We examined the propagation of nonanalytical points encoded on temporally Gaussian-shaped optical pulses in fast and slow light in an optical ring resonator at $\lambda$ = 1.5 μm. The temporal peak of the Gaussian pulse was either advanced or delayed, reflecting anomalous or normal dispersions in the ring resonator, relevant to under- or overcoupling conditions, respectively. The nonanalytical points were neither advanced nor delayed but appeared as they entered the ring resonator. The nonanalytical points could be interpreted as information; therefore, the experimental results suggested that information velocity is equal to the light velocity in vacuum or the background medium, independent of the group velocity. The transient behaviors at the leading and trailing edges of the nonanalytical points are discussed in terms of optical precursors.

We investigate the teleportation scheme of two-mode squeezed states proposed by Adhikari et al. [S. Adhikari et al., Phys. Rev. A 77, 012337 (2008)]. It uses four-mode entangled states to teleport two-mode squeezed states. The fidelity between the original two-mode squeezed states and teleported ones is calculated. The maximal fidelity value of Adhikari's protocol is $0.38$, which is incompatible with the fidelity definition with the maximal value 1. In our opinion, one reason is that they calculate the fidelity for multimodes Gaussian states using the fidelity formula for single-mode ones. Another reason is that the covariance matrix of output states should be what is obtained after applying the linear unitary Bogoliubov operations (two cascaded Fourier transformations) on the covariance matrix given in Eq. (12) in their paper. These two reasons result in the incomparable results. In addition, Adhikari's protocol can be simplified to be easily implemented.

We investigate the high-order harmonic generation of atoms and molecules exposed in strong and short-wavelength (shorter than 800 nm) laser fields. Our simulations show that the electronic rescattering trajectory depends strongly on the property of the Coulomb potential. Using numerical schemes, we identify the important role of excited states in the emission times of harmonics from molecules. We propose a model, which considers the initial position when electrons tunnel out from the potential, to explain the electronic response in intense and relatively high-frequency laser fields.

We analyze the behavior of the anomalous density as function of the radial distance at different temperatures in a variational framework. We show that the temperature dependence of the anomalous density agrees with the Hartree-Fock-Bogoliubov (HFB) calculations. Comparisons between the normal and anomalous fractions at low temperature show that the latter remains higher and, consequently, the neglect of the anomalous density may destabilize the condensate. These results are compatible with those of Yukalov. Surprisingly, the study of the anomalous density in terms of the interaction parameter shows that the dip in the central density is destroyed for sufficiently weak interactions. We explain this effect.

We study the transition from hydrodynamic to collisionless behavior in collective modes of ultracold trapped Fermi gases. To that end, we solve the Boltzmann equation for the trapped Fermi gas via the moments method. We showed previously that it is necessary to go beyond second-order moments if one wants to reproduce the results of a numerical solution of the Boltzmann equation. Here, we will give the detailed description of the method including fourth-order moments. We apply this method to the case of realistic parameters, and compare the results for the radial quadrupole and scissors modes at unitarity to experimental data obtained by the Innsbruck group. It turns out that the inclusion of fourth-order moments clearly improves the agreement with the experimental data. In particular, the fourth-order moments reduce the effect of collisions and therefore partially compensate the effect of the enhanced in-medium cross section at low temperatures.

We study the counting statistics of ultracold bosonic atoms that are released from an optical lattice. We show that the counting probability distribution of the atoms collected at a detector located far away from the optical lattice can be used to characterize the initially trapped states. We consider trapped superfluid and insulating states with different occupation patterns. We analyze how the correlations between the modes that develop during the expansion in the gravitational field appear in the counting distribution and find that the ratio of the detector size with respect to the expanded wave function determines whether short-range or long-range correlations of the initial state are reflected in the counting statistics. We find that detectors which are large compared to the size of the expanded wave function distinguish insulating and superfluid phases irrespective of the occupation pattern. We show that using detectors that are small compared to the size of the expanded wave function, occupation patterns in insulating and supersolid states can be distinguished. Finally, we show how the magnetic phase patterns are dramatically reflected in the number distribution.

A laser diode subject to a phase-conjugate optical feedback can exhibit rich nonlinear dynamics and chaos. We report here on two bifurcation mechanisms that appear when increasing the amount of light being fed back to the laser. First, we report on a full suppression of chaos from a crisis induced by a saddle-node bifurcation on self-pulsing, so-called external-cavity-mode solutions (ECMs). Second, the feedback-dependent torus and saddle-node bifurcations on ECMs may be responsible for large regions of bistability between ECMs of different and high (beyond gigahertz) frequencies.

We conduct theoretical studies on electromagnetically induced absorption via incoherent collisions in an inhomogeneously broadened ladder-type three-level system with the density-matrix approach. The effects of the collision-induced coherence decay rates as well as the probe laser field intensity on the probe field absorption are examined. It is shown that with the increase of the collisional decay rates in a moderate range, a narrow dip due to electromagnetically induced transparency superimposed on the Doppler-broadened absorption background can be turned into a narrow peak under the conditions that the probe field intensity is not very weak as compared to the pump field, which results from the enhancement of constructive interference and suppression of destructive interference between one-photon and multiphoton transition pathways. The physical origin of the collision-assisted electromagnetically induced absorption is analyzed with a power-series solution of the density-matrix equations.

We consider different classes of quenched disordered quantum $XY$ spin chains, including a quantum $XY$ spin glass and a quantum $XY$ model with a random transverse field, and investigate the behavior of genuine multiparty entanglement in the ground states of these models. We find that there are distinct ranges of the disorder parameter that give rise to a higher genuine multiparty entanglement than in the corresponding systems without disorder: an order-from-disorder phenomenon in genuine multiparty entanglement. Moreover, we show that such a disorder-induced advantage in the genuine multiparty entanglement is useful: It is almost certainly accompanied by a order-from-disorder phenomenon for a multiport quantum dense coding capacity with the same ground state used as a multiport quantum network.

It is shown that an optical arrangement in which two identical counterpropagating Laguerre-Gaussian doughnut beams LG($l$,0) and LG$(-l,0)$ with orthogonal linear polarizations ${\widehat{\mathbf{e}}}_x$ and ${\widehat{\mathbf{e}}}_y$ can lead to azimuthal polarization gradients and an as yet undiscovered azimuthal Sisyphus effect. It is demonstrated that this effect can be utilized in the creation and control of a persistent current of superfluid atoms circulating in a toroidal trap. Such a physical system has recently been highlighted as the basis for an atomic superconducting quantum interference device (SQUID) and ultimately for the realization of atom circuits.

We present a general method, based on a multiple-scale approach, for deriving the perturbative solutions of the scaling equations governing the expansion of superfluid ultracold quantum gases released from elongated harmonic traps. We discuss how to treat the secular terms appearing in the usual naive expansion in the trap asymmetry parameter $\epsilon$ and calculate the next-to-leading correction for the asymptotic aspect ratio, with significant improvement over the previous proposals.

In a dipolar Fermi gas, the dipole-dipole interaction between fermions can be turned into a dipolar Ising interaction between pseudospins in the presence of an ac electric field. When trapped in a two-dimensional optical lattice, this dipolar Fermi gas has a very rich phase diagram at zero temperature, due to the competition between antiferromagnetism and superfluidity. At half-filling, the antiferromagnetic state is the favored ground state. The superfluid state appears as the ground state at a smaller filling factor. In between there is a phase-separated region. The order parameter of the superfluid state can display different symmetries depending on the filling factor and interaction strength, including the $d$-wave ($d$), the extended $s$-wave ($xs$), or their linear combination ($xs+id$). Implications for the current experiment are discussed.

We study coherence of a trapped bosonic cloud with attractive finite-range interaction in a tight harmonic trap. One-body density and pair-distribution function in the ground state for different trap sizes are calculated. We also calculate healing length and the correlation length which signify the presence of high spatial coherence in a very tight trap leading to the destruction of the condensate for a fixed particle number. This is in marked variance with the usual collapse of the attractive metastable condensate when $N>N_{cr}$. Thus we investigate the critical frequency and critical size of the trap for the existence of attractive Bose-Einstein condensation. The finite-range interaction gives a nonlocal effect in the effective many-body potential, and we observe a high-density stable branch besides the known metastable branch. Moreover, the new branch shows universal behavior even in the very tight trap.

We consider the one-dimensional expansion of a system of interacting bosons, starting from a regular array. Without interactions the familiar Hanbury Brown and Twiss effect for bosons gives rise to a series of peaks in the density-density correlations of the expanded system. Infinitely repulsive particles likewise give a series of dips, a signature of the underlying description in terms of free fermions. In the intermediate case of finite interaction, the noise correlations consist of a set of Fano resonance lineshapes, with an asymmetry parameter determined by the scattering phase shift of a pair of particles and a width depending on the initial momentum spread of the particles.

Based on the narrow linewidth ${}^1{S}_0-{}^3{P}_1$ transition of the bosonic ${}^{88}$Sr atom, we investigate the dispersion relation for light propagating in a three-dimensional (3D) atomic dipolar lattice. Two factors determine the presence of the photonic band gap (PBG): lattice geometry and the atomic polarization. For a two-level atomic system, the former factor plays a major role. An omnidirectional PBG is predicted in the non-Bravais diamond lattice while no PBG exists in the Bravais lattices. However, since a PBG happens around the resonant ${}^1{S}_0-{}^3{P}_1$ line, the probe beam suffers a significant atomic absorption, which is the major drawback in Bragg scattering. We propose applying an external field to couple the atomic ${}^3{P}_1-{}^3{D}_1$ transition so as to modify the atomic polarization of the ${}^1{S}_0-{}^3{P}_1$ transition. In the three level ${}^1{S}_0-{}^3{P}_1-{}^3{D}_1$ atomic system, the atomic polarization strongly affects the band structure besides the lattice geometry. Dual PBGs can exist in simple cubic and face-centered cubic lattices, but the original PBG, in the diamond lattice composed of the two-level atoms, vanishes. Meanwhile, since two PBGs can be pushed far away from the atomic resonant ${}^1{S}_0-{}^3{P}_1$ line, the atomic absorption is significantly suppressed, which enhances the reflection efficiency in Bragg scattering.

We demonstrate that a generalized nonlinear Schrödinger equation (NSE), which includes dispersion of the intensity-dependent group velocity, allows for exact solitary solutions. In the limit of a long pulse duration, these solutions naturally converge to a fundamental soliton of the standard NSE. In particular, the peak pulse intensity times squared pulse duration is constant. For short durations, this scaling gets violated and a cusp of the envelope may be formed. The limiting singular solution determines then the shortest possible pulse duration and the largest possible peak power. We obtain these parameters explicitly in terms of the parameters of the generalized NSE.

The angular dependence of the momentum flow of a polychromatic plane wave scattered from deterministic and random collections of particles is determined, within the occuracy of the first-order Born approximation, as a function of individual and collective properties of particles. The results are of importance for optimization of optical tweezers.