The formation of antihydrogen in the laser-assisted charge-transfer reaction between the antiproton and positronium is investigated within the first Born approximation, including dressing of the asymptotic scattering states by the laser field. Through a quantitative study it is shown that, if the field intensity is sufficiently high so as to begin influencing the charge-exchange cross section, the positronium will break up and therefore lower the production of antihydrogen. This conclusion is in contrast to results recently reported by Li et al. [Phys. Rev. A 59, 1697 (1999)], where an enhancement effect is predicted using a similar model for the antihydrogen formation, but neglecting the role of the photobreakup channel.

We point out that the conclusion of the preceding comment on antihydrogen formation in laser-assisted antiproton-positronium collision [preceding Comment, Phys. Rev. A 61] is invalid for an intense field case, and show that the positronium will not be broken up by the laser before the collision takes place in the suggested experimental setup. Thus we cannot rule out the possibility of the cross-section enhancement for such a laser-assisted reaction.

We have made a systematic observation for integrated cross sections that vibrational excitation modes are found to show quite different energy dependence in the 3.8-eV shape resonance peak in a ${\mathrm{CO}}_2$ molecule. This feature arises from the different interaction scheme for excitation to each mode, i.e., the weak polarization interaction for excitation to the symmetric stretching mode, weak dipole interaction to the bending mode, and strong dipole interaction to the asymmetric stretching mode. The asymmetric stretching shows a sharp increase in differential cross sections (DCSs) toward small-angle scattering due to the strong dipole interaction, which washes out resonance features seen at intermediate angles. DCSs for the symmetric and bending modes clearly show conspicuous features at a wide range of the scattering angle around 3.8-eV shape resonance.

A precision calculation of the two-photon detachment cross section for ${\mathrm{H}}^{-}$ is performed by means of a non-Hermitian Floquet theory. A highly accurate initial state wave function is used along with fully correlated saddle-point wave functions for the intermediate and final states. The cross section is investigated for energies below the $n=2\mathrm{}$ and $n=3\mathrm{}$ thresholds. The peak cross section predicted for the ${}^{1}D$ resonance is $3.10\times {10}^{-49}$ ${\mathrm{cm}}^4$ sec at 10.873 2 eV. It represents an ab initio theoretical prediction in complete agreement with the experimental result of 3.2 $(+1.8,-1.2)\times {10}^{-49}$ ${\mathrm{cm}}^4$ sec at 10.873 2(27) eV.

Alkaline-earth-like atoms possess singlet and triplet manifolds coupled through the ground state. The weak and hence narrow linewidth intercombination transition can provide a powerful probe of singlet-singlet magneto-optic trap (MOT) dynamics. We demonstrate in situ probing of an ytterbium MOT and discuss how cloud position, size, and temperature can be determined.

The paper studies the effect of a weak periodic driving on metastable Wannier-Stark states. The decay rate of the ground Wannier-Stark states as a continuous function of the driving frequency is calculated numerically. The theoretical results are compared with the experimental data of Wilkinson et al. [Phys. Rev. Lett. 76, 4512 (1996)] obtained for cold sodium atoms in an accelerated optical lattice.

Employing a far-off resonance optical-dipole trap (FORT), we attained a phase-space density exceeding 0.1, or an order to quantum degeneracy. Strontium atoms were magneto-optically cooled and trapped using the spin-forbidden ${}^1{S}_0{-}^3{P}_1$ transition and then compressed into a FORT that was designed to allow simultaneous Doppler cooling. We discussed that the phase-space density was finally limited by the light-assisted collisions occurring in the optical cooling.

This Brief Report quantifies the continuity properties of entanglement: how much does entanglement vary if we change the entangled quantum state just a little? This question is studied for the pure state entanglement of a bipartite system and for the entanglement of formation of a bipartite system in a mixed state.

Two-level systems were shown to be fully described by a single function, known sometimes as the Stueckelberg parameter. Using concepts from differential geometry, we give geometrical meaning to the Stueckelberg parameter and to other related quantities. As a result, a generalization of the Stueckelberg parameter is introduced, and a relation obtained between two-level systems and spatial one-dimensional curves in three-dimensional space. Previous authors used this Stueckelberg parameter to solve analytically several two-level models. We further develop this idea, and solve analytically three fundamental models, from which many other known models emerge as special cases. We present the detailed analysis of these models.

A mapping that relates the Wigner phase-space distribution function of a given stationary quantum mechanical wave function, a solution of the Schrödinger equation, to a specific solution of the Liouville equation, both subject to the same potential, is studied. By making this mapping, bound states are described by semiclassical distribution functions still depending on Planck’s constant, whereas for elastic scattering of a particle by a potential they do not depend on it, the classical limit being reached in this case. Following this method, the mapped distributions of a particle bound in the Pöschl-Teller potential and also in a modified oscillator potential are obtained.

We generalize Grover’s unstructured quantum search algorithm to enable it to work with arbitrary starting superpositions and arbitrary unitary operators. We show that the generalized quantum search algorithm, when cast in a special orthonormal basis, can be understood as performing an exact rotation of a starting superposition into a target superposition. We derive a formula for the success probability of the generalized quantum search algorithm after n rounds of amplitude amplification. We then use this formula to determine the optimal strategy for a punctuated quantum search algorithm, i.e., one in which the amplitude amplified state is observed before the point of maximum success probability. On average, the optimal strategy is about 12% better than the naive use of Grover’s algorithm. The speedup obtained is not dramatic but it illustrates that a hybrid use of quantum computing and classical computing techniques can yield a performance that is better than either alone. In addition, we show that a punctuated quantum algorithm that takes the same average computation time as Grover’s standard algorithm only requires half the coherence time. We then extend the analysis to the case of a society of k quantum searches acting in parallel. We derive an analytic formula that connects the degree of parallelism with the expected computation time for k-parallel quantum search. The resulting parallel speedup scales as $O\left(\sqrt{k}\right),$ while the minimum number of agents needed to ensure success, k, decreases as the inverse of the square of the achievable coherence time. This result has practical significance for the design of rudimentary quantum computers that are likely to have a limited coherence time.

We present an improved and simplified version of “δ-kick cooling.” By application of a pulsed magnetic field, laser-cooled Rb atoms are cooled in one dimension, by a factor of 10 below the temperature of optical molasses. While phase-space density is conserved in this process, the method is more efficient than adiabatic expansion. We have achieved temperatures below 700 nK, but there is no fundamental limit to this cooling technique. The technique can also be used as an atom lens and as a spin-dependent probe. We show preliminary results suggesting interesting spin-temperature correlations within an optical molasses.

Atom lasers based on rf outcoupling from a trapped Bose-Einstein condensate can be described by a set of generalized, coupled Gross-Pitaevskii equations (GPEs). If not only one but two radio frequencies are used for outcoupling, the atoms emerging from the trap have two different energies and the total wave function of the untrapped spin state is a coherent superposition which leads to a pulsed atomic beam. We present results for such a situation obtained from a one-dimensional GPE model for magnetically trapped ${}^{87}\mathrm{Rb}$ in the $F=1\mathrm{}$ state. The wave function of the atomic beam can be approximated by a sum of two Airy functions. In the limit of weak coupling we calculate the intensity analytically.

We consider effects of a broadband squeezed reservoir on the second-order intensity correlation, and on squeezing properties in the resonance fluorescence of a bichromatically driven two-level atom in a cavity of moderate Q. Phase-dependent squeezed reservoir effects change the photon statistics, and lead to an amplification of the degree of squeezing. Squeezed reservoir effects in the second-order correlation function $g^{\mathrm{}\left(2\right)\mathrm{}}\left(\tau \right)$ are determined by two-photon emission processes which can be enhanced or suppressed in dependence on the squeezing phase.

Complete formulas for the QED corrections of the first order in $\alpha$ to the radiative recombination of an electron with a bare nucleus are derived. An analysis of the ultraviolet and infrared divergences is presented. Application of the formulas to the radiative electron capture process is discussed. Numerical results are presented for the vacuum-polarization correction evaluated in the Uehling approximation and for a part of the self-energy correction.

Entanglement swapping between Einstein-Podolsky-Rosen (EPR) pairs can be used to generate the same sequence of random bits in two remote places. A quantum key distribution protocol based on this idea is described. The scheme exhibits the following features. (a) It does not require that Alice and Bob choose between alternative measurements, therefore improving the rate of generated bits by transmitted qubit. (b) It allows Alice and Bob to generate a key of arbitrary length using a single quantum system (three EPR pairs), instead of a long sequence of them. (c) Detecting Eve requires the comparison of fewer bits. (d) Entanglement is an essential ingredient. The scheme assumes reliable measurements of the Bell operator.

We present the theoretical results of a comprehensive study of the photoionization from excited Mg atoms in the spectral region below the Mg II $3p{}^{2}P$ limit using a B-spline-based configuration interaction approach. The variation of the doubly excited ${}^{1,3}S,$ ${}^{1,3}P,$ ${}^{1,3}D,$ and ${}^{1,3}F$ resonance structures along the autoionization series is discussed. The theoretical resonant energy and width, derived from the energy variation of the scattering phase shift across each resonance, are also presented.

We present absolute measurements of electron-capture cross sections in atomic and molecular hydrogen by $\alpha$ particles with energies ranging from 0.5 to 2.75 MeV. The measured single-electron-capture cross sections as well as the ratio ${\sigma }_c^{{\mathrm{H}}_2}/{\sigma }_c^{\mathrm{H}}$ are compared with several available theories. In particular, a detailed analysis of the role played by two-electron processes in the single-electron capture of ${\mathrm{H}}_2$ is performed considering a distorted-wave model as a starting point.

We study the dynamical evolution of diamagnetic Zeeman states in hydrogen and sodium atoms ionized by half-cycle pulses. The eigenstates of the combined Coulomb-diamagnetic potential are determined by solving the Schrödinger equation using a grid-based pseudopotential method. We study states with principal quantum number n between $15-20$ in the $l$-mixing regime at a magnetic field of 6 T. Diamagnetic states that are initially localized parallel and perpendicular to the magnetic field are subjected to the electric field of a half-cycle pulse (HCP) and their time evolution is monitored. We calculate the total ionized fraction, and also the spectrum of the ionized photoelectrons, keeping the total momentum transferred by the HCP constant and varying the HCP width. We find differences in both the amount of ionization and the form of the photoelectron spectrum for the two classes of localized states. In the impulsive limit, where the width of the pulse is much smaller than typical time scales in the system, the differences are due to the different initial momentum distributions of the parallel and perpendicular states. For longer pulse widths, we find that ionization is supressed as compared with the impulsive limit. The states localized perpendicular to the magnetic field are found to be much more sensitive to the HCP width than the parallel states, which reflects the fact that the two classes of states interact with different parts of the diamagnetic potential during the HCP.

An alternative theoretical approach for solving the time-dependent Schrödinger equation for atoms in an intense laser field is presented. In this method the time-dependent wave function is expanded in a basis set but the expansion coefficients are determined by linear-least-squares fitting of the wave function on discrete mesh points in configuration space, thus avoiding the need of evaluating a large number of matrix elements. We illustrate the method by computing wave functions, above-threshold ionization spectra, and harmonic generation spectra of a model atom and compare the results with those obtained using the split-operator method.