-and-prepare channels. There are two possible approaches to this problem. The first one is simple and does not utilize entanglement. We focus only on the discrimination of classical probability distributions, which are outputs of the channels. We find necessary and sufficient criterion for perfect discrimination in this case. A more advanced approach requires the usage of entanglement. We quantify the distance between two measurements in terms of the diamond norm (called sometimes the completely bounded trace norm). We provide an exact expression for the optimal probability of correct distinction and relate it to the discrimination of unitary channels. We also state a necessary and sufficient condition for perfect discrimination and a semidefinite program which checks this condition. Our main result, however, is a cone program which calculates the distance between the measurements and hence provides an upper bound on the probability of their correct distinction. As a by-product, the program finds a strategy (input state) which achieves this bound. Finally, we provide a full description for the cases of Fourier matrices and mirror isometries.

We consider particle-hole symmetric photonic graphene with balanced gain and loss. We show that edge states with purely imaginary eigenvalues appear along the zigzag edge. We propose an idea that these edge states are protected by spontaneously broken particle-hole symmetry. We discuss that the edge states are topological in the sense that the exceptional rings are robust against symmetry protecting disorder. If the disorder is too strong to restore the spontaneously broken particle-hole symmetry, then such protection of the edge states is lost.

The question of how Bell nonlocality behaves in bipartite systems of higher dimensions is addressed. By employing the probability of violation of local realism under random measurements as the figure of merit, we investigate the nonlocality of entangled qudits with dimensions ranging from $d=2$ up to $d=10$. We proceed in two complementary directions. First, we study the specific Bell scenario defined by the Collins-Gisin-Linden-Massar-Popescu (CGLMP) inequality. Second, we consider the nonlocality of the same states under a more general perspective by directly addressing the space of joint probabilities (computing the frequencies of behaviours outside the local polytope). In both approaches we find that the nonlocality decreases as the dimension $d$ grows, but in quite distinct ways. While the drop in the probability of violation is exponential in the CGLMP scenario, it presents, at most, a linear decay in the space of behaviors. Furthermore, in the latter approach the states that produce maximal numeric violations in the CGLMP inequality present low probabilities of violation in comparison to maximally entangled states, so no anomaly is observed. Finally, the nonlocality of states with nonmaximal Schmidt rank is investigated.

Continuous-variable quantum key distribution (CVQKD) is considered to be an alternative to classical cryptography for secure communication. However, its transmission distance is restricted to metropolitan areas, given that it is affected by the channel excess noise and losses. In this paper, we present a scheme for implementing long-distance CVQKD using separable Gaussian states. This tunable QKD protocol requires separable Gaussian states, which are squeezed and displaced, along with the assistance of classical communication and available linear optics components. This protocol originates from the entanglement of one mode and the auxiliary mode used for distribution, which is first destroyed by local correlated noises and restored subsequently by the interference of the auxiliary mode with the second distant separable correlated mode. The displacement matrix is organized by two six-dimensional vectors and is finally fixed by the separability of the tripartite system. The separability between the ancilla and Alice and Bob's system mitigates the enemy's eavesdropping, leading to tolerating higher excess noise and achieving longer transmission distance.

We introduce an efficient scheme to correct errors due to the finite squeezing effects in continuous-variable cluster states. Specifically, we consider the typical situation where the class of algorithms consists of input states that are known. By using the knowledge of the input states, we correct the errors and significantly improve the fidelity in the context of continuous-variable cluster states. We illustrate the error correction scheme for single- and two-mode unitaries implemented by spatial continuous-variable cluster states. We show that there is no resource advantage to error correcting multimode unitaries implemented by spatial cluster states. However, the generalization to multimode unitaries implemented by temporal continuous-variable cluster states shows significant practical advantages since it costs only a small fixed number of optical elements (squeezer, beam splitter, etc.) for an arbitrary number of modes.

We study entropic uncertainty relations by using stepwise linear functions and quadratic functions. Two kinds of improved uncertainty lower bounds are constructed: the state-independent one based on the lower bound of Shannon entropy and the tighter state-dependent one based on the majorization techniques. The analytical results for qubit and qutrit systems with two or three measurement settings are explicitly derived, with detailed examples showing that they outperform the existing bounds. The case with the presence of quantum memory is also investigated.