This work concerns radiation trapping of the 123-nm krypton and 147-nm xenon lines, in the presence of a lighter rare gas, as well as the study of the decay processes of the relevant resonant states due to inelastic collisions. It deals with Kr-Ar, Xe-Kr, and Xe-Ar mixtures. Pulsed, brief selective excitation of the $\mathrm{Xe}{(}^3{P}_1)$ or $\mathrm{Kr}{(}^3{P}_1)$ resonant states by three-photon absorption was achieved by means of a tunable dye laser. Spectral and temporal analysis was then performed. The time resolved luminescence, filtered in the vacuum ultraviolet region, obeys a decay law described by the sum of two exponential terms which are attributed to the deexcitation of the ${}^3{P}_1$ and ${}^3{P}_2$ states. The decay constants are estimated by the maximum likelihood method applied to a Poisson statistical law. In order to correctly determine the collision rate constants, it is important to account for variations of the apparent lifetime according to the gas concentration. Analysis of both systematic experimental errors and statistical errors leads to a good estimation of the accuracy of the results obtained. For each mixture, the variation of the time constants as a function of partial pressures allows a determination of the resonance broadening coefficient and van der Waals broadening coefficient of the transition studied, as well as the two- and three-body inelastic collision rates relative to the ${}^3{P}_1$ and ${}^3{P}_2$ states. There exists no energy transfer between the two gases. For binary rare-gas mixtures, where only the heavier gas is excited, homonuclear and heteronuclear three-body reactions account for the decay of the ${}^3{P}_1$ state. Nevertheless, for the ${}^3{P}_2$ state we observe both two- and three-body collisions. In order to simulate resonance radiation trapping, a numerical method based upon Monte Carlo techniques was used. Calculations were first performed and validated for an infinite cylinder. The difference between calculation and experiment was less than 1%. Then the program was adapted to our real experimental conditions and applied to the binary mixtures studied. A good agreement was found between experiments and calculations. Furthermore, our program allows us to obtain information not easily obtained experimentally.

We have applied close-coupling calculations to calculate the single-ionization cross section of helium by antiproton impact at collision energies above 10 keV. The atomic states of helium are expanded by three sets of eigenstate(s) of ${\mathrm{He}}^{+}:$ $\{1s\},$ $\{1s,2s,2p\},$ and $\{1s,2s,2p,3s,3p,3d\}.$ The calculations are in good convergence within these models. The present cross sections are in good agreement with the experimental data of Andersen et al. [Phys. Rev. A 40, 7366 (1989); 41, 6359 (1990)] and Hvelplund et al. [J. Phys. B 27, 925 (1994)] at energies above 40 keV. However, our treatment gives larger cross sections than the data of Hvelplund et al. for lower energies and the disagreement is no less than a factor of 2 near 10 keV.

Single-photon ionization of atomic singlet oxygen $\left[\mathrm{O}{(}^{1}D\right)]$ is investigated between 12 and 18 eV. $\mathrm{O}{(}^{1}D)$ is efficiently prepared by laser photolysis of ozone in the Hartley band regime $(\lambda =270\mathrm{}$ nm). It is subsequently ionized by using monochromatic radiation, which is generated in a laser-produced plasma. Time-of-flight mass spectra are recorded and the photoion yield of ${\mathrm{O}}^{+}$ is measured. Direct photoionization of $\mathrm{O}{(}^{1}D)$ is not observed between the onset of the first ${(}^4{S}^o)$ and the second ${(}^2{D}^o)$ ionization threshold. Intense resonant features occur below the ${}^2{D}^o$ threshold. These are assigned to LS-forbidden autoionization of Rydberg singlet states by the ${}^4{S}^o$ continuum. The relative intensities of autoionization resonances are compared to the corresponding optical oscillator strengths. The results are discussed in terms of the autoionization efficiency of Rydberg states.

Theoretical results are presented for double photoionization and ionization with excitation of the metastable $2{}^{1,3}S$ states of helium. We employ the momentum space formulation of the close-coupling theory to describe nonperturbatively the electron-electron interaction in the final state. A large Laguerre basis is used to obtain convergent close-coupling (CCC) results. In addition, we employ a B-spline basis for the target states. The presented results cover continuously the photon energy range from the double-photoionization threshold to the asymptotic limit of infinite photon energy. Near the threshold our data generally support the calculations of van der Hart et al. [Phys. Rev. A 57, 3641 (1998)]. At large photon energies our results merge continuously with the asymptotic values derived from the metastable state $2{}^{1,3}S$ wave functions. Proportionality between the double-to-single photoionization cross-sections ratio and the cross section of electron-impact ionization of the ion is tested to examine the relative contributions of different mechanisms of the two-electron photoionization in the ground and metastable states of the helium atom.

A theory of pump-probe spectroscopy is developed in which optical fields drive two-quantum, Raman-like transitions between ground-state sublevels. Three fields are incident on an ensemble of atoms. Two of the fields act as the pump field for the two-quantum transitions. The absorption or gain of an additional probe field is monitored as a function of its detuning from one of the fields which constitutes the pump field. Although the probe absorption spectrum displays features common to those found in pump-probe spectroscopy of single-quantum transitions, new interference effects are found to modify the spectrum. Many of these features can be explained within the context of a dressed-atom picture.

We propose a method for achieving highly efficient transfer between the vibrational states in a diatomic molecule. The process is mediated by strong laser pulses and can be understood in terms of light-induced potentials and their vibrational eigenstates. In addition to describing a specific molecular system, our results show how, in general, one can manipulate the populations of the different quantum states in double-well systems.

Dark soliton states of Bose-Einstein condensates in harmonic traps are studied both analytically and computationally by the direct solution of the Gross-Pitaevskii equation in three dimensions. The ground and self-consistent excited states are found numerically by relaxation in imaginary time. The energy of a stationary soliton in a harmonic trap is shown to be independent of density and geometry for large numbers of atoms. Large-amplitude field modulation at a frequency resonant with the energy of a dark soliton is found to give rise to a state with multiple vortices. The Bogoliubov excitation spectrum of the soliton state contains complex frequencies, which disappear for sufficiently small numbers of atoms or large transverse confinement. The relationship between these complex modes and the snake instability is investigated numerically by propagation in real time.

We study the relativistic dynamics of a single electron in a plane-wave laser field and a static magnetic field oriented along the laser propagation direction. A set of exact solutions, that demonstrate acceleration to TeV energies in vacuum by the mechanism of resonance between the electron cyclotron frequency and the Doppler-shifted laser frequency, is presented. Forward velocity focusing is demonstrated, the radiation losses are shown to be negligible, and the frequency distribution of the weak radiation scattered by the electron undergoing the acceleration is discussed.

It is a well-known and remarkable fact that in certain coincidence photon-counting experiments with cw-pumped parametric down-converters, the effects of group-velocity dispersion arising from media interposed between source and detectors are completely canceled, even if the media physically affect only one of the photons of the pair. Recently Perina et al. [Phys. Rev. A 59, 2359 (1999)] showed that this phenomenon does not occur when certain classical timing information is available about the arrival of individual photons at the detectors, as is the case when the photon pairs are produced via spontaneous parametric down-conversion using an ultrashort pump pulse. In this paper we show that the nonlocal cancellation of dispersion for such a source of entangled photons can be restored in principle by proper engineering of the source properties. In particular, we describe techniques for recovering interference in coincidence-counting experiments by suppressing distinguishing information without the post selection of photons. Moreover, a precise classical timing signal coincident with the photon pair is still available.

Relativistic configuration-interaction (RCI) calculations of the correlation energies of the $2s-{2p}_{3/2}$ and the ${2s}^2{}^1{S}_0-2s2p{}^1{P}_1$ transitions in high-Z Li-like and Be-like ions, respectively, are reexamined. Experimental quantum electrodynamic (QED) corrections are obtained by subtracting RCI correlation energies from high-precision x-ray energy measurements. It is found that our QED results are in good agreement with experiment and that screening and relaxation corrections are well accounted for by the use of the Dirac-Kohn-Sham potentials in QED calculations of the one-loop radiative diagrams. Our present QED energies are also in good agreement with recent higher-order QED calculations of the $2s-{2p}_{1/2}$ transition in high-Z Li-like ions.

We describe measurements of the interspecies trap-loss-rate parameter ${\beta }_{\mathrm{NaRb}}$ in a magneto-optical trap containing both Na and Rb atoms. Our results reveal the presence of interspecies heteronuclear hyperfine-changing collisions.

Using an l-insensitive quantum defect theory, we derive a set of universal properties, both spectral and scattering, that are shared by diatomic systems with an asymptotic van der Waals interaction and having a zero-energy bound or a quasibound state of angular momentum $l_b.$ The ${}^{133}\mathrm{Cs}$ dimer is studied as an example. Systems that are close to having a bound or a quasibound state at the threshold are also discussed.

The order of high harmonics emitted by an atom in an intense laser field is limited by the so-called cutoff frequency. Solving the time-dependent Schrödinger equation, we show that this frequency can be increased considerably by a parabolic confining potential, if the confinement parameters are suitably chosen. Furthermore, due to confinement, the radiation intensity remains high throughout the extended emission range. All features observed can be explained with classical arguments.

Buz̆ek and Hillery have recently analyzed the possibility of cloning imperfectly arbitrary states using a universal quantum cloning machine. They have analyzed their result using the Hilbert-Schmidt norm, although a better measure of fidelity is the Bures metric. In this paper, we repeat the Buz̆ek and Hillery result using the Bures metric and show that we can still obtain the same result with an improved measure. However, unlike the Hilbert-Schmidt norm, this computation may be extended to the case of continuous variable, infinite-dimensional spaces, or even Hilbert spaces of larger dimensions. For completeness, a similar analysis using the trace norm is also performed.

Quantum-information processing by liquid-state NMR spectroscopy uses pseudopure states to mimic the evolution and observations on true pure states. A new method of preparing pseudopure states is described, which involves the selection of spatially labeled states of an ancilla spin with which the spin system of interest is correlated. This permits a general procedure to be given for the preparation of pseudopure states on any number of spins, subject to the limitations imposed by the loss of signal from the selected subensemble. The preparation of a single pseudopure state is demonstrated by carbon and proton NMR of ${}^{13}\mathrm{C}$-labeled alanine. With a judicious choice of magnetic-field gradients, the method further allows encoding of up to $2^N$ pseudopure states in independent spatial modes in an $N+1\mathrm{}$ spin system. Fast encoding and decoding schemes are demonstrated for the preparation of four such spatially labeled pseudopure states.

We consider entanglement-swapping schemes with general (rather than maximally) entangled bipartite states of arbitary dimension shared pairwise between three or more parties in a chain. The intermediate parties perform generalized Bell measurements with the result that the two end parties end up sharing an entangled state which can be converted into maximally entangled states. We obtain an expression for the average amount of maximal entanglement concentrated in such a scheme and show that in a certain reasonably broad class of cases this scheme is provably optimal and that, in these cases, the amount of entanglement concentrated between the two ends is equal to that which could be concentrated from the weakest link in the chain.

We present a general method to construct fault-tolerant quantum logic gates with a simple primitive, which is an analog of quantum teleportation. The technique extends previous results based on traditional quantum teleportation [Gottesman and Chuang, Nature (London) 402, 390 (1999)] and leads to straightforward and systematic construction of many fault-tolerant encoded operations, including the $\pi /8\mathrm{}$ and Toffoli gates. The technique can also be applied to the construction of remote quantum operations that cannot be directly performed.

We have discussed some general properties of the wave functions of ${\mathrm{H}}_2^{+}$ and its isoelectronic ions, in terms of confocal elliptic coordinates. Based on the asymptotic properties when the electron is far away from the nuclei, and the coalescence properties when it is close to one of the nuclei, we have developed simple, model wave functions for several states. These wave functions are simple enough to provide a clear insight into the structure of the wave functions, and yet accurate enough to provide energies close to the exact values.

A coupled Hartree-Fock procedure has been employed to estimate the frequency shift in the infrared spectra of the S and R enantiomers of the CHFClBr molecule due to the parity-violating electroweak interaction. The calculations indicate that a resolving power $\nu /\Delta \nu \approx 1\times {10}^{16},$ i.e., three orders of magnitude larger than that obtained in recent experiments, would be necessary to detect the parity-violating effects. The largest frequency shift was found for C-Cl stretching.

The R matrix with pseudostates (RMPS) method was used with inclusion of relativistic effects to calculate differential cross sections and several spin asymmetries for elastic electron scattering from Cs atoms in the theoretically most difficult “intermediate energy regime” between one and five times the ionization threshold. Comparison of the RMPS predictions with experimental benchmark data for 7, 13, and 20 eV is satisfactory, thereby indicating the applicability of the method to collision systems involving heavy targets. Furthermore, justification for the apparent success of nonrelativistic calculations for the differential cross sections and the exchange asymmetry is given.