We present a theoretical study on the ionization of a H atom by antiproton impact in a wide energy range (from 0.1 to 1000 keV). Taking a semiclassical approximation, in which the relative motion of the antiproton with respect to the atomic nucleus is described by classical mechanics, the time evolution of the electron motion is described by quantum mechanics. The time evolution of the electronic wave function is propagated by the split-operator method with a generalized pseudospectral method in the energy representation. Particular attention is paid to the numerical accuracy and numerical convergence. The maximum numerical uncertainty is estimated to be less than 3% at the lower-energy side by comparison of the ionization cross sections calculated with three schemes. The trajectory effect is also studied by comparison with the ionization cross sections calculated with a straight-line trajectory and a curved trajectory. The calculated ionization cross sections of H atoms by antiproton impact are compared with other calculations. Our calculated results are in good agreement with the experimental measurement.

We extend the kinetic theory of deep laser cooling of multilevel atoms by counterpropagating laser waves to a case of linearly polarized waves and compare the results with those for circularly polarized waves. Considering the excitation of a $\mathrm{}(3+5)-\mathrm{level}$ atom by two counterpropagating waves with orthogonal linear polarizations $(\mathrm{lin}\perp \mathrm{lin}$ configuration), we show that even-order stepwise processes composed of the two-photon optical processes are responsible for narrow structures near zero velocity in the ground-state atomic coherence, atomic populations, and optical coherences. The stepwise processes composed of the two-photon structures are shown to produce velocity structures much narrower than the two-photon structures produced by a single ${\sigma }^{+}-{\sigma }^{-}$ configuration [Phys. Rev. A 60, 3148 (1999)]. A Fokker-Planck type kinetic equation is derived for a model of a $\mathrm{}(3+5)-\mathrm{level}$ atom interacting with two linearly polarized waves in a $\mathrm{lin}\perp \mathrm{lin}$ configuration. It is found that stepwise multiphoton coherent processes enhance considerably the radiation force at low velocities and the friction produced by the force at a negative detuning. Contrary to a case of a ${\sigma }^{+}-{\sigma }^{-}$ configuration the stepwise processes are found to increase the diffusion coefficient. The stepwise even-order processes caused by two linearly polarized waves are identified as the basic physical mechanism responsible for deep laser cooling of atoms in a $\mathrm{lin}\perp \mathrm{lin}$ configuration. The temperature of laser-cooled atoms is derived for a model of a $\mathrm{}(3+5)-\mathrm{level}$ atom excited by a $\mathrm{lin}\perp \mathrm{lin}$ configuration and found to be asymptotically close to that for a case of ${\sigma }^{+}-{\sigma }^{-}$ configuration at large detuning.

The paper reports the realization of the process of two-dipole Dicke superradiance in a planar optical microcavity. Two dye molecules, located at a mutual distance |R| within the transverse resonant mode of the microcavity, are excited by two independent femtosecond pulses. The superradiant photon emission by the two systems and the consequent enhancement of the time decay of the dipole excitation is investigated. Furthermore, the spatial counterpart of superradiance is revealed by the realization of the Bose-Einstein partition statistics within the detection of photons emitted over the two output sides of the microcavity. A general quantum analysis of the process is given in the paper.

We consider a photonic crystal (PC) doped with four-level atoms whose intermediate transition is coupled near resonantly with a photonic band-gap edge. We show that two photons, each coupled to a different atomic transition in such atoms, can manifest strong phase or amplitude correlations: One photon can induce a large phase shift on the other photon or trigger its absorption and thus operate as an ultrasensitive nonlinear photon switch. These features allow the creation of entangled two-photon states and have unique advantages over previously considered media: (i) no control lasers are needed; (ii) the system parameters can be chosen to cause full two-photon entanglement via absorption; (iii) a number of PCs can be combined in a network.

We present the gas phase measurement of the static electric dipole polarizability of ${\mathrm{C}}_{70}$ fullerenes. The value is $102\pm 14{\AA }^3.$ The increase in polarizability from ${\mathrm{C}}_{60}$ to ${\mathrm{C}}_{70}$ is discussed and compared to different theoretical predictions.

We demonstrate sympathetic cooling between magnetically trapped ${}^{85}\mathrm{Rb}$ and ${}^{87}\mathrm{Rb}$ atoms. Up to $6\times {10}^6$ ${}^{85}\mathrm{Rb}$ atoms are cooled via elastic collisions with a reservoir of $8\times {10}^8$ ${}^{87}\mathrm{Rb}$ atoms. The temperature of the binary atomic ensemble is adjusted by using radio-frequency evaporation of ${}^{87}\mathrm{Rb}.$ Temperatures as low as $T=100\mathrm{}\hspace{0.5em}\mathrm{nK}$ have been realized in the binary ensemble. Through a cross-dimensional mixing approach we have been able to measure the interspecies ${}^{87}\mathrm{Rb}{-}^{85}\mathrm{Rb}$ scattering cross section $\sigma =1.52\mathrm{}\left(34\right)\mathrm{}\times {10}^{-15}\hspace{0.5em}{\mathrm{m}}^2.$

Two independent mode-locked femtosecond lasers are synchronized to an unprecedented precision. The rms timing jitter between the lasers is 4.3 fs, observed within a 160-Hz bandwidth over minutes. Multistage phase-locked loops help to preserve this ultrahigh timing resolution throughout the entire delay range between pulses (10 ns). We also demonstrate that the same level of synchronization can be achieved with two lasers at different repetition frequencies.

New radiative lifetimes of eight levels in Pr III have been measured using the time-resolved laser-induced fluorescence method. A tunable frequency-doubled dye laser with 1-ns pulse duration was used to excite selectively doubly ionized praseodymium in a laser-produced plasma. The experimental results agree generally well with theoretical data, provided core-polarization effects are considered in the calculations in an adequate way.

Using a high-frequency field superposed to a linearly polarized bichromatic laser field composed by a wave with frequency $\omega$ and a wave with frequency $2\omega ,$ we show it is possible to enhance the intensity of a group of high harmonics in orders of magnitude. These harmonics have frequencies about 30% higher than the monochromatic-cutoff frequency, and, within the three-step-model framework, correspond to a set of electron trajectories for which tunneling ionization is strongly suppressed. Particular features in the observed enhancement suggest that the high-frequency field provides an additional mechanism for the electron to reach the continuum. This interpretation is supported by a time-frequency analysis of the harmonic yield. The additional high-frequency field permits the control of this group of harmonics leaving all other sets of harmonics practically unchanged, which is an advantage over schemes involving only bichromatic fields.

We present a theoretical analysis of the coherent acceleration of atomic Bose-Einstein condensates. A first scheme relies on the “conveyor belt” provided by a frequency-chirped optical lattice. For potentials shallow enough that the condensate is not fragmented, the acceleration can be interpreted in terms of sequential Bragg scattering, with the atomic sample undergoing transitions to a succession of discrete momentum sidemodes. The narrow momentum width of these sidemodes offers the possibility to accelerate an ultracold atomic sample such as, e.g., a Bose-Einstein condensate without change in its momentum distribution. This is in contrast to classical point particles, for which this kind of acceleration leads to a substantial heating of the sample. A second scheme is based on the idea of a synchronous particle accelerator consisting of a spatial array of quadrupole traps. Pulsing the trapping potential creates a traveling trap that confines and accelerates the atomic system. We study this process using the concept of phase stability.

We develop a three-dimensional analysis of the phase separation of two-species Bose-Einstein condensates in the presence of vorticity within the Thomas-Fermi approximation. We find different segregation features according to whether the more repulsive component is in a vortex or in a vortex-free state. An application of this study is aimed at describing systems formed by two almost immiscible species of rubidium-87 that are commonly used in Bose-Einstein condensation experiments. In particular, in this work we calculate the density profiles of condensates for the same conditions as the states prepared in the experiments performed at JILA [Matthews et al., Phys. Rev. Lett. 83, 2498 (1999)].

We present time-resolved measurements of the ballistic trajectory of ultrashort optical wave packets circulating near the surface of a dielectric microcavity. The wave-packet dynamics are determined by using two-color two-photon induced fluorescence of Coumarin 510 within the microcavity as a correlator. Ballistic motion of the wave packets in whispering gallery modes and rainbow trajectories has been deduced from the time delay between collisions of the two wave packets. The cavity ringdown time is measured for each of the wave packets, which retain their integrity even after several round trips.

Predictions for systems in entangled states cannot be described in local realistic terms. However, after admixing some noise such a description is possible. We show that for two quantum systems described by N-dimensional Hilbert spaces (quNits) in a maximally entangled state the minimal admixture of noise increases monotonically with N. The results are a direct extension of those of Kaszlikowski et al. [Phys. Rev. Lett. 85, 4418 (2000)], where results for $N<~9$ were presented. The extension up to $N=16\mathrm{}$ is possible when one defines for each N a specially chosen set of observables. We also present results concerning the critical detectors efficiency beyond which a valid test of local realism for entangled quNits is possible.

We present a measure of quantum entanglement which is capable of quantifying the degree of entanglement of a multipartite quantum system. This measure, which is based on a generalization of the Schmidt rank of a pure state, is defined on the full state space, and is shown to be an entanglement monotone, that is, it cannot increase under local quantum operations with classical communication and under mixing. For a large class of mixed states this measure of entanglement can be calculated exactly, and it provides a detailed classification of mixed states.

The energy levels of lithiumlike bismuth are shown to be accurately described in a representation-independent manner when all diagrams involving one and two photons, with the exception of the two-loop Lamb shift, are evaluated. Comparison with the experimental value of the ${2p}_{3/2}-{2s}_{1/2}$ splitting then shows that, assuming three-photon effects are negligible, the contribution of the two-loop Lamb shift is 0.175(39) eV.

Electron capture and excitation process in collisions of doubly charged ions of ${\mathrm{He}}^{2+}$ with neutral He in either a metastable ${}^{1}S$ or ${}^{3}S$ state are investigated using a theoretical model based on the adiabatic representation of the collision complex. Both ab initio and model potential calculations are used to treat the adiabatic states of the ${\mathrm{He}}_2^{2+}$ molecular ion. A fully quantum mechanical treatment of the dynamics is adopted. Cross sections are calculated for energies ranging from ${10}^{-2}$ to 400 eV/amu. The singlet and triplet metastable states exhibit very pronounced differences in the low eV energy range. Whereas, electron capture and excitation occur simultaneously in collisions with $2{}^{1}S$ metastable He, electron capture is dominant in collisions with $2{}^{3}S$ metastable He.

Experimental data are presented for the scattering of cold electrons by ${\mathrm{CO}}_2,$ for both integral and backward scattering, down to energies of a few meV with an energy resolution of 0.95–1.5 meV (full width at half maximum) in the electron beam. The experimental data show evidence for the phenomenon of virtual-state scattering, a subject of extensive theoretical study. The measured scattering cross sections rise rapidly at low energy, to a limiting value in integral scattering in excess of 120 ${\mathrm{\AA }}^2$. In addition, analysis shows that scattering is s-wave dominated and that the s-wave scattering length is negative. All features are characteristic of virtual state scattering.

We show that the “two-potential” formalism of conventional distorted-wave rearrangement theory, which is formally valid only for short-range interactions, can be used to evaluate amplitudes for the ionization of atomic hydrogen by electron impact. The triply differential cross sections calculated using this method validate earlier results obtained by extrapolating the quantum mechanical flux. Although it uses the same time-independent wave functions, this method offers significant advantages over flux extrapolation. It is more accurate, can be applied in practical calculations over a broader range of collision energies, and unlike the flux-extrapolation method, can be applied for arbitrary values of energy sharing between the ejected electrons. Since this rearrangement formalism provides the complete scattering amplitude for ionization, it can be used to calculate any differential cross section.

Inverting laboratory measurements of quantum-mechanical observables to recover the underlying molecular potential typically produces nonunique solutions. Without quantifying the full family of potentials consistent with the measurements, it is impossible to fully determine how experimental error and limited data affect the inversion, or to assess the quality of the recovered potential. Here, we present a global, nonlinear algorithm for extracting molecular potentials from measurements of quantum-mechanical observables. The method utilizes a mapping technique to learn the relationship between a broad domain of potentials and their resulting observables to facilitate the inversion. Once constructed, the maps reduce the arduous task of repeatedly solving the Schrödinger equation for each trial potential tested during the inversion and permit the use of normally expensive, global optimization procedures to thoroughly explore the distribution of potentials consistent with the data. As a demonstration, the new algorithm is applied to quantum collision cross sections to illustrate the effect of experimental error and finite resolution of the scattering observables on the recovered potential. A series of simulated inversions were performed to examine these issues along with the inversion of laboratory differential cross-section data for He+Ne scattering. These illustrations show that laboratory errors can have a nonlinear effect on the family of extracted potentials. Furthermore, the examples provide a benchmark for the capabilities of the proposed algorithm to stably reveal the full distribution of potentials consistent with the data. The algorithm may be applied to other observables and molecular systems with more spatial coordinates.

The energy lost by a heavy projectile, with charge $Z_P,$ moving in a free-electron gas is studied within the framework of the dielectric formalism. In this model, the potential induced by the projectile is expanded in a perturbative series, and terms up to second order in $Z_P$ are conserved. The obtained quadratic potential is expressed as a function of the first-order dielectric response or Lindhard dielectric function. We apply the formalism to the calculation of stopping for different fixed charges (protons, neutral hydrogen, and antiprotons) moving in aluminum. Energy-loss distributions are investigated, and in the case of antiprotons, the second-order term is modified to avoid negative probabilities. The total stopping power, calculated taking into account the inner-shell contribution and different charge states in equilibrium, is compared with experimental data. The induced electronic density is also studied, and results agree with those derived from the density-functional theory.