The configuration-interaction (CI) method is applied to the calculation of the structures of a number of positron binding systems, including $e^{+}\mathrm{Be}$, $e^{+}\mathrm{Mg}$, $e^{+}\mathrm{Ca}$, and $e^{+}\mathrm{Sr}$. These calculations were carried out in orbital spaces containing about 200 electron and 200 positron orbitals up to $\ell =12$. Despite the very large dimensions, the binding energy and annihilation rate converge slowly with $\ell$, and the final values do contain an appreciable correction obtained by extrapolating the calculation to the $\ell \to \infty$ limit. The binding energies were 0.00317 hartree for $e^{+}\mathrm{Be}$, 0.0170 hartree for $e^{+}\mathrm{Mg}$, 0.0189 hartree for $e^{+}\mathrm{Ca}$, and 0.0131 hartree for $e^{+}\mathrm{Sr}$.

We present a theoretical and experimental investigation of homonuclear ionizing collisions of laser-cooled metastable $\left(2{}^{3}S_1\right)$ helium atoms, considering both the fermionic ${}^3\mathrm{He}$ and bosonic ${}^4\mathrm{He}$ isotopes. The theoretical description combines quantum threshold behavior, Wigner's spin-conservation rule, and quantum-statistical symmetry requirements in a single-channel model, complementing a more complete close-coupling theory that has been reported for collisions of metastable ${}^4\mathrm{He}$ atoms. The model is supported with measurements (in the absence of light fields) of ionization rates in magneto-optically trapped samples that contain about $3\times {10}^8$ atoms of a single isotope. The ionization rates are determined from measurements of trap loss due to light-assisted collisions combined with comparative measurements of the ion production rate in the absence and presence of trapping light. Theory and experiment show good agreement.

We report on an experimental realization of a two-dimensional magneto-optical trap (2D-MOT) that allows the generation of cold atomic beams of ${}^{39}\mathrm{K}$ and ${}^{41}\mathrm{K}$ bosonic potassium isotopes. The high measured fluxes up to $1.0\times {10}^{11}\mathrm{atoms}∕\mathrm{s}$ and low atomic velocities around $33\mathrm{m}∕\mathrm{s}$ are well suited for a fast and reliable three-dimensional magneto-optical trap loading, a basilar feature for new generation experiments on Bose-Einstein condensation of dilute atomic samples. We also present a simple multilevel theoretical model for the calculation of the light-induced force acting on an atom moving in a MOT. The model gives a good agreement between predicted and measured flux and velocity values for our 2D-MOT.

The development of soft x-ray lasers collisionally pumped by a relatively long prepulse and short main pulse has markedly improved the effectiveness of these devices. As a result it is possible to envisage their use in a well equipped university laboratory. In this note we break down the various elements of the interaction and pumping into their simple components in order to see how relatively simple analytic concepts can be used to identify the best mode of operation. However detailed simulation is still required to identify the actual experimental operating conditions. The scaling results are compared with simulation, confirming the value of this analysis.

We investigate quantum properties of phase-locked light beams generated in a nondegenerate optical parametric oscillator (NOPO) with an intracavity waveplate. This investigation continues our previous analysis presented in Phys. Rev. A 69, 053814 (2004), and involves problems of continuous-variable quadrature entanglement in the spectral domain, photon-number correlations as well as the signatures of phase-locking in the Wigner function. We study the role of phase-localizing processes on the quantum correlation effects. The peculiarities of phase-locked NOPO in the self-pulsing instability operational regime are also cleared up. The results are obtained in the $P$-representation as a quantum-mechanical calculation in the framework of stochastic equations of motion, as well as by numerical simulation based on the method of quantum state diffusion.

Standard quantum cryptographic protocols are not secure if one assumes that nonlocal hidden variables exist and can be measured with arbitrary precision. The security can be restored if one of the communicating parties randomly switches between two standard protocols.

In a recent letter Li et al. [Phys. Rev. Lett. 94, 173001 (2005)] have studied experimentally the role of dipole-dipole interactions in the ionization of Rydberg atoms and the formation of a cold plasma. The authors associated their observations with atomic motion due to such interaction. In this work, we adapt the dynamical model proposed by De Oliveira et al. [Phys. Rev. Lett. 90, 143002 (2003)] based on a two atom interaction, which reproduce qualitatively well some experimental results observed by Li and co-workers. This comparison presents clear evidences of motion in such ultralong range potential between cold-Rydberg atoms, which may be important for some applications, as quantum computation.

We report on the implementation of evaporative cooling of a magnetically guided beam by adsorption on a ceramic surface. We use a transverse magnetic field to shift locally the beam towards the surface, where atoms are selectively evaporated. With a $5\text{−}\mathrm{mm}$-long ceramic piece, we gain a factor of $1.5\pm 0.2$ on the phase-space density. Our results are consistent with a 100% efficiency of this evaporation process. The flexible implementation that we have demonstrated, combined with the very local action of the evaporation zone, makes this method particularly suited for the evaporative cooling of a beam.

Lorentz transformations of spin density matrices for a particle with positive mass and spin $1∕2$ are described by maps of the kind used in open quantum dynamics. They show how the Lorentz transformations of the spin depend on the momentum. Since the spin and momentum generally are not independent, the maps generally are not completely positive and act in limited domains. States with two momentum values are considered, so the maps are for the spin qubit correlated with the qubit made from the two momentum values, and results from the open quantum dynamics of two coupled qubits can be applied. Inverses are used to show that every Lorentz transformation completely removes the spin polarization, and so completely removes the information, from a number of spin density matrices. The size of the spin polarization that is removed is calculated for particular cases.

We consider translationally invariant states of an infinite one-dimensional chain of qubits or spin-$\frac{1}{2}$ particles. We maximize the entanglement shared by nearest neighbors via a variational approach based on finitely correlated states. We find an upper bound of nearest-neighbor concurrence equal to $\mathcal{C}=0.434095$ which is $0.09\%$ away from the bound ${\mathcal{C}}_{\mathrm{W}}=0.434467$ obtained by a completely different procedure. The obtained state-maximizing nearest-neighbor entanglement seems to approximate the maximally entangled mixed states. Further we investigate in detail several other properties of the so-obtained optimal state.

A version of the Bethe-Salpeter equation appropriate for calculating recoil corrections in highly charged hydrogenlike ions is presented. The nucleus is treated as a scalar particle of charge $Z$, and the electron treated relativistically. The known recoil corrections of order $m^2∕M{\left(Z\alpha \right)}^4$ are derived in both this formalism and in NRQED.

The combination of density-functional theory with other approaches to the many-electron problem through the separation of the electron-electron interaction into a short-range and a long-range contribution is a promising method, which is raising more and more interest in recent years. In this work some properties of the corresponding correlation energy functionals are derived by studying the electron-electron coalescence condition for a modified (long-range-only) interaction. A general relation for the on-top (zero electron-electron distance) pair density is derived, and its usefulness is discussed with some examples. For the special case of the uniform electron gas, a simple parametrization of the on-top pair density for a long-range only interaction is presented and supported by calculations within the “extended Overhauser model.” The results of this work can be used to build self-interaction corrected short-range correlation energy functionals.

We study the stationary nonlinear Schrödinger equation, or Gross-Pitaevskii equation, for a one-dimensional finite square-well potential. By neglecting the mean-field interaction outside the potential well it is possible to discuss the transport properties of the system analytically in terms of ingoing and outgoing waves. Resonances and bound states are obtained analytically. The transmitted flux shows a bistable behavior. Novel crossing scenarios of eigenstates similar to beak-to-beak structures are observed for a repulsive mean-field interaction. It is proven that resonances transform to bound states due to an attractive nonlinearity and vice versa for a repulsive nonlinearity, and the critical nonlinearity for the transformation is calculated analytically. The bound-state wave functions of the system satisfy an oscillation theorem as in the case of linear quantum mechanics. Furthermore, the implications of the eigenstates on the dymamics of the system are discussed.

We describe a scheme for measurement of the mean photon flux at an arbitrary optical sideband frequency using homodyne detection. Experimental implementation of the technique requires an acousto-optic modulator in addition to the homodyne detector, and does not require phase locking. The technique exhibits polarization and frequency and spatial mode selectivity, as well as much improved speed, resolution, and dynamic range when compared to linear photodetectors and avalanche photodiodes, with potential application to quantum-state tomography and information encoding using an optical frequency basis. Experimental data also support a quantum-mechanical description of vacuum noise.

The results of an investigation concerning the sensitivity of conventional unfolding methods applied to electronic-vibrational electron-energy-loss spectra to the transmission efficiency of electron spectrometers are presented. This investigation was made in an effort to understand differences in the differential cross sections for excitation of low-lying electronic states determined experimentally by various groups using electronic-vibrational energy-loss spectra of ${\mathrm{N}}_2$. In these experiments, very similar spectral unfolding methods were used, which relied on similar Franck-Condon factors. However, the overall analyses of the electron scattering spectra (by the individual groups) resulted in large differences among the differential cross sections determined from these energy-loss spectra. The transmission response of the experimental apparatus to different-energy scattered electrons has often been discussed as a key factor that caused these disagreements. The present investigation shows in contrast that the effect of transmission is smaller than that required to independently explain such differences, implying that other systematic effects are responsible for the existing differences between measurements.

In a recent work [Phys. Rev. Lett.94, 143201 (2005)], we reported an accumulation of three-body resonant states attached to $n=2$ and higher two-body thresholds. A more careful investigation revealed that there are resonances of the same kind above the $n=1$ threshold as well. This suggests that the resonances attached to the thresholds are Efimov resonances.

Precision measurement of small separations between two atoms or molecules has been of interest since the early days of science. Here, we discuss a scheme which yields spatial information on a system of two identical atoms placed in a standing wave laser field. The information is extracted from the collective resonance fluorescence spectrum, relying entirely on far-field imaging techniques. Both the interatomic separation and the positions of the two particles can be measured with fractional-wavelength precision over a wide range of distances from about $\lambda ∕550$ to $\lambda ∕2$.

We formulate an expression for the accessible information in mirror-symmetric ensembles of real qubit states. This expression is used to make a detailed study of optimum quantum measurements for extracting the accessible information in three-state, mirror-symmetric ensembles. Distinct measurement regimes are identified for these ensembles with optimal measurement strategies involving different numbers of measurement operators, similar to results found for minimum error discrimination. Our results generalize known results for the accessible information in two pure states and in the trine ensemble.

We have investigated the $s$-wave elastic scattering of antihydrogen atoms off atomic alkali-metal targets (Li, Na, K, and Rb) at thermal energies $({10}^{-16}–{10}^{-4}\mathrm{a.u.})$ using an atomic orbital expansion technique. The elastic cross sections of these systems at thermal energies are found to be very high compared to $\overline{\mathrm{H}}\text{−}\mathrm{H}$ and $\overline{\mathrm{H}}\text{−}\mathrm{He}$ systems. The theoretical models employed in this study are so chosen to consider long-range forces dynamically in the calculation. The mechanism of cooling suggests that Li may be considered to be a good candidate as a buffer gas for enhanced cooling of antihydrogen atoms to ultracold temperature.