We give a quantum benchmark for teleportation and quantum storage experiments suited for pure and mixed test states. The benchmark is based on the average fidelity over a family of phase-covariant states and certifies that an experiment cannot be emulated by a classical setup, i.e., by a measure-and-prepare scheme. We give an analytical solution for qubits, which shows important differences with standard state estimation approach, and compute the value of the benchmark for coherent and squeezed states, both pure and mixed.

The geometric (Berry) phase of a two-level system in a dissipative environment is analyzed by using the second-quantized formulation, which provides a unified and gauge-invariant treatment of adiabatic and nonadiabatic phases and is thus applicable to a quantitative analysis of transitional regions away from ideal adiabaticity. In view of the recent experimental observation of the Berry phase in a superconducting qubit, we illustrate our formulation for a concrete adiabatic case in the Ohmic dissipation. The correction to the total phase together with the geometry-dependent dephasing time is given in a transparent way. The behavior of the geometric phase away from ideal adiabaticity is also analyzed in some detail.

We propose schemes to perform the deterministic dense coding and faithful teleportation with multipartite graph states. We also find the sufficient and necessary condition of a viable graph state for the proposed schemes. That is, for the associated graph, the reduced adjacency matrix of the Tanner-type subgraph between senders and receivers should be invertible.

Rates of depopulation by blackbody radiation (BBR) and effective lifetimes of alkali-metal $nS$, $nP$, and $nD$ Rydberg states have been calculated in a wide range of principal quantum numbers $n\le 80$ at the ambient temperatures of 77, 300, and 600 K. Quasiclassical formulas were used to calculate the radial matrix elements of the dipole transitions from Rydberg states. Good agreement of our numerical results with the available theoretical and experimental data has been found. We have also obtained simple analytical formulas for estimates of effective lifetimes and BBR-induced depopulation rates, which well agree with the numerical data.

We theoretically investigate the high-order harmonic generation when the spatially extended systems are irradiated by a two-color laser field. By controlling electron ionization, the $\omega +2\omega$ excitation scheme creates efficient harmonics radiation beyond $I_p+3.17U_p$ with both odd and even harmonics. Calculations show that the harmonics beyond $I_p+3.17U_p$ are well phase locked, moreover, the chirp rate is only 3 as/eV. Using a 5 fs driving pulse, the bandwidth supporting isolated attosecond pulse in harmonic spectrum is 125 eV, and a 68 as single pulse can be obtained straightforwardly.

Recent experiments on imbalanced fermion gases have proved the existence of a sharp interface between a superfluid and a normal phase. We show that, at the lowest experimental temperatures, a temperature difference between normal (N) and superfluid (SF) phases can appear as a consequence of the blocking of energy transfer across the interface. Such blocking is a consequence of the existence of a SF gap, which causes low-energy normal particles to be reflected from the N-SF interface. Our quantitative analysis is based on the Hartree-Fock–Bogoliubov–de Gennes formalism, which allows us to give analytical expressions for the thermodynamic properties and characterize the possible interface scattering regimes, including the case of unequal masses. Our central result is that the thermal conductivity is exponentially small at the lowest experimental temperatures.

The atomic current in the Fourier-synthesized optical lattices under a constant external force is investigated theoretically. Based on a two-band model, the atomic current is derived by solving the Boltzmann equations. We find that the stationary atomic current changes with the probability of Landau-Zener tunneling, depending on the adjustable energy structure of the optical lattices. In contrast to the classical results of an electron in superlattices given by the Esaki-Tsu equations, the relation between the stationary atomic current and the strength of the external force in optical lattices is modified significantly. Both these characteristics can be taken as an effective way to observe the Landau-Zener tunneling in the optical lattices.

A model of two-species bosons moving on the sites of a lattice is studied at nonzero temperature, focusing on magnetic order and superfluid-insulator transitions. First, Landau theory is used to find the general structure of the phase diagram and in particular to demonstrate the presence of first-order transitions and hysteresis in the vicinity of a multicritical point. Second, an explicit thermodynamic phase diagram is calculated using an approach based on a field-theoretical description of the Bose-Hubbard model, which incorporates the crucial effects of particle-number fluctuations. The maximum transition temperature to a magnetically ordered Mott insulator is found to be limited by the presence of the superfluid phase.

We have found the asymmetric splitting of a high-order circularly polarized vortex-beam in a uniaxial crystal. The $l$-order vortex-beam splits into the same one and the beam with the $l-1$ vortices at the beam axis while one optical vortex is shifted along the direction perpendicular to the inclination plane of the beam. Such a vortex displacement causes the transverse shift of the partial beam. We consider this effect both in terms of the conservation law of the angular-momentum flux and on the base of the solutions to the paraxial wave equation. We revealed that the transverse shift of the crystal-traveling beam depends on neither a magnitude nor a sign of the vortex topological charge being defined only by a handedness of the initial circular polarization and a sign of the inclination angle of the beam.

Using a Green’s tensor method, we investigate the rate of resonance electronic energy transfer between two molecules near a dielectric cylinder. Both the case of a real and frequency-independent dielectric constant $ϵ$ and the case of a Drude-Lorentz model for $ϵ\left(\omega \right)$ are considered, the latter case including dispersion and absorption. If the donor is placed at a fixed position near the cylinder, we find that the energy transfer rate to the acceptor is enhanced compared to its vacuum value in a number of discrete hotspots, centered at the cylinder’s surface. In the absence of dispersion and absorption the rate of energy transfer may be enhanced at most a few times. On the other hand, for the Drude-Lorentz model the enhancement may be huge (up to ${10}^6$) and the hotspots are sharply localized at the surface. We show that these observations can be explained from the fact that in the resonance region of the Drude-Lorentz dielectric surface plasmons occur, which play the dominant role in transferring the electronic energy between the donor and the acceptor. The dependence of the energy transfer rate on the molecular transition frequency is investigated as well. For small intermolecular distances, the cylinder hardly affects the transfer rate, independent of frequency. For larger distances, the frequency dependence is quite strong, in particular in the stop-gap region. The role of the intermolecular distance in the frequency dependence may be explained qualitatively using Heisenberg’s uncertainty principle to calculate the spread in the frequencies of the intermediate photons.

We report on the observations of coherent random lasers in weakly scattering polymer films embedded with silver nanoparticles. The dominant oscillation cavity calculated from the ensemble-averaged power Fourier transform spectrum of single-shot emission spectra is shorter by more than two orders of magnitude than the scattering mean-free path, indicating that the oscillation cavity is loosely associated with multiple scattering. We suggest that the coherent feedback arises from the cooperative effect of light scattering and field enhancement in the vicinity of silver nanoparticles.

We investigate the existence and stability of gap vortices and multipole gap solitons in a kagome lattice with a defocusing nonlinearity both in a discrete case and in a continuum one with periodic external modulation. In particular, predictions are made based on expansion around a simple and analytically tractable anticontinuum (zero-coupling) limit. These predictions are then confirmed for a continuum model of an optically induced kagome lattice in a photorefractive crystal obtained by a continuous transformation of a honeycomb lattice.

The new symmetry axis of a well-behaved electromagnetic beam observed in a previous paper [Phys. Rev. A 78, 063831 (2008)] is not purely a mathematical concept. The experimental data reported by Hosten and Kwiat [Science 319, 787 (2008)] are shown to demonstrate the existence of such a symmetry axis that is neither perpendicular nor parallel to the propagation axis. Specifically, it is shown that the idea of this symmetry axis was implicitly used by Hosten and Kwiat in their calculations and exists in their results.

Identification of light modes usually requires careful considerations of the collecting geometry. This is particularly true for states carrying orbital angular-momentum modes from spontaneous parametric down-conversion due to the entanglement of the signal and idler fields. A detailed understanding of the generated modes in a general case allows the design of efficient detection setups. This is true for distinct cases, e.g., when multiple samplings are performed by repeated state generation or in single-photon cryptography or quantum computation, where a photon state is generated within a short-time window and a single measurement is performed. Aspects of nonorthogonality of signal and idler modes and effects of restrictions on the collecting geometry are discussed in this work.

The adiabatic approximation of time-dependent density-functional theory is studied in the context of nonlinear excitations of two-electron singlet systems. We compare the exact time evolution of these systems to the adiabatically exact one obtained from time-dependent Kohn-Sham calculations relying on the exact ground-state exchange-correlation potential. Thus, we can show under which conditions the adiabatic approximation breaks down and memory effects become important. The hydrodynamic formulation of quantum mechanics allows us to interpret these results and relate them to dissipative effects in the Kohn-Sham system. We show how the breakdown of the adiabatic approximation can be inferred from the rate of change of the ground-state noninteracting kinetic energy.

We study the influence of thermal Casimir-Polder forces on the near-surface trapping of cold polar molecules, with emphasis on LiH and YbF near a Au surface at room temperature. We show that even for a molecule initially prepared in its electronic and rovibrational ground state, the Casimir-Polder force oscillates with the molecule-wall separation. The nonresonant force and the evanescent part of the resonant force almost exactly cancel at high temperature which results in a saturation of the (attractive) force in this limit. This implies that the Casimir-Polder force on a fully thermalized molecule can differ dramatically from that obtained using a naive perturbative expansion of the Lifshitz formula based on the molecular ground-state polarizability. A dynamical calculation reveals how the spatial oscillations die out on a typical time scale of several seconds as thermalization of the molecule with its environment sets in.

The quantum optical response of high-density ultracold atomic systems is important to a wide range of fundamentally and technically important physical processes. We present here a microscopic analysis of the light scattering on such a system and compare it with a corresponding description based on macroscopic Maxwell theory. Results are discussed in the context of the spectral resonance structure of the scattering cross section, the time-dependent response under a range of conditions, and evolution of these quantities as the atomic density is varied. For high atomic densities, the microscopic theoretical treatment reveals a distributed and configuration-dependent narrow resonance structure. This structure is attributed to microcavity spatial structures associated with the dense and ultracold atomic gases.

The stability and collapse of fundamental unstaggered bright solitons in the discrete Schrödinger equation with the nonpolynomial on-site nonlinearity, which models a nearly one-dimensional Bose-Einstein condensate trapped in a deep optical lattice, are studied in the presence of the long-range dipole-dipole (DD) interactions. The cases of both attractive and repulsive contact and DD interaction are considered. The results are summarized in the form of stability-collapse diagrams in the parametric space of the model, which demonstrate that the attractive DD interactions stabilize the solitons and help to prevent the collapse. Mobility of the discrete solitons is briefly considered too.

We investigate a Bose-Einstein condensate (BEC) held in a quasi-one-dimensional Wannier-Stark lattice which is a combination of linear potential with an accelerating optical lattice. It is demonstrated that the system can be reduced to a periodically driven Gross-Pitaevskii one, in which we find the exact analytical solution and the regular and chaotic numerical solutions with accelerated atomic flow densities. The higher instabilities in the transport of BEC are predicted for the positive interatomic interaction. The results suggest an experimental scheme to generate and control the accelerated regular and chaotic matter waves.

We study the zero temperature mean-field phase diagram of the Bose-Hubbard model in the presence of local coupling between the bosons and an external bath. We consider a coupling that conserves the on-site occupation number, preserving the robustness of the Mott-insulator and superfluid phases. We show that the coupling to the bath renormalizes the chemical potential and the interaction between the bosons and reduces the size of the superfluid regions between the insulating lobes. For strong enough coupling, a finite value of hopping is required to obtain superfluidity around the degeneracy points where Mott-insulator phases with different occupation numbers coexist.