We investigate pairwise quantum correlation as measured by the quantum discord as well as its classical counterpart in the thermodynamic limit of anisotropic $\mathit{XY}$ spin-$1/2$ chains in a transverse magnetic field for both zero and finite temperatures. Analytical expressions for both classical and quantum correlations are obtained for spin pairs at any distance. In the case of zero temperature, it is shown that the quantum discord for spin pairs farther than second neighbors is able to characterize a quantum phase transition, even though pairwise entanglement is absent for such distances. For finite temperatures, we show that quantum correlations can be increased with temperature in the presence of a magnetic field. Moreover, in the $\mathit{XX}$ limit, thermal quantum discord is found to be dominant over classical correlation while the opposite scenario takes place for the transverse field Ising model limit.

For quantum mechanics of a charged particle in a classical external electromagnetic field, there is an apparent puzzle that the matrix element of the canonical momentum and Hamiltonian operators is gauge dependent. A resolution to this puzzle was recently provided by us [X.-S. Chen et al., Phys. Rev. Lett. 100, 232002 (2008)]. Based on the separation of the electromagnetic potential into pure-gauge and gauge-invariant parts, we have proposed a new set of momentum and Hamiltonian operators which satisfy both the requirement of gauge invariance and the relevant commutation relations. In this paper we report a check for the case of the hydrogen-atom problem: Starting from the Hamiltonian of the coupled electron, proton, and electromagnetic field, under the infinite proton mass approximation, we derive the gauge-invariant hydrogen-atom Hamiltonian and verify explicitly that this Hamiltonian is different from the Dirac Hamiltonian, which is the time translation generator of the system. The gauge-invariant Hamiltonian is the energy operator, whose eigenvalue is the energy of the hydrogen atom. It is generally time dependent. In this case, one can solve the energy eigenvalue equation at any specific instant of time. It is shown that the energy eigenvalues are gauge independent, and by suitably choosing the phase factor of the time-dependent eigenfunction, one can ensure that the time-dependent eigenfunction satisfies the Dirac equation.

For an anisotropic Heisenberg ($\mathit{XXZ}$) spin chain, we show that an open boundary induces a series of approximately self-similar features at different energy scales, high up in the eigenvalue spectrum. We present a nonequilibrium phenomenon related to this fractal structure, involving states in which a connected block near the edge is polarized oppositely to the rest of the chain. We show that such oppositely polarized blocks can be “locked” to the edge of the spin chain and that there is a hierarchy of edge-locking effects at various orders of the anisotropy. The phenomenon enables dramatic control of quantum-state transmission and magnetization control.

A simple scheme is proposed to achieve three-mode continuous-variable (CV) entanglement in a coupled triple-semiconductor quantum-well (TSQW) structure via tunneling-induced interference. In the present scheme, the TSQW structure is trapped into a triply resonant cavity, and the tunneling-induced interference effects considered here are the key to realizing entanglement. By numerically simulating the dynamics of the system, we show that the strength of tunneling-induced interference can effectively influence the period of entanglement, and the generation of entanglement does not depend intensively on the initial condition of the cavity field in our scheme. As a result, the present research provides an efficient approach to achieve three-mode CV entanglement in a semiconductor nanostructure, which may have an impact on the progress of solid-state quantum-information theory.

Electron-impact vibrational excitation of the hydrogen molecule has been revisited in the energy region from $1$ to $5$ eV. A crossed-beam double trochoidal electron spectrometer is used. Forward and backward scattered electrons from the $v=0\to 1$ excitation channel are separated by electron-beam modulation and time-of-flight detection technique. Present results are normalized and absolute values of differential cross sections at critical border angles of $0^{°}$ and ${180}^{°}$ are determined. In this way the differential cross-section measurements are completed in the full angular range from $0$${}^{°}$ to $180$${}^{°}$.

We propose a scheme to generate isolated sub-100-as pulses with 30-fs lasers. Our model is based on high-order harmonic generation from a helium atom irradiated by a two-color femtosecond laser field, in which the driving and the control fields are linearly polarized and the polarization angle between them is $\pi /4$. By mixing the polarization gating technique and optimization of two-color lasers, we destroy the symmetry of the driving field from two directions and realize quantum path control with multicycle lasers.

We analyze light-wave dynamics in artificial materials characterized by periodically modulated gain or loss on the wavelength scale. The study of the temporal dispersion in one-dimensionally modulated materials predicts mode-locked states and superluminal light propagation regimes in the parameter regions close to the locking regions. The study of spatial dispersion for a two-dimensional gain-loss modulation predicts nontrivial beam propagation effects such as self-collimation, angle-sensitive gain, and negative diffraction in such gain-loss-modulated materials.

Teleportation is a basic primitive for quantum communication and quantum computing. We address the problem of continuous-variable (unconditional and conditional) teleportation of a pure single-photon state and a mixed attenuated single-photon state generally in a nonunity-gain regime. Our figure of merit is the maximum negativity of the Wigner function, which demonstrates a highly nonclassical feature of the teleported state. We find that the negativity of the Wigner function of the single-photon state can be unconditionally teleported for an arbitrarily weak squeezed state used to create the entangled state shared in teleportation. In contrast, for the attenuated single-photon state there is a strict threshold squeezing one has to surpass to successfully teleport the negativity of its Wigner function. The conditional teleportation allows one to approach perfect transmission of the single photon for an arbitrarily low squeezing at a cost of decrease of the success rate. In contrast, for the attenuated single photon state, conditional teleportation cannot overcome the squeezing threshold of the unconditional teleportation and it approaches negativity of the input state only if the squeezing increases simultaneously. However, as soon as the threshold squeezing is surpassed, conditional teleportation still pronouncedly outperforms the unconditional one. The main consequences for quantum communication and quantum computing with continuous variables are discussed.

The spatial profile of ultracold atoms reflecting from an exponentially decaying magnetic potential depends on parameters such as the corrugation in the magnetic potential and the temperature of the atomic cloud. We report on experimental investigations of the effect of projection velocity which determines the strength of the interaction of the atom cloud with the magnetic potential and the effect of temperature of ultracold ${}^{87}\mathrm{Rb}$ atoms reflecting from a periodic one-dimensional corrugated magnetic potential. The magnetic potential is generated on an atom chip by a periodic permanent magnetic structure of period 10 $\mu$m. The amplitude of the corrugation is controlled by applying a uniform external-bias magnetic field.

We theoretically investigate the itinerant ferromagnetic transition of a spherically trapped ultracold Fermi gas with spin imbalance under strongly repulsive interatomic interactions. Our study is based on a self-consistent solution of the Hartree-Fock mean-field equations beyond the widely used local-density approximation. We demonstrate that, while the local-density approximation holds in the paramagnetic phase, after the ferromagnetic transition it leads to a quantitative discrepancy in various thermodynamic quantities even with large atom numbers. We determine the position of the phase transition by monitoring the shape change of the free-energy curve with increasing the polarization at various interaction strengths.

We investigate the dynamics of linear vortex lattices in anisotropic traps in two dimensions and show that the interplay between the rotation and the anisotropy leads to a rich but highly regular dynamics.

We investigate the many-particle and mean-field correspondence for a non-Hermitian $N$-particle Bose-Hubbard dimer where a complex on-site energy describes an effective decay from one of the modes. Recently a generalized mean-field approximation for this non-Hermitian many-particle system yielding an alternative complex nonlinear Schrödinger equation was introduced. Here we give details of this mean-field approximation and show that the resulting dynamics can be expressed in a generalized canonical form that includes a metric gradient flow. The interplay of nonlinearity and non-Hermiticity introduces a qualitatively new behavior to the mean-field dynamics: The presence of the non-Hermiticity promotes the self-trapping transition, while damping the self-trapping oscillations, and the nonlinearity introduces a strong sensitivity to the initial conditions in the decay of the normalization. Here we present a complete characterization of the mean-field dynamics and the fixed point structure. We also investigate the full many-particle dynamics, which shows a rich variety of breakdown and revival as well as tunneling phenomena on top of the mean-field structure.

In materials lacking inversion symmetry, both Pockels and Kerr electro-optic effects are simultaneously present, with the former effect generally dominating the latter one. The theoretical findings of this article provide the crystal physics community with concrete tabulated evidence showing that it is possible in principle to selectively bypass contributions from the linear effect(s) and directly obtain information only about the genuine (Kerr-like) quadratic effects in $90\%$ of the noncentrosymmetric point groups. The general idea and treatment used for the electro-optic effect can be extended and adapted to other optical or non-optical (phenomenological) purely quadratic effects in media lacking inversion symmetry.

The second-order spatial subwavelength interference pattern is observed in a modified Michelson interferometer with single-mode continuous-wave laser beams. By analyzing our subwavelength interference experiment based on Feynman’s path integral theory, a unified interpretation for all the second-order subwavelength interference is suggested.

Electrons and $\alpha$ particles injected midway between two ultrahigh intensity crossed laser beams of radial polarization are shown to be accelerated in vacuum to several gigaelectron volts and to have average energy gradients in excess of $150$ GeV/m. A unique model of the crossing beams is suggested, which maximizes the particle energy gain and minimizes the particle-beam diffraction.

We consider a way to generate operational inequalities to test nonclassicality (or quantumness) of multimode bosonic fields (or multiparty bosonic systems) that unifies the derivation of many known inequalities and allows to propose new ones. The nonclassicality criteria are based on Vogel’s criterion corresponding to analyzing the positivity of multimode $P$ functions or, equivalently, the positivity of matrices of expectation values of, e.g., creation and annihilation operators. We analyze not only monomials but also polynomial functions of such moments, which can sometimes enable simpler derivations of physically relevant inequalities. As an example, we derive various classical inequalities which can be violated only by nonclassical fields. In particular, we show how the criteria introduced here easily reduce to the well-known inequalities describing (a) multimode quadrature squeezing and its generalizations, including sum, difference, and principal squeezing; (b) two-mode one-time photon-number correlations, including sub-Poisson photon-number correlations and effects corresponding to violations of the Cauchy-Schwarz and Muirhead inequalities; (c) two-time single-mode photon-number correlations, including photon antibunching and hyperbunching; and (d) two- and three-mode quantum entanglement. Other simple inequalities for testing nonclassicality are also proposed. We have found some general relations between the nonclassicality and entanglement criteria, in particular those resulting from the Cauchy-Schwarz inequality. It is shown that some known entanglement inequalities can be derived as nonclassicality inequalities within our formalism, while some other known entanglement inequalities can be seen as sums of more than one inequality derived from the nonclassicality criterion. This approach enables a deeper analysis of the entanglement for a given nonclassicality.

We theoretically investigate the supercontinuum generation (SCG) on the basis of modulational instability (MI) in liquid-core photonic crystal fibers (LCPCF) with CS${}_2$-filled central core. The effect of saturable nonlinearity of LCPCF on SCG in the femtosecond regime is studied using an appropriately modified nonlinear Schrödinger equation. We also compare the MI induced spectral broadening with SCG obtained by soliton fission. To analyze the quality of the pulse broadening, we study the coherence of the SC pulse numerically. It is evident from the numerical simulation that the response of the saturable nonlinearity suppresses the broadening of the pulse. We also observe that the MI induced SCG in the presence of saturable nonlinearity degrades the coherence of the SCG pulse when compared to unsaturated medium.

We investigate theoretically the interaction of dark solitons in materials with a spatially nonlocal nonlinearity. In particular we do this analytically and for arbitrary degree of nonlocality. We employ the variational technique to show that nonlocality induces an attractive force in the otherwise repulsive soliton interaction.

The alignment dependence of the ionization behavior of ${\mathrm{H}}_2$ exposed to intense ultrashort laser pulses is investigated on the basis of solutions of the full time-dependent Schrödinger equation within the fixed-nuclei and dipole approximation. The total ionization yields as well as the energy-resolved electron spectra have been calculated for a parallel and a perpendicular orientation of the molecular axis with respect to the polarization axis of linear polarized laser pulses. For most, but not all considered laser peak intensities, the parallel aligned molecules are easier to ionize. Furthermore, it is shown that the velocity formulation of the strong-field approximation predicts a simple interference pattern for the ratio of the energy-resolved electron spectra obtained for the two orientations, but this is not confirmed by the full ab initio results.