The ratio of line strengths for the two argon transitions, ${3p}^5{(}^2{P}_{1/2}^o\left)4s\right[1/2{]}_1^o\to {3p}^5{(}^2{P}_{3/2}^o\left)4p\right[3/2{]}_2$ at wavelength 922.7 nm and ${3p}^5{(}^2{P}_{1/2}^o\left)4s\right[1/2{]}_1^o\to {3p}^5{(}^2{P}_{3/2}^o\left)4p\right[5/2{]}_2$ at wavelength 978.7 nm, is measured in an argon pulsed discharge with the use of a single-mode Ti:sapphire laser. The result 3.29(0.13) is in agreement with our theoretical prediction 3.23 and with a less accurate ratio 2.89(0.43) from the NIST database.

This paper presents an experimental study designed to address the long-standing question regarding the origin of very large positron annihilation rates observed for many molecules. We report a study of the annihilation, resolved as a function of positron energy $(\Delta E\sim 25\mathrm{meV},$ full width at half maximum) for positron energies from 50 meV to several eV. Annihilation measurements are presented for a range of hydrocarbon molecules, including a detailed study of alkanes, ${\mathrm{C}}_n{\mathrm{H}}_{2n+2},$ for $n=1\mathrm{}–9$ and 12. Data for other molecules are also presented: ${\mathrm{C}}_2{\mathrm{H}}_2,$ ${\mathrm{C}}_2{\mathrm{H}}_4;$ ${\mathrm{CD}}_4;$ isopentane; partially fluorinated and fluorinated methane $\left({\mathrm{CH}}_x{\mathrm{F}}_{4-x}\right);$ 1-fluorohexane $\left({\mathrm{C}}_6{\mathrm{H}}_{13}\mathrm{F}\right)$ and 1-fluorononane $\left({\mathrm{C}}_9{\mathrm{H}}_{19}\mathrm{F}\right).\mathrm{}$ A key feature of the results is very large enhancements in the annihilation rates at positron energies corresponding to the excitation of molecular vibrations in larger alkane molecules. These enhancements are believed to be responsible for the large annihilation rates observed for Maxwellian distributions of positrons in molecular gases. In alkane molecules larger than ethane $\left({\mathrm{C}}_2{\mathrm{H}}_6\right),$ the position of these peaks is shifted downward by an amount $\sim 20\mathrm{meV}$ per carbon. The results presented here are generally consistent with a physical picture recently considered in detail by Gribakin [Phys. Rev. A 61, 022720 (2000)]. In this model, the incoming positron excites a vibrational Feshbach resonance and is temporarily trapped on the molecule, greatly enhancing the probability of annihilation. The applicability of this model and the resulting enhancement in annihilation rate relies on the existence of positron-molecule bound states. In accord with this reasoning, the experimental results presented here provide the most direct evidence to date that positrons bind to neutral molecules. The shift in the position of the resonances is interpreted as a measure of the binding energy of the positron to the molecule. Other features of the results are also discussed, including large, qualitative changes in the annihilation spectra observed when hydrocarbon molecules are fluorinated.

The integrated Stokes parameters of the decay photons from the excited ${\mathrm{np}}^5\mathrm{}(n+1\mathrm{})p[5/2{]}_3,$ ${\mathrm{np}}^5{\mathrm{}(n+1\mathrm{})p}^{\prime }[3/2{]}_2,$ and ${\mathrm{np}}^5{\mathrm{}(n+1\mathrm{})p}^{\prime }[3/2{]}_1$ $(n=5\mathrm{}$ and 6) states of Kr and Xe, respectively, are measured from near threshold to 100 eV using incident polarized electrons. The effects of the cascade photons on the Stokes parameters have been investigated using a pulsed beam of polarized electrons. The cascade effects for the final excited states are more significant for the linear polarization $P_1$ parameter than for the circular polarization $P_3$ parameter and they do not always have depolarization effects. The integrated state multipoles, determined from the integrated Stokes parameters corrected for cascade effects, are closer to, but not in agreement with, the values from semirelativistic R matrix and relativistic distorted-wave approximation methods.

A simple means to realize conservative, all-optical atomic array illuminators is proposed. We show that, in the atom nanofabrication context, such devices can be used to substantially reduce imaging problems caused by spherical lens imperfections. Both one- and two-dimensional focusing setups are considered.

Recent experimental measurements of the time interval between detection of the two photons emitted in positron-electron annihilation have indicated that collapse of the spatial part of the photon’s wave function, due to detection of the other photon, does not occur. Although quantum nonlocality actually occurs in photons produced through parametric down-conversion, the recent experiments give strong evidence against measurement-induced instantaneous spatial localization of high-energy $\gamma$ photons. A quantum-mechanical analysis of the Einstein-Podolsky-Rosen problem is presented, which may help to explain the observed differences between photons produced through parametric down-conversion and photons produced through positron-electron annihilation. The results are found to concur with the recent experiments involving $\gamma$ photons.

The scattering of orthopositronium from Kr and Xe is investigated with the fixed core stochastic variational method. The scattering lengths for Ps-Kr and Ps-Xe scattering in the fixed core approximation are ${3.18a}_0$ and ${3.82a}_0,$ respectively. Inclusion of core-polarization potentials leads to a significant reduction in the scattering lengths. The Ps-Kr and Ps-Xe scattering lengths were ${1.98a}_0$ and ${2.29a}_0$ for polarization potentials that were tuned to an average of the model electron-atom and positron-atom scattering lengths for the respective atoms. Other choices of the polarization potential resulted in scattering lengths that could be larger by ${0.3a}_0$ or smaller by ${0.8a}_0.$ The pick-off annihilation parameter ${}_1{Z}_{\mathrm{eff}}$ has also been computed as part of the analysis. The present values of ${}_1{Z}_{\mathrm{eff}}$ are about 3 to 10 times smaller than the accepted experimental values since short-range electron-positron correlations were ignored when the annihilation matrix element was evaluated.

We investigate theoretically the generation of a persistent current in a Bose-Einstein condensate in a toroidal trap by means of phase imprinting. The phase-imprinting potential must be periodic around the torus, and will have parts that accelerate atoms in the direction opposite to the intended flow. We study the evolution of the average momentum and the momentum spread under a two-slope model potential numerically by using the Gross-Pitaevskii equation, and also in a simple classical-mechanics analogy. Because of the inevitable pollution caused by the opposite slopes of the potential, it is not possible to generate a pure persistent current. The same caveat presumably applies to vortices in higher-dimensional systems.

The quantum random walk has been much studied recently, largely due to its highly nonclassical behavior. In this paper, we study one possible route to classical behavior for the discrete quantum walk on the line: the presence of decoherence in the quantum “coin” which drives the walk. We find exact analytical expressions for the time dependence of the first two moments of position, and show that in the long-time limit the variance grows linearly with time, unlike the unitary walk. We compare this to the results of direct numerical simulation, and see how the form of the position distribution changes from the unitary to the usual classical result as we increase the strength of the decoherence.

We investigate strong-coupling effects on the superfluid phase transition in a gas of Fermi atoms with a Feshbach resonance. The Feshbach resonance describes a composite quasiboson that can give rise to an additional pairing interaction between the Fermi atoms. This attractive interaction becomes stronger as the threshold energy $2\nu$ of the Feshbach resonance two-particle bound state is lowered. In a recent paper, we showed that in the uniform Fermi gas, this tunable pairing interaction naturally leads to a crossover from a BCS state to a Bose-Einstein condensate (BEC) of the Nozières and Schmitt-Rink kind, in which the BCS-type superfluid phase transition continuously changes into the BEC type as the threshold energy is decreased. In this paper, we extend our previous work by including the effect of a harmonic trap potential, treated within the local-density approximation. We also give results for both weak and strong coupling to the Feshbach resonance. We show that the BCS-BEC crossover phenomenon strongly modifies the shape of the atomic density profile at the superfluid phase-transition temperature $T_{\mathrm{c}},$ reflecting the change of the dominant particles going from Fermi atoms to composite bosons. In the BEC regime, these composite bosons are shown to first appear well above $T_{\mathrm{c}}.$ We also discuss the “phase diagram” above $T_{\mathrm{c}}$ as a function of the tunable threshold energy $2\nu .$ We introduce a characteristic temperature $T^{*}\left(2\mathrm{}\nu \right)$ describing the effective crossover in the normal phase from a Fermi gas of atoms to a gas of stable molecules.

We consider states leading to the equality sign in the uncertainty inequalities associated with correlations in open quantum systems which have been recently derived by Ponomarenko and Wolf [Phys. Rev. A 63, 062106 (2001)]. The new inequalities involve fluctuations defined in terms of the square of the density operator that characterizes mixed states. We find the minimum-correlation states associated with the quadratures of single-mode and two-mode electromagnetic fields in a cavity and for the angular momentum operators which can describe atomic degrees of freedom. We show that while in the case of single-mode quadratures the functional form of the minimum-correlation state is uniquely specified, this is not so for the other pairs of noncommuting operators. In general, the states with the least amount of correlations are mixed and they exhibit squeezing.

The time evolution of wave packets in the Morse potential is investigated under the influence of the environment consisting of harmonic oscillators. These oscillators represent photon or phonon modes and are assumed to be in thermal equilibrium. Our model explicitly incorporates the fact that in the case of a nonequidistant spectrum the rates of the environment induced transitions are different for each transition. The nonunitary time evolution is visualized by the aid of the corresponding Wigner function. The time scale of decoherence is much shorter than that of dissipation, and gives rise to states that are mixtures of localized states along the phase-space orbit of the corresponding classical particle. This behavior is to a large extent independent of the coupling strength, the temperature of the environment, and also the initial state.

Universal quantum-cloning machines (UQCMs), sometimes called quantum cloners, generate many outputs with identical density matrices, with as close a resemblance to the input state as is allowed by the basic principles of quantum mechanics. Any experimental realization of a quantum cloner has to cope with the effects of decoherence which terminate the coherent evolution demanded by a UQCM. We examine how many clones can be generated within a decoherence time. We compare the time that a quantum cloner implemented with trapped ions requires to produce M copies from N identical pure state inputs and the decoherence time during which the probability of spontaneous emission becomes non-negligible. We find a method to construct an $\overrightarrow{N}M$ cloning circuit, and estimate the number of elementary logic gates required. It turns out that our circuit is highly vulnerable to spontaneous emission as the number of gates in the circuit is exponential with respect to the number of qubits involved.

We theoretically investigate the quantum dynamics of the center of mass of trapped atoms, whose internal degrees of freedom are driven in a $\Lambda$-shaped configuration with the lasers tuned at two-photon resonance. In the Lamb-Dicke regime, when the motional wave packet is well localized over the laser wavelength, transient coherent population trapping occurs, canceling transitions at the laser frequency. In this limit the motion can be efficiently cooled to the ground state of the trapping potential. We derive an equation for the center-of-mass motion by adiabatically eliminating the internal degrees of freedom. This treatment provides the theoretical background of the scheme presented in [G. Morigi et al., Phys. Rev. Lett. 85, 4458 (2000)] and implemented in [C.F. Roos et al., Phys. Rev. Lett. 85, 5547 (2000)]. We discuss the physical mechanisms determining the dynamics and identify parameters regimes, where cooling is efficient. We discuss implementations of the scheme to cases where the trapping potential is not harmonic.

The input-output relations for linear amplifiers and attenuators that have two input and two output channels are used to derive inequalities that relate their gain profiles and output noise spectra. The results generalize earlier derivations, which mainly focus their attention on single-channel devices, to the two-ended amplifiers and attenuators often used in practical communications systems. The present inequalities are satisfied by the results of previous calculations for specific model systems. It is shown that; in contrast to single-channel devices, a two-ended system can act as an amplifier for some input signals and an attenuator for others, even when all the signal frequencies are the same. The output from the two-channel amplifier has a minimum noise determined by the sum of the gains for both input channels, even when only one input channel is used and the other is in its vacuum state. The conditions on device construction needed to achieve equal gains for signals that arrive at the two ends of the device are determined. The present results reduce to those of single-channel theory in special cases where the two output channels are each separately fed by only one of the two input channels.

A common experimental setup in cavity quantum electrodynamics consists of a single two-level atom interacting with a single mode of the electromagnetic field inside an optical cavity. The cavity is externally driven and the output is continuously monitored via homodyne measurements. We derive formulas for the optimal rates at which these measurements provide information about (i) the quantum state of the system composed of atom and electromagnetic field, and (ii) the coupling strength between atom and field. We find that the two information rates are anticorrelated.

In this paper, some concepts of nonrelativistic many-particle quantum mechanics (e.g., product states, density matrix) are generalized to the relativistic domain using a framework called relativistic Schrödinger theory (RST). By using a general ansatz, the RST framework is simplified considerably and some of its field equations are solved directly. The RST approach is then compared with nonrelativistic quantum mechanics (QM) for the case of a product state (conventional QM) and its RST analog. It is shown that relativistic wave equations can be derived from the RST formalism, so that they coincide in the nonrelativistic limit with the well-known Hartree equations.

We describe an experimental scheme of preparing multipartite W class of maximally entangled states between many atomic ensembles. The scheme is based on laser manipulation of atomic ensembles and single-photon detection, and well fits the status of the current experimental technology. In addition, we show one of the applications of the kind of W class states, teleporting an entangled state of atomic ensembles with unknown coefficients to more than one distant parties, either one of which equally likely receives the transmitted state.

Using the spectral representation of Green’s function, we calculate the density of states and extract parameters of resonances in the scattering system. The method is implemented for the resonances in the $\mathrm{dt}\mu$ molecule below the $t\mu$ $\mathrm{}(n=2\mathrm{})\mathrm{}$ threshold.

The double ionization of He by electron impact at 580 eV has been studied in a coplanar symmetric $(e,(3\mathrm{}-1\left)e\right)$ experiment, in which two fast electrons of 250 eV are detected. In this way a momentum transfer as large as 6 a.u. is achieved. The results are compared with the predictions of a theoretical model based on the impulse (knock-out) approximation. The calculations, which include radial correlation of electrons in the He ground state wave function, better describe the experiment than those with an uncorrelated wave function.

Angle-resolved resonant Auger-electron spectroscopy has been carried out on the nitrogen molecule at selected photon energies around 419 eV, where a $1s$ core electron and two valence electrons are promoted into the lowest unoccupied molecular orbital ${1\mathrm{\pi }}_g.$ Significant enhancement of a specific band, which cannot be disentangled in direct photoionization, is observed at a binding energy of 37.6 eV, with a value of the anisotropy parameter $\beta$ much smaller than 2. We assign this new band to the transition to a doubly excited cationic state of ${\mathrm{N}}_2,$ in which two of the excited valence electrons remain in the $1{\pi }_g$ orbital, proposing a “double spectator” type decay mechanism. This observation shows how to preferentially probe multiply excited configurations of cations using multiple resonant excitation.