Experimental data with digital masks and a theoretical analysis are presented for an imaging scheme that we call time-correspondence differential ghost imaging (TCDGI). It is shown that by conditional averaging of the information from the reference detector but with the negative signals inverted, the quality of the reconstructed images is in general superior to all other ghost imaging (GI) methods to date. The advantages of both differential GI and time-correspondence GI are combined, plus less data manipulation and shorter computation time are required to obtain equivalent quality images under the same conditions. This TCDGI method offers a general approach applicable to all GI techniques, especially when objects with continuous gray tones are involved.

We propose a quantum simulation of a two-level atom coupled to a single mode of the electromagnetic field in the ultrastrong-coupling regime based upon resonant Raman transitions in an atom interacting with a high finesse optical cavity mode. We show by numerical simulation the possibility of realizing the scheme with a single rubidium atom, in which two hyperfine ground states make up the effective two-level system, and for cavity QED parameters that should be achievable with, for example, microtoroidal whispering-gallery-mode resonators. Our system also enables simulation of a generalized model in which a nonlinear coupling between the atomic inversion and the cavity photon number occurs on an equal footing with the (ultrastrong) dipole coupling and can give rise to critical-type behavior even at the single-atom level. Our model takes account of dissipation, and we pay particular attention to observables that would be readily observable in the output from the system.

We consider maximal violations of the Leggett-Garg inequality, obtained by maximizing over all possible measurement operators, in relation to nonunitary aspects of the system dynamics. We model the action of an environment on a qubit in terms of generic quantum channels and relate the maximal value of the Leggett-Garg correlator to the channel parameters. We focus on unital channels and hence on decoherence. In certain important cases, exact relations between the channel parameters and maximal violations can be found. Moreover, we demonstrate the existence of distinct thresholds for the channel parameters, below which no violation of the Leggett-Garg inequality can occur.

We study the influence of topological defects on the spin current as well as the spin Hall effect. We find that the nontrivial deformation of the space time due to topological defects can generate a spin-dependent current which then induces an imbalanced accumulation of spin states on the edges of the sample. The corresponding spin Hall conductivity has also been calculated for the topological defect of a cosmic string. Compared to the ordinary value, a correction which is linear with mass density of the cosmic string appears. Our approach to the dynamics of nonrelativistic spinors in the presence of a topological defect is based on the Foldy-Wouthuysen transformation. The spin current is obtained by using the extended Drude model, which is independent of the scattering mechanism.

Unlike correlation of classical systems, entanglement of quantum systems cannot be distributed at will: if one system $A$ is maximally entangled with another system $B$, it cannot be entangled at all with a third system $C$. This concept, known as the monogamy of entanglement, is manifest when the entanglement of $A$ with a pair $BC$ can be divided as contributions of the entanglement between $A$ and $B$ and $A$ and $C$, plus a term ${\tau }_{ABC}$ involving genuine tripartite entanglement and so expected to be always positive. A very important measure in quantum information theory, the entanglement of formation (EOF), fails to satisfy this last requirement. Here we present the reasons for that and show a set of conditions that an arbitrary pure tripartite state must satisfy for the EOF to become a monogamous measure, i.e., for ${\tau }_{ABC}\ge 0$. The relation derived is connected to the discrepancy between quantum and classical correlations, ${\tau }_{ABC}$ being negative whenever the quantum correlation prevails over the classical one. This result is employed to elucidate features of the distribution of entanglement during a dynamical evolution. It also helps to relate all monogamous instances of the EOF to the squashed sntanglement, an entanglement measure that is always monogamous.

We investigate quantum states that possess both maximum entanglement and maximum discord between the pertinent parties. Since entanglement (discord) is defined only for bipartite (two-qubit) systems, we use an appropriate sum over all bipartitions as the associated measure. The ensuing definition—not new for entanglement—is thus extended here to quantum discord. Also, additional dimensions within the parties are considered (qudits). We also discuss quantum correlations that induce Mermin's Bell-inequality violation for all multiqubit systems. One finds some differences when quantum mechanics is defined over the field of real or of complex numbers.

For harmonic-trapped atomic systems, we report system-independent nonadiabatic features in the response to interaction ramps. We provide results for several different systems in one, two, and three dimensions: bosonic and fermionic Hubbard models realized through optical lattices, a Bose-Einstein condensate, a fermionic superfluid, and a Fermi liquid. The deviation from adiabaticity is characterized through the heating or excitation energy produced during the ramp. We find that the dependence of the heat on the ramp time is sensitive to the ramp protocol but has aspects common to all systems considered. We explain these common features in terms of universal dynamics of the system size or cloud radius.

An integral Jones matrix for oblique light propagation through a deformed crystal is derived using the differential Jones matrix approach. We show that generally at oblique light propagation a crystal monotonically deformed along its normal should show both linear and circular birefringence, even without twist deformation. The capability of the approach is illustrated by the calculation of the integral extended Jones matrices for oblique light propagation through several deformed liquid crystal nematic cells. In particular, we predict that at oblique light propagation in certain directions the nontwisted hybrid nematic cell (in which the tilt angle of the optic axis with respect to the crystal plate normal smoothly varies through the thickness) shows optical activity, a phenomenon of spatial light dispersion akin to so-called weak optical activity in nonchiral polar solid crystals.

Regular quasiprobabilities are introduced for the aim of characterizing quantum correlations of multimode radiation fields. Negativities of these quantum-correlation quasiprobabilities are necessary and sufficient for any quantum correlation encoded in the multimode Glauber-Sudarshan $P$ function. The strength of the method is demonstrated for a two-mode, phase-randomized, squeezed-vacuum state. It has no entanglement, no quantum discord, a positive Wigner function, and a classical reduced single-mode representation. Our method clearly visualizes the quantum correlations of this state.

The generation of terahertz radiation from an ensemble of aligned air molecules in intense two-color laser fields has been investigated. Our experiments show that terahertz radiation yields are strongly modulated by the alignment of nitrogen and oxygen molecules. We explain this phenomenon in the context of the plasma current model combined with alignment-dependent ionization, in which nitrogen molecules aligned parallel to the laser field yield more ionization and plasma currents, resulting in enhanced terahertz radiation. Our measurements also provide the ratio of the ensemble-averaged ionization rates of nitrogen molecules aligned parallel versus orthogonal to the laser field.

We study theoretically nonadiabatic rotational excitation dynamics and field-free orientation of a polar molecule in a ramped laser field by using time-dependent quantum mechanics. By adjusting the time of rising and falling edge of the ramped pulse, the population of the specific rotational states can be controlled. We describe the effect of the different shaped pulses (ramp, Gaussian, square pulses) in controlling the molecular dynamics. Further, it is also shown that enhancement or suppression of molecular orientation can be coherently manipulated by controlling various laser parameters like intensity, time delay, and rotational temperature, etc.

We theoretically investigate the high-order-harmonic generation (HHG) from the oriented gas-phase CO molecule exposed to the combination of an intense few-cycle chirped laser and a unipolar pulse. We show that the HHG is dramatically extended and an ultrabroad extreme ultraviolet supercontinuum spectra in the plateau is generated by adding a unipolar pulse to the few-cycle chirped laser at a proper time. Both the classical trajectory simulation and quantum time-frequency analysis show that a dominant short quantum path is selected to contribute the HHG spectra in the plateau. By superposing the supercontinuum harmonics centered at several different central frequencies, we can directly produce several isolated pulses in which the shortest one achieves 38 as.

We have experimentally observed the two lowest vibrational levels of Cs${}_2$ 0${}_g^{-}$ purely-long-range states. The photoassociation spectroscopy of ultracold cesium atoms with rotational structure presents clear identification of these lowest levels. Values of radiative lifetimes ${\tau }_{3/2}=30.41\pm 0.06$ ns and ${\tau }_{1/2}=34.81\pm 0.07$ ns of the 6$p{}^2{P}_{1/2}$and 6$p{}^2{P}_{3/2}$atomic levels and van der Waals coefficient $C_6=6852\pm 25$ a.u. of the ground-state cesium molecule are extracted, respectively.

We investigate the entanglement properties of the equilibrium and nonequilibrium quantum dynamics of two-dimensional (2D) and three-dimensional (3D) Fermi gases, by computing entanglement entropies of extended space regions, which generally show multiplicative logarithmic corrections to the leading power-law behaviors, corresponding to the logarithmic corrections to the area law. We consider 2D and 3D Fermi gases of $N$ particles constrained within a limited space region, for example, by a hard-wall trap, at equilibrium at $T=0$, i.e., in their ground state, and compute the first few terms of the asymptotic large-$N$ behaviors of entanglement entropies and particle fluctuations of subsystems with some convenient geometries, which allow us to significantly extend their computation. Then we consider their nonequilibrium dynamics after instantaneously dropping the hard-wall trap, which allows the gas to expand freely. We compute the time dependence of the von Neumann entanglement entropy of space regions around the original trap. We show that at small time it is characterized by the relation $S\approx {\pi }^{2}V/3$ with the particle variance, and multiplicative logarithmic corrections to the leading power law, i.e., $S\sim t^{1-d}\ln (1/t)$.

The unique atomic properties of samariumlike ions, not yet measured experimentally, are theoretically predicted and studied in this paper. Excitation energies, oscillator strengths, transition probabilities, and lifetimes are calculated for $(5s^2+5p^2+5d^2+5s5d+5s5g+5p5f)$–$(5s5p+5s5f+5p5d+5p5g)$ electric dipole transitions in Sm-like ions with nuclear charge $Z$ ranging from 74 to 100. Relativistic many-body perturbation theory (RMBPT), including the Breit interaction, is used to evaluate retarded $E1$ matrix elements in length and velocity forms. The calculations start from a $1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}3d^{10}4s^{2}4p^{6}4d^{10}4f^{14}$ Dirac-Fock potential. First-order perturbation theory is used to obtain intermediate coupling coefficients, and the second-order RMBPT is used to determine the matrix elements. The contributions from negative-energy states are included in the second-order $E1$ matrix elements to achieve agreement between length-form and velocity-form amplitudes. The resulting transition energies and transition probabilities, and lifetimes for Sm-like W${}^{12+}$ are compared with results obtained by the relativistic Hartree-Fock approximation (cowan code) to estimate contributions of the $4f$-core-excited states. Trends of excitation energies and oscillator strengths as the function of nuclear charge $Z$ are shown graphically for selected states and transitions. This work provides a number of yet unmeasured atomic properties of these samariumlike ions for various applications and as a benchmark for testing theory.

We studied magneto-optical resonances caused by excited-state level crossings in a nonzero magnetic field. Experimental measurements were performed on the transitions of the $D_2$ line of rubidium. These measured signals were described by a theoretical model that takes into account all neighboring hyperfine transitions, the mixing of magnetic sublevels in an external magnetic field, the coherence properties of the exciting laser radiation, and the Doppler effect. Good agreement between the experimental measurements and the theoretical model could be achieved over a wide range of laser power densities. We further showed that the contrasts of the level-crossing peaks can be sensitive to changes in the frequency of the exciting laser radiation as small as several tens of megahertz when the hyperfine splitting of the exciting state is larger than the Doppler broadening.

We investigate the single-photon transport in a single-mode optical fiber coupled to an optomechanical system in the single-photon strong-coupling regime. The single-photon transmission amplitude is analytically obtained with a real-space approach and the effects of cavity and mechanical dissipations are studied via master-equation simulations. Based on the theoretical framework, we further propose a heralded probabilistic scheme to generate mechanical NOON states with arbitrary phonon numbers by measuring the sideband photons. The efficiency and fidelity of the scheme are discussed finally.

The effect of a transverse magnetic field (TMF) on the absorption spectra of degenerate two-level systems in the $D_2$ line of ${}^{87}$Rb is investigated both analytically and numerically. We compare the effect of the TMF on the absorption of a $\sigma$ polarized pump in the Hanle configuration with that of a ${\sigma }^{-}$ probe in the presence of a ${\sigma }^{+}$ pump in the pump-probe configuration, and we show that the absorption spectra in both configurations is split in the presence of a TMF and that the splitting is proportional to its magnitude. The population redistribution in the ground state due to the TMF is reinforced by collisional effects, leading to increased splitting of the coherent population trapping (CPT) dip in the absorption spectrum. We explain the appearance of the splitting by setting the quantization axis along the total magnetic field. Then, in the Hanle configuration, the direction of the quantization axis changes as the longitudinal magnetic field is scanned, so that the laser polarization also changes. This leads to the creation of new two-photon detuned $\Lambda$ subsystems. The evolution of the populations and coherences of the ground-state Zeeman sublevels is illustrated using the angular momentum probability surface. In addition to being split, the CPT dip in the pump-probe configuration is also shifted by the longitudinal magnetic field, so that the effects of the longitudinal and transverse magnetic fields can be distinguished from each other.

We consider the problem of photon creation from a vacuum inside an ideal cavity with vibrating walls in the resonance case, taking into account the interaction between the resonant field mode and a detector modeled by a quantum harmonic oscillator. The frequency of wall vibrations is taken to be twice the cavity normal frequency, modified due to the coupling with the detector. The dynamical equations are solved with the aid of the multiple scales method. Analytical expressions are obtained for the photon mean numbers and their variances for the field and detector modes, which are supposed to be initially in the vacuum quantum states. We analyze different regimes of excitation, depending on the ratio of the modulation depth of the time-dependent cavity eigenfrequency to the coupling strength between the cavity mode and detector. We show that statistical properties of the detector quantum state (variances of the photon numbers, photon distribution function, and the degree of quadrature squeezing) can be quite different from that of the field mode. Besides, the mean number of quanta in the detector mode increases with some time delay, compared with the field mode.