Configuration interaction (CI) calculations in atoms with two-valence electrons, carried out in the $V^{\left(N-2\right)}$ Hartree-Fock potential of the core, are corrected for core-valence interactions using many-body perturbation theory (MBPT). Two variants of the mixed CI+MBPT theory are described and applied to obtain energy levels and transition amplitudes for Be, Mg, Ca, and Sr.

The stability and ion loss in a linear electrostatic ion trap are investigated experimentally and by numerical simulations. The decay of a stored 4.2-keV ${\mathrm{Ar}}^{+}$ beam was monitored for various potential configurations of the trap. Both classical trajectory calculations and optical models were used to numerically study the stability and dynamics of the trapped ions. Two different modes of trapping were identified with very different dynamical properties. It is found that the ion-loss processes are controlled by collisions both with residual gas species and among the stored ions.

The motion of an ion bunch trapped between two electrostatic mirrors in an ion-trap resonator is analyzed both experimentally and via numerical simulations. It is found that under certain conditions it is possible to achieve either synchronization of the ion motion, which causes the size of the bunch to remain constant, or enhanced diffusion, which leads to rapid debunching. These effects are analyzed and criteria for synchronization are proposed.

A two-parameter semiempirical theory of positron scattering and annihilation is developed and used to investigate the behavior of positrons interacting with the rare gases and metal vapors. The two-parameter theory is able to do a reasonable job of reproducing existing cross section and annihilation data for the rare gases. A model-potential calculation that correctly predicts the behavior of the phase shifts will also predict the energy dependence of $Z_{\mathrm{eff}}\mathrm{}\left(k\right)\mathrm{}$ even if the magnitude is incorrect. Analysis of the $Z_{\mathrm{eff}}$ versus temperature data of Kurz et al. [Phys. Rev. Lett. 77, 2929 (1996)] suggests scattering lengths of $-5.6\pm {1.0a}_0,$ $-10.3\pm {2.0a}_0,$ and $-56\pm {15a}_0$ for Ar, Kr, and Xe, respectively. Existing bound-state calculations can be used to fix the values of the semi-empirical parameters for a number of metal vapors, resulting in predicted $Z_{\mathrm{eff}}$ of 119, 36, and 94 for Be, Mg, and Cu at threshold. In addition to the calculations, expressions relating the threshold form of $Z_{\mathrm{eff}}\mathrm{}\left(k\right)\mathrm{}$ to the complex scattering length are presented.

We explore the feasibility of extending the quantum-fluid dynamics (QFD) approach for quantitative investigation of nonlinear optical processes of many-electron quantum systems in intense laser fields. Through the amalgamation of the QFD and density-functional theory (DFT), a single time-dependent hydrodynamical equation of motion can be derived. This equation has the form of a generalized nonlinear Schrödinger equation (GNLSE) but includes the many-body effects through a local time-dependent exchange-correlation potential. The time-dependent generalized pseudospectral method is extended to the solution of the GNLSE in spherical coordinates, allowing nonuniform spatial discretization and efficient, accurate solution of the hydrodynamical density and wave function in space and time. The procedure is applied to the study of multiphoton ionization (MPI) and high-order harmonic generation (HHG) of He and Ne atoms in intense laser fields. Excellent agreement with other recent self-interaction-free time-dependent DFT calculations is obtained for He, while for Ne, good agreement is achieved. Four different exchange-correlation energy functionals are used in the study with an aim to explore the roles of exchange and correlation on MPI/HHG processes in details. The method offers a conceptually appealing and computationally practical approach for nonperturbative treatment of strong-field processes of many-electron systems beyond the time-dependent Hartree-Fock level.

Vortex configurations in rotating Bose-Einstein condensed gases trapped in power-law and anharmonic potentials are studied. When the confining potential is steeper than harmonic in the plane perpendicular to the axis of rotation, vortices with quantum numbers larger than one are energetically favorable if the interaction is weak enough. Features of the wave function for small and intermediate rotation frequencies are investigated numerically.

We present a complete analytical solution for a single four-level atom strongly coupled to a cavity field mode and driven by external coherent laser fields. The four-level atomic system consists of a three-level subsystem in an electromagnetically induced transparency configuration, plus an additional atomic level; this system has been predicted to exhibit a photon blockade effect. The solution is presented in terms of polaritons. An effective Hamiltonian obtained by this procedure is analyzed from the viewpoint of an effective two-level system, and the dynamic Stark splitting of dressed states is discussed. The fluorescence spectrum of light exiting the cavity mode is analyzed and relevant transitions identified.

The Bersons-Kulsh analytical form factor of the short-pulse approximation along with its transformation from the parabolic to spherical bases are used as a tool alternative to numerical solution of the time-dependent Schrödinger equation to explain the recently measured [Wetzels et al., Eur. Phys. J. D 14, 157 (2001)] dynamics of the Rydberg wave packets generated from the initial $n=40,$ 41 rubidium states by the impact of half-cycle electrical pulses of a frequency spectrum from the terahertz regime. When a comparison was possible, both theoretical approaches were found to give the same results and offer the same level of explanation of the measured dynamics but the analytical approach had the advantage of being simpler in use. In addition, this analytical approach was applied to an experimental situation that was not given a recent theoretical attention and results of qualitative agreement with experimental findings were obtained.

The Fock space of a system of indistinguishable particles is isomorphic (in a nonunique way) to the state space of a composite, i.e., many modes, quantum system. One can then discuss quantum entanglement for fermionic as well as bosonic systems. We exemplify the use of this notion—central in quantum information—by studying some, e.g., Hubbard, lattice fermionic models relevant to condensed matter physics.

Here we present a quantum electrodynamics model involving a large-detuned single-mode cavity field and n identical two-level atoms. One of its applications for the generation of multiparticle entangled states of various kinds (Greenberger-Horne-Zeilinger states and different class of so-called W states) is analyzed. The theoretical prediction for the model of $n=2\mathrm{}$ is made that is consistent with the experimental result by considering the possible three-atom collisions.

We use a variation of the Bethe-Salpeter equation to complete the calculation of the one-photon annihilation contribution to the hyperfine interval of positronium at order $m{\alpha }^6.$ Our results are in accord with a quite different calculation independently done using an effective field theory approach. This completes the evaluation of all the $m{\alpha }^6$ terms. We give the total theoretical value for this interval and compare with experiment.

An atomic polarizing white-color interferometer composed of the wave packets in the $m=+1\mathrm{}$ and $m=-1$ states of the excited ${}^3{P}_1$ state of Ca, has been developed using a thermal atomic beam excited by a pair of two resonant lights at two separate zones under a homogeneous magnetic field. The interferometer generated the Ramsey fringes as a detuning of the rf frequency between two resonant lights from the Zeeman-frequency shift, while it generates white-color interference fringes as a detuning of laser frequency. The interferometer was used for the direct and short-time measurement of the Aharonov-Casher phase by removing the main part of dc Stark phase shift and without the influence of the frequency fluctuation of a laser. The measured dependence of the Aharonov-Casher phase on the electric field agreed with the predicted one within a relative uncertainty of 2.9%. The tensor polarizability in the ${}^3{P}_1$ state was determined to be ${\alpha }_2=2.623\mathrm{}\pm 0.015\hspace{0.5em}\mathrm{k}\mathrm{H}\mathrm{z}/(\mathrm{k}\mathrm{V}/\mathrm{c}\mathrm{m}{)}^2$ from the frequency shift between the $m=+1\mathrm{}$ and $m=-1$ states.

We describe a protocol for continuously protecting unknown quantum states from decoherence that incorporates design principles from both quantum error correction and quantum feedback control. Our protocol uses continuous measurements and Hamiltonian operations, which are weaker control tools than are typically assumed for quantum error correction. We develop a cost function appropriate for unknown quantum states and use it to optimize our state-estimate feedback. Using Monte Carlo simulations, we study our protocol for the three-qubit bit-flip code in detail and demonstrate that it can improve the fidelity of quantum states beyond what is achievable using quantum error correction when the time between quantum error-correction cycles is limited.

We present a local-hidden-variable model for positive-operator-valued measurements (an LHVPOV model) on a class of entangled generalized Werner states, thus demonstrating that such measurements do not always violate a Bell-type inequality. We also show that, in general, if the state ${\rho }^{\prime }$ can be obtained from $\rho$ with certainty by local quantum operations without classical communication, then an LHVPOV model for the state $\rho$ implies the existence of such a model for ${\rho }^{\prime }.$

We consider the effects of certain forms of decoherence applied to both adiabatic and nonadiabatic geometric phase quantum gates. For a single qubit we illustrate path-dependent sensitivity to anisotropic noise and for two qubits we quantify the loss of entanglement as a function of decoherence.

The doubly excited ${}^1{\Sigma }_g^{+}$ and ${}^3{\Sigma }_u^{+}$ states of the ${\mathrm{H}}_2$ molecule converging to the $\mathrm{H}\mathrm{}(n=2\mathrm{})+\mathrm{H}{(n}^{\prime }=2)$ limit have been calculated using explicitly correlated basis functions, giving accurate results out to a nuclear separation sufficiently large to apply asymptotic expansions for the potential energy. The autoionization widths have been obtained using the complex-scaling method. For nuclear separations larger than 22 a.u., asymptotic expressions for the interaction energies have been obtained from perturbation theory.

The influence of the instrumental function on the Cu $K{\alpha }_1$ emission line was investigated for the case of a double-crystal spectrometer. The magnitude of broadening for both Si(220) and Si(440) was calculated for a Lorentzian emission line with the width of 1–5 eV; the broadening for Si(220) is 0.12–0.18 eV while that for Si(440) is only 0.015–0.043 eV. The former is too large to be neglected, so the correction for the instrumental function is important. The spectrum affected by the instrumental function seems to keep the shape of Lorentzian though its width is larger. The fact indicates that the Lorentzian fitting analysis is effective if the appropriate correction for width is done.

We report absolute optical emission cross sections for 31 vibrational transitions of the $A{}^2{\Sigma }^{+}{(v}^{\prime })–X{}^2\Pi {(v}^{″})$ system of nitric oxide for electron-impact energies from threshold to 740 eV using the optical method and a crossed-beam geometry. Our measurements include emission bands originating in the $v^{\prime }=0,$ 1, 2, 3 vibrational levels and terminating in the $v^{″}$ levels from 0 up to 11. We find good agreement between our emission cross sections, our measurements of the emission intensities from an rf discharge, and the intensities predicted by the Franck-Condon principle. We also obtain apparent level cross sections for the $v^{\prime }=0,$ 1, 2, 3 vibrational levels of the $A{}^2{\Sigma }^{+}$ state; despite the presence of a cascade, these level cross sections are proportional to the direct excitation rates predicted by the Franck-Condon principle. We determine the total apparent cross section for the $A{}^2{\Sigma }^{+}$ electronic state to reach its peak value of $40\times {10}^{-19}{\mathrm{cm}}^2$ at 30 eV.

The electron spectrum of Ar has been studied in the binding energy region of the ${2p}^{-1}{3p}^{-1}\mathrm{np}$ states. These states are populated in direct $2p$ photoionization accompanied by the $3\overrightarrow{p}\mathrm{np}$ shake-up transitions. In addition, at photon energies corresponding to the $2\overrightarrow{s}4p,5p$ excitations they gain intensity through the ${2s}^{-1}n\overrightarrow{p}{2p}^{-1}{3p}^{-1}\mathrm{np}$ resonant Auger decay channels. The two channels can interfere, leading to changes in the intensities of the Ar $2p$ photoelectron satellites. Experimental results are compared with the results of one-step calculations.

The properties of Green’s functions and various correlation functions of density and spin operators are considered in a homogeneous spin-1 Bose gas in different phases. The dielectric formalism is worked out and the partial coincidence of the one particle and collective spectra is pointed out below the temperature of Bose-Einstein condensation. As an application, the formalism is used to give two approximations for the propagators and the correlation functions and the spectra of excitations including shifts and widths due to the thermal cloud.