An optical microtrap is realized on a dielectric surface by crossing a tightly focused laser beam with a horizontal evanescent-wave atom mirror. The nondissipative trap is loaded with $\sim {10}^5$ cesium atoms through elastic collisions from a cold reservoir provided by a large-volume optical surface trap. With an observed 300-fold local increase of the atomic number density approaching ${10}^{14}{\mathrm{cm}}^{-3},$ unprecedented conditions of cold atoms close to a surface are realized.

We investigate the system of a particle moving on a half line $x>~0$ under the general walls at $x=0\mathrm{}$ that are permitted quantum mechanically. These quantum walls, characterized by a parameter L, are shown to be realized as a limit of regularized potentials. We then study the classical aspects of the quantum walls by seeking a classical counterpart that admits the same time delay in scattering with the quantum wall, and also by examining the WKB exactness of the transition kernel based on the regularized potentials. It is shown that no classical counterpart exists for walls with $L<\mathrm{}0,$ and that the WKB exactness can hold only for $L=0\mathrm{}$ and $L=\infty .$

The Casimir effect in a dispersive and absorbing multilayered system is considered adopting the (net) vacuum-field pressure point of view to the Casimir force. Using the properties of the macroscopic field operators appropriate for absorbing systems and a convenient compact form of the Green function for a multilayer, a straightforward and transparent derivation of the Casimir force in a lossless layer of an otherwise absorbing multilayer is presented. The resulting expression, in terms of the reflection coefficients of the surrounding stacks of layers, is of the same form as that obtained by Zhou and Spruch for a purely dispersive multilayer using the (surface) mode summation method [Phys. Rev. A 52, 297 (1995)]. Owing to the recursion relations that the generalized Fresnel coefficients satisfy, this result can be applied to more complex systems with planar symmetry. This is illustrated by calculating the Casimir force on a dielectric (metallic) slab in a planar cavity with realistic mirrors. Also, a relationship between the Casimir force and energy in two different layers is established.

The entanglement of clouds of $\mathrm{N}{=10\mathrm{}}^{11}$ atoms recently experimentally verified is expressed in terms of the fluctuation algebra introduced by [P. Goderis, A. Verbeure, and P. Vets, Commun. Phys. $128,$ 533 (1990)]. A mean-field Hamiltonian describing the coupling to a laser beam leads to different time evolutions if considered on microscopic or mesoscopic operators. Only the latter creates nontrivial correlations that finally after a measurement lead to entanglement between the clouds.

One of the main requirements in linear optics quantum computing is the ability to perform single-qubit operations that are controlled by classical information fed forward from the output of single-photon detectors. These operations correspond to predetermined combinations of phase corrections and bit flips that are applied to the postselected output modes of nondeterministic quantum logic devices. Corrections of this kind are required in order to obtain the correct logical output for certain detection events, and their use can increase the overall success probability of the devices. In this paper, we report on the experimental demonstration of the use of this type of feed-forward system to increase the probability of success of a simple nondeterministic quantum logic operation from approximately $\frac{1}{4}$ to $\frac{1}{2}.$ This logic operation involves the use of one target qubit and one ancilla qubit which, in this experiment, are derived from a parametric down-conversion photon pair. Classical information describing the detection of the ancilla photon is fed forward in real time and used to alter the quantum state of the output photon. A fiber-optic delay line is used to store the output photon until a polarization-dependent phase shift can be applied using a high-speed Pockels cell.

Recently it was realized that linear optics and photodetectors with feedback can be used for theoretically efficient quantum information processing. The first of three steps toward efficient linear optics quantum computation is to design a simple postselected gate that implements a nonlinear phase shift on one mode. Here a computational strategy is given for finding postselected gates for bosonic qubits with helper photons. A more efficient conditional sign flip gate is obtained. What is the maximum efficiency for such gates? This question is posed and it is shown that the probability of success cannot be 1.

The ${\mathrm{Ar}}^{9+}-\mathrm{He}$ charge-exchange collision system is studied at low kinetic energy $(E=2.25\mathrm{}\mathrm{k}\mathrm{e}\mathrm{V}/\mathrm{a}\mathrm{m}\mathrm{u}),$ using two complementary instrumental techniques: x-ray and vacuum ultraviolet photon spectroscopy and Auger-electron spectroscopy. It is shown that single capture populates mostly $n=4\mathrm{}$ and 5 in Ne-like argon ${1s}^2{2s}^2{2p}^5\mathrm{nl}.\mathrm{}$ The analysis and identification of the lines show the cascade population of ${1s}^2{2s2p}^{6}3l$ states. These are not expected to be seen since in the entrance channel the projectile has the core ${1s}^2{2s}^2{2p}^5{}^2{P}_J^o.$ They are identified by photon emission. Double capture populates mostly Na-like core excited states corresponding to the configurations ${1s}^2{2s}^2{2p}^5{3l5l}^{\prime }.$ They are identified and it is seen that the continuum ${2p}^6{}^1{S}_0$ to which they Auger decay is the only one available in this case.

Configuration-average distorted-wave calculations are carried out for electron-impact single ionization of all ionization stages of krypton. Contributions to the cross sections are included from both direct ionization and excitation autoionization. Good agreement with experimental crossed-beams measurements is found for many of the moderately charged ionization stages. Maxwellian-averaged rate coefficients are calculated for each ionization stage and then archived for future use in studies of ionization balance in astrophysical and laboratory plasmas.

A molecular three-continuum-type approximation is developed to study the $\mathrm{}(e,2e)\mathrm{}$ reaction for ${\mathrm{H}}_2$ targets. The molecular nature of the target is treated within the framework of a two-effective-center approximation. The correlate motion of the particles in the final channel of the reaction is taken into account by an adequate product of Coulomb functions. Triple differential cross sections are computed. A good agreement with the available experiments is obtained.

We develop a simple analytical model based on a variational method to explain the properties of trapped cylindrically symmetric Bose-Einstein condensates of varying degrees of anisotropy, well into the regimes of effective one dimension and effective two dimensions. Our results are accurate in regimes where the Thomas-Fermi approximation breaks down and they are shown to be in agreement with recent experimental data. We also present analytic expressions for various ground-state properties and quasiparticle excitations valid for highly anisotropic condensates.

We study superradiance from a fully inverted medium in the presence of two channels for atomic relaxation. Spontaneous emission from individual atoms is included and competes with the collective emission responsible for superradiance. We compare exact results within the quantum trajectory formalism with a previous approximation [J. P. Clemens and H. J. Carmichael, Phys. Rev. A 65, 023815 (2002)] which neglects the reexcitation of an atom after it spontaneously emits. The approximation underestimates the decoherence during the second half of an emitted pulse.

Spontaneous emission in a planar microcavity by two identical and spatially separated two-level atoms, both of them initially excited, is considered under fourth-order perturbation approximation in atom-field coupling constants. A delay-differential equation with proper retardation times for the probability of both atoms in the upper state is derived and expressions for the emitted-light intensity and spectrum are given. It is numerically shown that, while two-atom cooperation is enhanced for small interatomic distances, the two-atom vacuum Rabi oscillation is somewhat suppressed compared to the single-atom oscillation level when the interatomic separation is close to the cavity transverse coherence length.

The theories of noncommutative dynamical entropy and quantum symbolic dynamics for quantum-dynamical systems are analyzed from the point of view of quantum information theory. Using a general quantum-dynamical system as a communication channel, one can define different classical capacities depending on the character of resources applied for encoding and decoding procedures and on the type of information sources. It is shown that for Bernoulli sources, the entanglement-assisted classical capacity, which is the largest one, is bounded from above by the quantum-dynamical entropy defined in terms of operational partitions of unity. Stronger results are proved for the particular class of quantum-dynamical systems—quantum Bernoulli shifts. Different classical capacities are exactly computed and the entanglement-assisted one is equal to the dynamical entropy in this case.

This paper presents simulations of the state vector dynamics for a pair of atomic samples that are being probed by phase-shift measurements on an optical beam passing through both samples. We show how measurements, which are sensitive to different atomic components, serve to prepare states that are close to being maximally entangled.

The ratio of double- to single-electron photoionization cross section is calculated in the ultrarelativistic photon frequency region, namely where the photon energy is much larger than the electron rest mass. This ratio can be presented as a sum of the shake-off term and the quasifree term. They are expressed via essentially different integrals of the initial-state two-electron wave function. The shake-off term is calculated in the dipole approximation. The quasifree contribution is nondipole and is determined by the probability of finding both ionizing electrons at the same point in the initial state. The calculations are performed for the negative hydrogen ion, the helium atom, and the heliumlike ions in the ground and excited states using high-precision nonvariational wave functions. The nuclear charge dependence of this correction for heliumlike ions in the ground and the four lowest ${}^{1}S$ excited states is calculated. The possibility to detect the quasifree contribution experimentally is discussed.

We calculate the critical temperature of a trapped interacting Bose-Fermi gas mixture by means of a first-order perturbation method in a local-density approximation. An explicit analytical expression is derived for the dependence of the critical temperature on the numbers of bosons and fermions, the strengths of boson-boson and boson-fermion interactions, and the trapping frequencies.

As typically implemented, single-photon sources cannot be made to produce single photons with high probability, while simultaneously suppressing the probability of yielding two or more photons. Because of this, single-photon sources cannot really produce single photons on demand. We describe a multiplexed system that allows the probabilities of producing one and more photons to be adjusted independently, enabling a much better approximation of a source of single photons on demand.

We present a quantum algorithm that allows one to simulate chaos-assisted tunneling in deep semiclassical regime on existing quantum computers. This opens additional possibilities for investigation of macroscopic quantum tunneling and realization of semiclassical Schrödinger cat oscillations [E. Schrödinger, Naturwissenschaften 32, 807 (1935)]. Our numerical studies determine the decoherence rate induced by noisy gates for these oscillations and propose a suitable parameter regime for their experimental implementation.

The dynamics of Bose-Einstein condensates of a gas of bosonic particles with long-range dipole-dipole interactions in a harmonic trap is studied. Sufficient analytical criteria are found both for catastrophic collapse of Bose-Einstein condensates and for long-time condensate existence. Analytical criteria are compared with variational analysis.

The Bogoliubov transformation in thermofield dynamics, an operator formalism for the finite-temperature quantum field theory, is generalized to describe a field in arbitrary confined regions of space and time. Starting with the scalar field, the approach is extended to the electromagnetic field and the energy-momentum tensor is written via the Bogoliubov transformation. In this context, the Casimir effect is calculated for zero and nonzero temperature, and therefore it can be considered as a vacuum condensation effect of the electromagnetic field. This aspect opens an interesting perspective for using this procedure as an effective scheme for calculations in the studies of confined fields, including interacting fields.