Einstein-Podolsky-Rosen (EPR) steering is a quantum mechanical phenomenon that allows one party to steer the state of a distant party by exploiting their shared entanglement. It has potential applications in secure quantum communication. In this paper, we present two swapping schemes of Gaussian EPR steering, single-channel and dual-channel schemes, by the technique of entanglement swapping. Two space-separated independent EPR steering states without a direct interaction present EPR steering after deterministic swapping. By comparing the EPR steering of the single-channel and dual-channel schemes, we show that the transmission distance of the single-channel scheme is much longer than that of the symmetric dual-channel scheme. Different from entanglement swapping, one-way EPR steering is presented after swapping over lossy channels. The most interesting thing is that the change of the EPR steering direction is observed in the dual-channel scheme. We also show that excess noise in a quantum channel will shorten the transmission distance of the swapping, even leading to the sudden death of EPR steering. The presented schemes provide a technical reference for remote quantum communications with EPR steering.

We present a study of the photoelectron spectra for the ionization process driven by counterrotating and corotating circularly polarized fundamental and odd-order harmonic fields. The main features of the spectra, such as symmetric lobed structures, are understood using simple arguments based on the strong field approximation (SFA) picture of ionization. A deviation from this picture is, most notably, the presence of the low-energy structures (LES) in the spectra. We show that the Rydberg states populated as a result of the combined absorption of the photons from the fundamental and harmonic fields are responsible for the origin of LES.

We study controllable friction in a system consisting of a dark soliton in a one-dimensional Bose-Einstein condensate coupled to a noninteracting Fermi gas. The fermions act as impurity atoms, not part of the original condensate, that scatter off of the soliton. We study semiclassical dynamics of the dark soliton, a particlelike object with negative mass, and calculate its friction coefficient. Surprisingly, it depends periodically on the ratio of interspecies (impurity-condensate) to intraspecies (condensate-condensate) interaction strengths. By tuning this ratio, one can access a regime where the friction coefficient vanishes. We develop a general theory of stochastic dynamics for negative-mass objects and find that their dynamics are drastically different from their positive-mass counterparts: they do not undergo Brownian motion. From the exact phase-space probability distribution function (i.e., in position and velocity), we find that both the trajectory and lifetime of the soliton are altered by friction, and the soliton can undergo Brownian motion only in the presence of friction and a confining potential. These results agree qualitatively with experimental observations by Aycock et al. [Proc. Natl. Acad. Sci. USA 114, 2503 (2017)] in a similar system with bosonic impurity scatterers.

We explore the far-from-equilibrium dynamics of Bose gases in a universal regime associated to nonthermal fixed points. While previous investigations concentrated on scaling functions and exponents describing equal-time correlations, we compute the additional scaling functions and dynamic exponent $z$ characterizing the frequency dependence or dispersion from unequal-time correlations. This allows us to compare the characteristic condensation and correlation times from a finite-size scaling analysis depending on the system's volume.

We investigate some aspects of the Bose-Einstein condensation (BEC) of quantum gases in the presence of inhomogeneous conditions. We consider three-dimensional (3D) quantum gases trapped by an external potential when the temperature is sufficiently low to show a BEC-phase region around the center of the trap. If the trap is sufficiently large, different phases may coexist in different space regions, when moving from the center of the trap. We show that the quantum gas develops a peculiar critical behavior at the boundary of the BEC region, whose scaling behavior is controlled by the universality class of the homogeneous BEC transition. We provide numerical evidence of this phenomenon, for lattice atomic gases modeled by the 3D Bose-Hubbard Hamiltonian.

We investigate single-photon scattering properties in a one-dimensional waveguide coupled to a quantum emitter's chain with dipole-dipole interaction (DDI). The photon transport is extremely sensitive to the location of the evanescently coupled atoms. The analytical expressions of reflection and transmission amplitudes for the chain containing two emitters with DDI are deduced by using a real-space Hamiltonian. Two cases, where the two emitters symmetrically or asymmetrically couple to the waveguide, are discussed in detail. It shows that the reflection and transmission typical spectra split into two peaks due to the DDI. The Fano minimum in the spectra can be used to estimate the strength of the DDI. Furthermore, the DDI makes spectra strongly asymmetric and creates a transmission window in the region where there was zero transmission. The scattering spectra for the chain consisting of multiple emitters are also given. Our key finding is that DDI can broaden the frequency bandwidth for high reflection when the chain consists of many emitters.

The enhancement of light extraction efficiency (LEE), spontaneous emission rate (SER), and far-field directivity is of crucial importance in single-photon sources. For this reason, we propose here a partially reflecting metasurface cavity (PRMC) concept that simultaneously enhances these three parameters in a planar cavity configuration. The LEE is enhanced by enforcing the emitted light to radiate perpendicularly to the device so as to suppress wave trapping loss, the SER is enhanced by properly confining the field near the source, and the far-field directivity is enhanced by maximizing the radiation aperture of the device. The PRMC structure is designed using a recent surface susceptibility metasurface synthesis technique and demonstrated by full-wave simulation to provide LEE and SER enhancements by factors of 4.0 and 1.9, respectively, and a half-power beam width of $22.5^{\circ }$. Therefore, this PRMC structure has a promising potential for single-photon source with higher efficiency and single-photon purity.

A simple superbunching pseudothermal light source is introduced based on common instruments such as laser, lenses, pinholes, and ground glasses. $g^{\left(2\right)}\left(0\right)=3.66\pm 0.02$ is observed in the suggested scheme by employing two rotating ground glasses. Quantum and classical theories are employed to interpret the observed superbunching effect. It is predicted that $g^{\left(2\right)}\left(0\right)$ can reach $2^N$ if $N$ rotating ground glasses were employed. These results are helpful to understand the physics of superbunching. The proposed superbunching pseudothermal light may serve as a type of light to study the second- and higher-order coherence of light and have potential application in improving the visibility of thermal light ghost imaging.

Nonlinear interfaces separating two diffusive Kerr-type media exhibit dual switching between total internal reflection and transmission. This property is found within a weakly nonlocal regime when both a nonparaxial treatment of the problem and a full two-dimensional model for carrier diffusion are assumed. The theoretical model is shown to predict an effective cubic-quintic nonlinearity with competing terms that produces such property. The validity of the analysis is contrasted with a full set of numerical simulations.

We theoretically investigate an ultrastrong-coupled micromaser based on Rydberg atoms interacting with a superconducting LC resonator, where the common rotating-wave approximation and slowly varying envelope approximation are no longer applicable. The effect of counter-rotating terms on masing dynamics is studied in detail. We find that the intraresonator electric energy declines and the microwave oscillation frequency shifts significantly in the regime of ultrastrong coupling. Additionally, the micromaser phase fluctuation is suppressed, resulting in a reduced spectral linewidth.

We present a study of the quantum properties of two light fields used in parametric four-wave mixing in a Raman-type atomic system. The system realizes an effective Hamiltonian of beam-splitter-type coupling between the light fields, which allows one to control squeezing and amplitude distribution of the light fields, as well as realizing their entanglement. The scheme can be feasibly applied to engineer the quantum properties of two single-mode light fields in properly chosen input states.

The interactive dynamics of two relativistic laser beams copropagating in underdense plasmas is studied using a coupled model equation for the relativistic laser propagation. It is shown that the relative phase difference between the two laser pulses plays a significant role in their interaction processes. When the relative phase varies, the two laser beams display different features, such as attraction, repulsion, and energy shift. In particular, energy flow from the phase-advanced beam to the spot domain of the phase-delayed beam is observed when the relative phase difference is between zero and $\pi$. When the relative phase is larger than $\pi /2$, repulsion is dominant and the interaction gradually becomes weak. When the relative phase difference is smaller than $\pi /2$, attraction becomes dominant and, as the phase difference decreases, the phase-advanced beam shifts most of its energy into the spot domain of the phase-delayed beam. These conclusions are verified by our three-dimensional particle-in-cell simulations. This provides an efficient way to manipulate the energy distribution of relativistically intense laser pulses in plasmas by adjusting their relative phase.

A Kochen-Specker (KS) set is a specific set of projectors and measurement contexts that prove the Bell-Kochen-Specker contextuality theorem. The simplest known KS sets in Hilbert space dimensions $d=3,4,5,6,8$ are reproduced, and several methods by which a new KS set can be constructed using one or more known KS sets in lower dimensions are reviewed and improved. These KS sets and improved methods enable the construction of explicitly critical new KS sets in all dimensions, where critical refers to the irreducibility of the set of contexts. The simplest known critical KS sets are derived in all even dimensions $d\ge 10$ with at most nine contexts and 30 projectors, and in all odd dimensions $d\ge 7$ with at most 13 contexts and 39 projectors. These results show that neither the number of contexts nor the number of projectors in a minimal KS set scales with dimension $d$.

Double Compton scattering of high-energy twisted photons is investigated within the framework of relativistic quantum electrodynamics. We investigate the dependence of the angular distributions of the scattered photons on the parameters of the incident photon beam, such as momentum cone opening angle and projection of orbital angular momentum. Numerical calculations of the angular distributions of the scattered photons are presented for incoming twisted photons and compared to the standard case of incident plane-wave photons. The dependence of the angular distributions of the double-Compton-scattered photons for initially twisted photons prepared in a superposition of two vortex states is also presented.

We introduce the concept of fidelity for dynamical maps in an open quantum system scenario. We derive an inequality linking this quantity to the distinguishability of the inducing environmental states. Our inequality imposes constraints on the allowed set of dynamical maps arising from the microscopic description of system plus environment. Remarkably, the inequality involves only the states of the environment and the dynamical map of the open system and, therefore, does not rely on the knowledge of either the microscopic interaction Hamiltonian or the environmental Hamiltonian characteristic parameters. We demonstrate the power of our result by applying it to two different scenarios: quantum programming and quantum probing. In the first case, we use it to derive bounds on the dimension of the processor for approximate programming of unitaries. In the second case we present an intriguing proof-of-principle demonstration of the ability to extract information on the environment via a quantum probe without any a priori assumption on the form of the system-environment coupling Hamiltonian.

A major obstacle to attaining the fundamental precision limit of phase estimation in interferometry is the identification and implementation of the optimal measurement. Here we demonstrate that this can be accomplished by the use of three conventional measurements among interferometers with Bayesian estimation techniques. Conditions that hold for the precision limit to be attained with these measurements are obtained by explicitly calculating the Fisher information. Fortunately, these conditions are naturally satisfied in most interferometric experiments. We apply our results to an atomic spectroscopy experiment and examine the robustness of phase sensitivity for the two-axis counter-twisted state suffering from detection noise.

We introduce a class of inequalities based on low-order correlations of operators to detect entanglement in bipartite systems. The operators may either be Hermitian or non-Hermitian and are applicable to any physical system or class of states. Testing the criteria on example systems reveals that they outperform other common correlation-based criteria, such as those by Duan-Giedke-Cirac-Zoller and Hillery-Zubairy. One unusual feature of the criteria is that the correlations include the density matrix itself, which is related to the purity of the system. We discuss how such a term could be measured in relation to the criteria.

Given a preferred orthonormal basis $B$ in the Hilbert space of a quantum system, we define a measure of the coherence-generating power of a unitary operation with respect to $B$. This measure is the average coherence generated by the operation acting on a uniform ensemble of incoherent states. We give its explicit analytical form in any dimension and provide an operational protocol to directly detect it. We characterize the set of unitaries with maximal coherence-generating power and study the properties of our measure when the unitary is drawn at random from the Haar distribution. For a large state-space dimension a random unitary has, with overwhelming probability, nearly maximal coherence-generating power with respect to any basis. Finally, extensions to general unital quantum operations and the relation to the concept of asymmetry are discussed.

Given an orthonormal basis $B$ in a $d$-dimensional Hilbert space and a unital quantum operation $\mathcal{E}$ acting on it, one can define a nonlinear mapping that associates with $\mathcal{E}$ a $d\times d$ real-valued matrix that we call the coherence matrix of $\mathcal{E}$ with respect to $B$. This is the Gram matrix of the coherent part of the basis projections of $B$ under $\mathcal{E}$. We show that one can use this coherence matrix to define vast families of measures of the coherence-generating power (CGP) of the operation. These measures have a natural geometrical interpretation as separation of $\mathcal{E}$ from the set of incoherent unital operations. The probabilistic approach to CGP discussed in P. Zanardi et al. [Phys. Rev. A 95, 052306 (2017).] can be reformulated and generalized, introducing, alongside the coherence matrix, another $d\times d$ real-valued matrix, the simplex correlation matrix. This matrix describes the relevant statistical correlations in the input ensemble of incoherent states. Contracting these two matrices, one finds CGP measures associated with the process of preparing the given incoherent ensemble and processing it with the chosen unital operation. Finally, in the unitary case, we discuss how these concepts can be made compatible with an underlying tensor product structure by defining families of CGP measures that are additive.

Multistep laser resonance ionization spectroscopy of lutetium (Lu) has been performed at TRIUMF's off-line laser ion source test stand. The even-parity Rydberg series $6s^{2}nd{}^2{D}_{3/2}, 6s^{2}nd{}^2{D}_{5/2}$, and $6s^{2}ns{}^2{S}_{1/2}$ were observed converging to the $6s^2$ ionization potential. The experimental results have been compared to those of previous work. Fifty-one levels of Rydberg series $6s^{2}nd{}^2{D}_{5/2}$ and 52 levels of Rydberg series $6s^{2}ns{}^2{S}_{1/2}$ were reported. Additionally, six even-parity autoionization (AI) series converging to Lu ionic states $5d6s{}^3{D}_1$ and $5d6s{}^3{D}_2$ were observed. The level energies of these AI states were measured. The configurations of the AI states were assigned by relativistic multichannel theory within the framework of multichannel quantum defect theory.