The physical origin and properties of quasistationary static-field-induced states (SFISs) are discussed. SFISs can be supported by a combination of any atomic or molecular potential and an external static electric field and do not have counterparts in the absence of the field. A resonant tunneling ionization (RTI) mechanism of molecules proceeding via SFISs is analyzed for a one-dimensional model. It is shown that SFISs manifest themselves as peaks of the tunneling ionization rate both in static and time-dependent fields in the adiabatic regime. The peaks are located along the RTI ridges in the plane of internuclear distance $R$ and the momentary value of electric field $F$. The RTI via SFISs appears to be one of the general ionization mechanisms of molecules.

In a joint experimental and theoretical endeavor, we explore the laser-induced dissociation and ionization dynamics of H${{}_2}^{+}$ beams using sub-10-fs, 800 nm laser pulses. Our theory predicts considerable control over the branching ratio of two-photon and three-photon above-threshold dissociation (ATD) by gating the dissociation pathway on a few-femtosecond timescale. We are able to experimentally demonstrate this control. Moreover, our theory also shows the importance of the highly excited H(2$l$) states of H${{}_2}^{+}$ that contribute to ATD structure in dissociation. As is the case for dissociation, we find that ionization is also sensitive to the effective laser interaction time.

We study Fermi-liquid properties of a weakly interacting two-dimensional gas of single-component fermionic polar molecules with dipole moments $d$ oriented perpendicularly to the plane of their translational motion. This geometry allows the minimization of inelastic losses due to chemical reactions for reactive molecules and, at the same time, provides a possibility of a clear description of many-body (beyond mean-field) effects. The long-range character of the dipole-dipole repulsive interaction between the molecules, which scales as $1/r^3$ at large distances $r$, makes the problem drastically different from the well-known problem of the two-species Fermi gas with repulsive contact interspecies interaction. We solve the low-energy scattering problem and develop a many-body perturbation theory beyond the mean field. The theory relies on the presence of a small parameter $k_F{r}_{*}$, where $k_F$ is the Fermi momentum and $r_{*}=m{d}^2/{\hslash }^2$ is the dipole-dipole length, with $m$ being the molecule mass. We obtain thermodynamic quantities as a series of expansion up to the second order in $k_F{r}_{*}$ and argue that many-body corrections to the ground-state energy can be identified in experiments with ultracold molecules, as it has been recently done for ultracold fermionic atoms. Moreover, we show that only many-body effects provide the existence of zero sound and calculate the sound velocity.

Motivated by recent experiments, we investigate the system of isotropically interacting bosons with Rashba spin-orbit coupling. At the noninteracting level, there is a macroscopic ground-state degeneracy due to the many ways bosons can occupy the Rashba spectrum. Interactions treated at the mean-field level restrict the possible ground-state configurations, but there remains an accidental degeneracy not corresponding to any symmetry of the Hamiltonian, indicating the importance of fluctuations. By finding analytical expressions for the collective excitations in the long-wavelength limit and through numerical solution of the full Bogoliubov-de Gennes equations, we show that the system condenses into a single-momentum state of the Rashba spectrum via the mechanism of order by disorder. We show that in three dimensions the quantum depletion for this system is small, while the thermal depletion has an infrared logarithmic divergence, which is removed for finite-size systems. In two dimensions, on the other hand, thermal fluctuations destabilize the system.

We investigate the interplay of temperature and trap effects in particle systems at their quantum critical regime, such as cold bosonic atoms in optical lattices at the transitions between Mott-insulator and superfluid phases. The theoretical framework is provided by the one-dimensional Bose-Hubbard model in the presence of an external trapping potential, and the trap-size scaling theory describing the large trap-size behavior at a quantum critical point. We present numerical results for the low-temperature behavior of the particle density and the density-density correlation function at the Mott transitions, and within the gapless superfluid phase, in the hard-core limit.

We study the three-dimensional bosonic $t$-$J$ model, i.e., the $t$-$J$ model of “bosonic electrons,” at finite temperatures. This model describes the $s=\frac{1}{2}$ Heisenberg spin model with the anisotropic exchange coupling $J_{\perp }=\alpha J_z$ and doped bosonic holes, which is an effective system of the Bose-Hubbard model with strong repulsions. The bosonic “electron” operator $B_{r\sigma }$ at the site $r$ with a two-component (pseudo)spin $\sigma (=1,2)$ is treated as a hard-core boson operator. By means of Monte Carlo simulations, we study its finite-temperature phase structure including the $\alpha$ dependence, the possible phenomena-like appearance of checkerboard long-range order, supercounterflow, superfluidity, phase separation, etc. The obtained results, which clarify the relation between various phases, may be taken as predictions about experiments of two-component cold bosonic atoms in a cubic optical lattice.

We present a rigorous theory of rotating degenerate optical cavities and a coupled-resonator optical waveguide (CROW) including the impact of material losses associated with practical cavities and resonators. The losses are modeled as a perturbation of the material's relative permittivity by adding to it a small imaginary component. The Sagnac frequency shift in a single lossy cavity is shown to be lower than that of a lossless one. For CROWs, the rotation-induced gap formed in the center of the transmission function of a lossy device is reduced compared to that of a lossless one. The inclusion of propagation losses in the analysis of the CROW reveals a relatively insensitive region (a dead zone) in the response of a finite device at low rotation rates. A periodic modulation of the resonators’ resonant frequencies is shown to be an effective artificial CROW biasing technique to overcome this problem. This biasing does not require any active control.

We show the existence of stable two- and three-dimensional vortex solitons carrying multiple, spatially separated, single-charge topological dislocations nested in a common vortex-ring core. Such nonlinear states are supported by elliptical gain landscapes in focusing nonlinear media with two-photon absorption. The separation between the phase dislocations is dictated mostly by the geometry of the gain landscape, and it only slightly changes upon variation of the gain or absorption strength.

We use singular value decomposition to derive a tight lower bound for geometric discord of arbitrary bipartite states. In a single shot this also leads to an upper bound of measurement-induced nonlocality which in turn yields that for Werner and isotropic states the two measures coincide. We also emphasize that our lower bound is saturated for all $2\otimes n$ states. Using this we show that both the generalized Greenberger-Horne-Zeilinger and $W$ states of $N$ qubits satisfy monogamy of geometric discord. Indeed, the same holds for all $N$-qubit pure states which are equivalent to $W$ states under stochastic local operations and classical communication. We show by giving an example that not all pure states of four or higher qubits satisfy monogamy.

We provide a simple argument showing that in the limit of infinite acceleration, the entanglement in a fermionic-field bipartite system must be independent of the choice of Unruh modes. This implies that most tensor product structures used previously to compute field entanglement in relativistic quantum information cannot give rise to physical results.

Dakic, Vedral, and Brukner [B. Dakic, V. Vedral, and Č. Brukner, Phys. Rev. Lett. 105, 190502 (2010)] gave a geometric measure of quantum discord in a bipartite quantum state as the distance of the state from the closest classical quantum (or zero discord) state and derived an explicit formula for a two-qubit state. Further, Luo and Fu [S. Luo and S. Fu, Phys. Rev. A 82, 034302 (2010)] obtained a generic form of this geometric measure for a general bipartite state and established a lower bound. In this Brief Report we obtain a rigorous lower bound to the geometric measure of quantum discord in a general bipartite state which dominates that obtained by Luo and Fu.

We theoretically investigate the high-order harmonic generation and isolated attosecond pulse generation from a two-dimensional model of an Ar${}^{+}$ cluster in a synthesized two-color laser pulse. A scheme to generate an isolated attosecond pulse is proposed by using a 4-fs 800-nm fundamental field in combination with a wavelength-adjustable weaker pulse. When the number of ions surrounding the central atom is 24 and the wavelength-adjustable weaker pulse is chosen to be 1200 nm with the intensity $1\times {10}^{14}$ W/cm${}^2$, an almost linearly polarized isolated attosecond pulse with the duration of 45 as is obtained by superposing a bandwidth of 100 eV near the cutoff of high-order harmonic generation. Moreover, we illustrate the interference effect of the paths in terms of the time-frequency analysis of high-order harmonic generation.

We perform a time-delay analysis of the strong field ionization of atomic hydrogen in the tunneling regime. We obtain values for the time delay by solving the time-dependent Schrödinger equation, and use these values as parameters to define the corresponding classical trajectories. We demonstrate that almost all of these trajectories tend to cluster together to a few starting points. The identification of these starting points and their corresponding starting times allows one to answer the question as to when and where the atomic photoionization process actually begins.

We consider the effects of multistate transitions on the tunneling racemization of chiral molecules. This requires going beyond simple two-state models of enantiomers and to include transitions within a multiple-level quantum-mechanical system. We derive an effective two-level description which accounts for transitions from the enantiomers to an arbitrary number of excited states as an application of the Weisskopf-Wigner approximation scheme. Modifications to the optical activity from these additional states are considered in general terms under the assumption of CPT invariance and then under $T$ invariance. Some formal dynamical analogies between enantiomers and the neutral $K$-meson system are discussed.

We have performed experimental quantum state tomography of NOON states with up to four photons. The measured states are generated by mixing photons from a classical coherent state with down-converted photon pairs. We show that the fidelity between the produced states and the ideal NOON states is high. The fidelity is limited by the overlap of the two-photon down-converted state with any two photons originating from the coherent state, for which we introduce and measure a figure of merit. A second limitation on the fidelity set by the total setup transmission is discussed. We also apply the same tomography procedure for characterizing correlated photon hole states.

Non-Markovian evolutions are responsible for a wide variety of physically interesting effects. Here, we study nonlocality of the nonclassical state of a system consisting of a qubit and an oscillator exposed to the effects of non-Markovian evolutions. We find that the different facets of non-Markovianity affect nonlocality in different and nonobvious ways, ranging from pronounced insensitivity of the Bell function to quite spectacular evidence of information kickback.

The bound state and low-energy scattering properties of two oriented dipoles are investigated for both bosonic and fermionic symmetries. Interestingly, a universal scaling emerges for the expectation value of the angular momentum for deeply bound two-dipole states. This scaling traces to the pendulum motion of two dipoles in the strong dipole regime. The low-energy scattering phase shifts of two dipoles also show universal behavior. These universal observations make connections to the scaling laws reported by Wang et al. [Phys. Rev. Lett. 106, 233201 (2011); Phys. Rev. Lett. 107, 233201 (2011)] for three dipoles.

We put forward a dissipative preparation scheme for strongly correlated photon states. Our approach is based on a two-photon loss mechanism that is realized via a single four-level atom inside a bimodal optical cavity. Each elementary two-photon emission event removes one photon out of each of the two modes. The dark states of this loss mechanism are given by NOON states and arbitrary superpositions thereof. We find that the steady state of the two cavity modes exhibits entanglement and, for certain parameters, a mixture of two coherent entangled states is produced. We discuss how the quantum correlations in the cavity modes and the output fields can be measured.

A class of nondiffracting beams, termed here a high-order Bessel nonvortex beam of fractional type $\alpha$ (HOBNVBs-F$\alpha$) is discussed. The nonvortex beam is created by superposing two equivalent solutions of copropagating equiamplitude Bessel vortex beams of fractional type $\alpha$ having a positive and a negative order (or topological charge), respectively. Since the resulting beam is generally of nonvortex nature, it may not carry orbital angular momentum. Furthermore, a vector wave analysis is developed to determine the spatial Cartesian components of the electric and magnetic fields stemming from Maxwell's vector equations and the Lorenz gauge condition. The results are useful in the study of wave scattering and radiation force in lasers operating with HOBNVBs-F$\alpha$.