We study the waveguiding induced in a weak probe propagating in a medium of two-level atoms driven by a copropagating strong pump. We show that when the transverse profile of the pump exhibits solitonlike oscillations, similar oscillations can be induced in the transverse profile of the probe. When the probe waist size is initially narrower than that of the pump, the probe adjusts its width after a short propagation distance, to that of the pump. We show that the effect of the weak beam, which is created during propagation by four-wave mixing, on the waveguiding of the probe, becomes important when the medium is optically thick. This suggests that the importance of four-wave mixing in modifying induced waveguiding should be considered in other waveguiding schemes.

We compute the influence on the spontaneous-emission rate for a two-level atom when it is located between two parallel plates of a different nature, namely a perfectly conducting plate $\left(\overrightarrow{\epsilon }\infty \right)$ and an infinitely permeable one $\left(\overrightarrow{\mu }\infty \right).\mathrm{}$ We also discuss the case of two infinitely permeable plates. We compare our results with those found in the literature for the case of two perfectly conducting plates.

Isolated electrons resting above a helium surface are predicted to have a bound spectrum corresponding to a one-dimensional hydrogen atom. But in fact, the observed spectrum is closer to that of a quantum-defect atom. Such a model is discussed and solved in analytic closed form.

The magnetic methods most successful so far for tight, stable confinement of cold neutral atoms in the dark explore three-dimensional-nonuniform magnetic configurations. A different method, using revolving planar-nonuniform fields, is developed in the present work. The paper gives an idea of the method, describes its basic features, and shows its superiority, such as (i) a greater product of curvatures of the trap well in the three spatial directions by a factor of two orders of magnitude given the same magnitude of field gradient, (ii) tight, stable, and more isotropic confinement in both the trap center and an easy-to-change orbit, (iii) vast possibilities to change the form of the trap well, and (iv) the possibility to separately detect the effective spin and translational motion temperatures of the cloud via observations of its density distribution.

We manifest the origin of the wrong conclusion made by several groups of authors on the absence of Coulomb corrections to the cross section of the $e^{+}{e}^{-}$ pair production in ultrarelativistic heavy-ion collisions. The source of the mistake is connected with an incorrect passage to the limit in the expression for the cross section. When this error is eliminated, the Coulomb corrections do not vanish, and agree with the results obtained within the Weizsäcker-Williams approximation.

We propose a scheme and a protocol for quantum teleportation of a single-mode optical field state, based on the entanglement produced by the quantum nondemolition (QND) interaction. We show that the recently attained results in the QND technique allow one to perform the teleportation in the quantum regime. We also show that an application of QND coupling to squeezed fields will significantly improve the quality of teleportation for a given degree of squeezing.

We develop theoretical and numerical tools for the quantification of entanglement in systems with continuous degrees of freedom. Continuous variable entanglement swapping is introduced and based on this idea we develop methods of entanglement purification for continuous variable systems. The success of these entanglement purification methods is then assessed using these tools.

Doubly excited intrashell resonance states of positronium negative ion, ${\mathrm{Ps}}^{-}$ associated with the $N=4\mathrm{}$ and $N=5\mathrm{}$ thresholds of Ps atom are calculated using the method of complex-coordinate rotation. Products of Slater-type orbitals are used to represent the two-electron wave functions. All possible intrashell states with angular momenta up to $L=6\mathrm{}$ and $L=8\mathrm{}$ for $N=4\mathrm{}$ and $N=5\mathrm{}$ thresholds, respectively, have been identified. Supermultiplet structures showing the rotational and vibrational character of the doubly excited resonance states of ${\mathrm{Ps}}^{-}$ are constructed.

The concept of implicit exchange-correlation energy functionals [optimized potential method (OPM)] is applied to the helium and neon dimers. Using the lowest order correlation functional obtained within Kohn-Sham perturbation theory we find energy surfaces in reasonable quantitative agreement with the exact results, thus demonstrating that the OPM allows a seamless description of van der Waals bond systems.

We have calculated the static properties of the ground state of the ${\mathrm{HD}}^{+}$ ion and its lowest-lying P state without making use of the Born-Oppenheimer approximation, as was done in the case of $H_2^{+}$ and $D_2^{+}$ [A. K. Bhatia, Phys. Rev. A 58, 2787 (1998)]. The ion is treated as a three-body system whose ground state is spherically symmetric. The wave function is of generalized Hylleraas type, but it is necessary to include high powers of the internuclear distance to localize the nuclear motion. We obtain good values of the energies of the ground S state and lowest P state and compare them with earlier calculations. Expectation values are obtained for various operators, the Fermi contact parameters, and the permanent quadrupole moment. The cusp conditions are also calculated. The polarizability is then calculated using second-order perturbation theory with intermediate P pseudostates. Since the nuclei in ${\mathrm{HD}}^{+}$ are not of equal mass there is dipole coupling between the lowest two rotational states, which are almost degenerate. This situation is carefully analyzed, and the Stark shift is calculated variationally as a function of the applied electric field.

The process of electron-positron pair production by linearly polarized photons may be used as a polarimeter to perform mobile measurement of linear photon polarization. In the limit of high photon energies ω the distributions of the recoil-electron momentum and azimuthal angle do not depend on the photon energy in the laboratory frame. We calculate the power corrections of order $m/\omega$ to the above distributions and estimate the deviation from the asymptotic result for various values of ω.

Partial and total photoionization cross sections of ${\mathrm{N}}_2$ molecule are calculated using the generalization of the random-phase approximation (RPA) which earlier has been successfully applied to the description of the atomic photoionization processes. According to this method, at first the Hartree-Fock (HF) ground-state wave functions are calculated in prolate spheroidal coordinates using the fixed-nuclei approximation. With their help the zero order basis set of single particle Hartree-Fock wave functions containing both discrete excited states and continuous spectrum is calculated in the field of a frozen core of a singly charged ion. The calculations are performed for all four valence shells of ${\mathrm{N}}_2$ molecule, $3{\sigma }_g,$ $1{\pi }_u,$ $2{\sigma }_u,$ and $2{\sigma }_g,$ with the intershell correlations fully taken into account within the RPA method. It is demonstrated that different intershell correlations, especially between three outer shells, play an important role in photoionization process. Examples of the influence of intershell correlations on several transitions are presented. Partial and total photoionization cross sections of ${\mathrm{N}}_2$ molecule obtained by this method in the photon energy range from ionization threshold up to 70 eV are in a good agreement with the existing experimental data and with the recent RPA calculations [Cacelli et al., Phys. Rev. A 57, 1895 (1998)].

The bow-tie model is a particular generalization of the famous two-state Landau-Zener model to an arbitrary number of states. Recently, solution of the bow-tie model was found by the contour integral method [V. N. Ostrovsky and H. Nakamura, J. Phys. A 30, 6939 (1997)]. However, its physical interpretation remained unclear, because the model implies simultaneous strong interaction of all the states apparently irreducible to any simpler pattern. We introduce here a generalized bow-tie model that contains an additional parameter $\varepsilon$ and an additional state. It includes the conventional bow-tie model as a particular limiting case when $\overrightarrow{\varepsilon }0.$ In the generalized model all the state-to-state transitions are reduced to the sequence of pairwise transitions and the two-state Landau-Zener model is applied to each of them. Such a reduction is well justified at least in the opposite limit of large parameter $\varepsilon .$ Several paths connect initial and final states; the contributions of different paths are summed up coherently to obtain the overall transition probability. The special $\mathrm{ET}$ (energy and time) symmetry intrinsic for the generalized bow-tie model results in a particular property of interference phases: only purely constructive or purely destructive interference is operative. The complete set of transition probabilities is obtained in a closed form. Importantly, the results do not depend on the parameter $\varepsilon .$ In the limit of conventional bow-tie model all previously derived results are reproduced. This amounts to rationalization of the bow-tie model by its interpretation in terms of multipath successive two-state transitions.

Absolute cross sections have been measured for single ionization of ${\mathrm{Mn}}^{q+}$ $(5\mathrm{}<~q<~8)$ by electron impact at energies ranging from the ionization thresholds to 4 keV using a new dynamic-crossed-beams apparatus. The experimental cross sections are essentially featureless, but show evidence for significant contributions due to $3\overrightarrow{p}\mathrm{nl}$ $(n>~4)$ inner-shell excitation followed by autoionization for ${\mathrm{Mn}}^{5+}$ and ${\mathrm{Mn}}^{6+},$ and $3\overrightarrow{s}\mathrm{nl}$ $(n>~5)$ excitation-autoionization for ${\mathrm{Mn}}^{7+}$ and ${\mathrm{Mn}}^{8+}.$ The measurements for ${\mathrm{Mn}}^{6+}$ and ${\mathrm{Mn}}^{8+}$ and their interpretation are complicated by the presence of an undetermined fraction of the primary ion beams in metastable states.

The motion of atoms inside a long hollow cylindrical waveguide with a circular cross section is investigated. The guide is assumed to have subwavelength dimensions, in which case the spontaneous decay process is effected only by emission of a few possible cavity modes. The characteristics of the atomic motion in the guide are explored in the presence of an excited waveguide mode. We show that the atomic motion in this case is determined by an axial channelling force and a trapping dipole force, plus a quantized light torque associated with the orbital angular momentum property of excited waveguide modes of order $\mathcal{l}>0.\mathrm{}$ It is predicted that in addition to its axial motion, an atom subject to such a mode should be trapped radially in a vibrational state and should exhibit interesting rotational features due to the light torque, including a rotational frequency shift.

Influence of rescattering process on the two-color laser ionization is investigated by using an improved two-step quasistatic model, in which the Coulomb focusing effect is considered. We focus on phase-dependent rescattering process and “phase-control.” It is found that the rescattering leads to further forward/backward asymmetry, breaks the symmetry of rate-phase relation about $\phi =\pm \pi /2,$ and accounts for the departure of the prediction of a simple two-step model from the experimental data [Phys. Rev. Lett. $73,$ 1344 (1994); Phys. Rev. A54, 4271 (1996)]. Our results are in good agreement with the experimental observations.

A quadrupolar static magnetic field used as a phase object in a Stern-Gerlach atom interferometer produces phase shifts proportional to the distance to the zero-field line. As a consequence the transverse intensity profile of the atomic beam beyond the interferometer is modulated by an interference pattern which is a “phase portrait” of the field configuration. This pattern—a central bright spot surrounded by annular fringes—can be translated as a whole in any transverse direction by adding a homogeneous field to the quadrupolar field. These effects have been investigated experimentally with a beam of metastable hydrogen atoms ${\mathrm{H}}^{*}\mathrm{}\left(2s\right),$ either by measuring the atom flux through a fixed slit or by scanning the beam profile through a movable hole. The results are in good agreement with the theoretical predictions.

The influence of atomic interactions on time-dependent tunneling processes of Bose-Einstein condensates is investigated. In a variety of contexts the relevant condensate dynamics can be described by a Landau-Zener equation modified by the appearance of nonlinear contributions. Based on this equation it is discussed how the interactions modify the tunneling probability. In particular, it is shown that for certain parameter values, due to a nonlinear hysteresis effect, complete adiabatic population transfer is impossible however slowly the resonance is crossed. The results also indicate that the interactions can cause significant increase as well as decrease of tunneling probabilities that should be observable in currently feasible experiments.

Comoving potentials or refractive indices acting on the propagation of waves of various nature are shown to exhibit a resonant effect giving rise to a general property we call genericity in the sense that they provide a phase shift with any desired dependence on the momentum. Application in atom optics to balance the natural spreading of matter waves in a vacuum is given. Optical analogies are also examined in view of applications in coherent control of short pulses.

We theoretically study the optical properties of a Fermi-Dirac gas in the presence of a superfluid state. We calculate the leading quantum-statistical corrections to the standard column density result of the electric susceptibility. We also consider the Bragg diffraction of atoms by means of light-stimulated transitions of photons between two intersecting laser beams. Bardeen-Cooper-Schrieffer pairing between atoms in different internal levels magnifies incoherent scattering processes. The absorption linewidth of a Fermi-Dirac gas is broadened and shifted. Bardeen-Cooper-Schrieffer pairing introduces a collisional local-field shift that may dramatically dominate the Lorentz-Lorenz shift. For the case of the Bragg spectroscopy the static structure function may be significantly increased due to superfluidity in the near-forward scattering.