Experiment and numerical simulations have shown that the heating of atomic clusters can be enhanced by using a correctly timed sequence of two high-intensity pulses of different frequencies. We have measured ion energies from Xe clusters irradiated with two high-intensity femtosecond laser pulses, one at the laser fundamental frequency (780 nm) and one at its second harmonic (390 nm), focused to an intensity of $\sim {10}^{15}{\mathrm{\hspace{0.5em}}\mathrm{W}\mathrm{}\mathrm{cm}}^{\mathrm{-}2}.$ The ion energies are found to depend on the relative delay of the two pulses but not on their relative polarizations. For the optimum value of the delay between the two laser pulses, the maximum ion energy is doubled from 100 to 200 keV. The laser energy required to obtain a given ion energy is significantly less when the cluster is irradiated with two pulses of different frequencies than a single laser pulse.

Two features with large rotational structure, observed at detunings ${\delta }_1=-2.14{\mathrm{cm}}^{-1},{\delta }_2=-6.15{\mathrm{cm}}^{-1}$ in the cesium photoassociation spectrum, are interpreted as due to the combined effect of tunneling through the barrier of the double-well $0_g^{-}{(6s+6p}_{3/2})$ potential, and channel mixing in the inner-well region. A precise value of the height of the barrier is given. The levels affected by these effects are expected to decay into low vibrational levels of the lowest triplet state, thus producing ultracold molecules.

We solve a stochastic master equation based on the theory of Savard et al. [T. A. Savard, K. M. O’Hara, and J. E. Thomas, Phys. Rev. A 56, R1095 (1997)] for heating arising from fluctuations in the trapping laser intensity. We compare with recent experiments of Ye et al. [J. Ye, D. W. Vernooy, and H. J. Kimble, Phys. Rev. Lett. 83, 4987 (1999)], and find good agreement with the experimental measurements of the distribution of trap occupancy times. The major cause of trap loss arises from the broadening of the energy distribution of the trapped atom, rather than the mean heating rate, which is a very much smaller effect.

We study the consequences of spontaneous emission interference in the propagation dynamics of a short laser pulse in a closed $\mathsf{V}$-type medium. We show that, under specific conditions, the (otherwise opaque) medium becomes transparent to the laser pulse. The influence of incoherent processes on the transparency is also investigated.

In a secure bit commitment protocol involving only classical physics, A commits either a $0$ or a $1$ to B. If quantum information is used in the protocol, A may be able to commit a state of the form $\alpha |0〉+\beta |1〉.$ If so, she can also commit mixed states in which the committed bit is entangled with other quantum states under her control. We introduce here a quantum cryptographic primitive, bit commitment with a certificate of classicality (BCCC), which differs from standard bit commitment in that it guarantees that the committed state has a fixed classical value. We show that no unconditionally secure BCCC protocol based on special relativity and quantum theory exists. We also propose complete definitions of security for quantum and relativistic bit commitment.

The time-dependent Dirac equation can be solved exactly for ionization induced by ultrarelativistic heavy-ion collisions. Ionization calculations are carried out in such a framework for a number of representative ion-ion pairs. For each ion-ion pair, the computed cross section consists of two terms: a constant energy independent term and a term whose coefficient is $\ln \gamma .$ Scaled values of both terms are found to decrease with increasing Z of the nucleus that is ionized.

We present a more efficient way of treating polarization effects in the scattering of low-energy electrons by molecules within the Schwinger multichannel (SMC) method. We propose to expand the scattering wave function in a set of functions of $N+1\mathrm{}$ electrons that describe the scattering in an effective way, which allows the use of a small number of functions to describe the polarization effects. As a first test, we apply the method to the scattering of electrons by the ${\mathrm{H}}_2$ molecule. We calculate elastic integral and differential cross sections, and we obtain excellent results with a reduction in the number of configurations of up to 98% when compared to the traditional method used in the SMC method. This is a substantial size reduction of all matrices involved in the SMC method and, as a consequence, it represents a promising technique for treating more complex molecular systems.

We used the Schwinger multichannel method with pseudopotentials [Bettega et al., Phys. Rev. A 47, 1111 (1993)] to study elastic scattering of low-energy electrons by the boron trihalides ${\mathrm{BCl}}_3,$ ${\mathrm{BBr}}_3,$ and ${\mathrm{BI}}_3,$ at the static-exchange approximation. We calculated elastic integral, differential, and momentum transfer cross section from 5 to 50 eV. In particular, our integral cross section for ${\mathrm{BCl}}_3$ agrees in shape with results of previous calculations by Isaacs et al. [Phys. Rev. A 58, 2881 (1998)]. The symmetry decomposition of the integral cross section in the $C_{2v}$ group is also presented. We discuss the existence of shape resonances for energies above 5 eV at the $A_1,$ $B_1,$ $B_2,$ and $A_2$ symmetries. We also investigated the low-energy cross section for the $B_2$ symmetry for these three molecules. For ${\mathrm{BCl}}_3$ our $B_2$ cross section shows good agreement with the results of Isaacs et al.

Numerical experiments for a two-dimensional hydrogen model interacting with very intense (far beyond the barrier suppression value) and linearly polarized laser fields are presented in nonrelativistic cases. The Schrödinger equation retaining the space dependence of the fields is solved (nondipole), comparing with results obtained in the standard dipole approximation, in situations where the validity of such an approximation can be questioned.

We study the stability of a Bose-Einstein condensate of harmonically trapped atoms with negative scattering length, specifically ${}^7\mathrm{Li}.$ Our method is to solve the time-dependent nonlinear Schrödinger equation numerically. For an isolated condensate, with no gain or loss, we find that the system is stable (apart from quantum tunneling) if the particle number N is less than a critical number $N_c.$ For $N>\mathrm{}{N}_c,$ the system collapses to high-density clumps in a region near the center of the trap. The time for the onset of collapse is on the order of one trap period. Within numerical uncertainty, the results are consistent with the formation of a “black hole” of infinite density fluctuations, as predicted by Ueda and Huang [Phys. Rev. A (to be published)]. We numerically obtain $N_c\approx 1251.$ We then include gain-loss mechanisms, i.e., the gain of atoms from a surrounding “thermal cloud,” and the loss due to two- and three-body collisions. The number N now oscillates in a steady state, with a period of about 145 trap periods. We obtain $N_c\approx 1260$ as the maximum value in the oscillations.

We discuss the possibility of observing superfluid phenomena in a quasi-one-dimensional weakly interacting Bose gas at finite temperature. The weakness of interaction in combination with generic properties of one-dimensional liquids can result in a situation where the relaxation time of the supercurrent flow is much longer than the system lifetime, and the behavior of the system is indistinguishable from that of a genuine superfluid.

In a recent paper [R. R. McGowan and W. C. Schieve, Phys. Rev. A 59, 778 (1999)], the authors claim that the detection of the outcoming atoms has a back-action effect on the spectrum of the micromaser field. We point out that this assertion is wrong. In particular, we discuss the connection between atom statistics and field statistics, which has been stated to be the ultimate goal of the cited paper.

We extend earlier theoretical work on the creation of autoionizing wave packets by a single laser pulse to the case where the autoionizing states are excited by two or more laser pulses. The calculations are performed using R-matrix methods combined with time-dependent multichannel quantum-defect theory. Atoms or molecules that are coherently excited into autoionizing states emit electrons in a series of pulses. We have found that the phase does not change rapidly across an electron pulse if the laser has little chirp. By choosing the delay between the two laser pulses, we can have the electron wave packets interfere. This interference can be exploited to partially control when the electron will be emitted and to partially control the time dependence of the channel into which it will be ejected. A comparison of the interference of different ejected wave packets gives information about the probability and phase to be initially excited into different channels.

Details of the dynamics of ultrastrong-field photoionization are studied for a one-dimensional model atom with a short-range potential. The wave packet’s drift due to its scattering on the binding potential is analyzed. The Kramers-Henneberger frame transformation is generalized in such a way that the drift is formally included and the packet’s shape becomes stable in the new frame. Such an approach in combination with the Ehrenfest description of the drift trajectory allows one to model the behavior of the packet and to interpret the details of the atomic stabilization.

Exact expressions for the electron trajectory and energy are discussed, which predict very high acceleration, in vacuum, by a laser field and a uniform magnetic field. The laser field is modeled by a ${\sin }^2$ pulse and the initial electron motion, propagation of the laser pulse, and the magnetic field are all chosen in the same direction. An acceleration gradient in the TeV/m range within a very short travel distance of the electron is shown to be possible when a resonance condition is initially met. The acceleration and radiation properties are investigated using a recent analytic solution [F. H. M. Faisal and Y. I. Salamin, Phys. Rev. A 60, 2505 (1999)] of the corresponding relativistic equations of motion in the laboratory frame. The radiation losses are shown to remain small under the resonance condition and the associated weak emission spectra are found to be characterized by highly irregular line distributions.

The $n=2S$ metastable-state hyperfine interval in atomic hydrogen has been measured to a precision of 160 ppb by driving the magnetic-dipole radio-frequency transition in a thermal beam. The $2S$ hyperfine interval is found to be $177556785\pm 29\mathrm{Hz}$ which is the most precise measurement of this interval.

We analyze the coherent dynamics of optically excited alkenes in a fully correlated $3d$ tight-binding model with extended Hubbard interactions. The scenario that emerges is that the steric repulsive interactions are the driving force behind ultrafast cis-trans photoisomerizations. This resolves the apparent discrepancy between values for the torsional stiffness obtained from band-structure potentials and from vibrational spectra. The mechanism is illustrated in quantitative detail for ethylene and is also shown to yield a promising scenario for the coherent dynamics of molecules like retinal.

Using the concept of light-induced potentials introduced by Garraway and Suominen [Phys. Rev. Lett. 80, 932 (1998)], we study alternative schemes of adiabatic population transfer in a diatomic molecule from the lowest vibrational level of the ground electronic state to the lowest vibrational level of a third electronic state via allowed optical transitions from the ground and third electronic states to a second electronic state. Our chirped-pulse rapid adiabatic passage scheme is implemented with short-duration optical pulses, and can be understood in terms of “road design engineering” of the light-induced time-dependent adiabatic potential upon which the dynamics occurs. We explicitly show that complete adiabatic vibronic population transfer occurs using chirped picosecond-duration pulses in the sodium dimer example.

The imaging of a neutral atomic beam by electrostatic force has been demonstrated. An ultracold neon atomic beam in the ${1s}_3\left[{\mathrm{}\left(2p\right)\mathrm{}}^5{3s:}^1{P}_0\right]$ state illuminated a transparent object, passed through a set of three electrostatic lenses, and formed the image on a microchannel plate detector with the magnification of unity. Each lens consisted of two parallel cylinders that functioned as a concave lens along one axis and as a convex lens along the other axis. The atomic polarizability of neon in the ${1s}_3$ state has been also determined to be $2.8(\pm 0.1)\times {10}^{-39}\hspace{0.5em}{\mathrm{Fm}}^2.$

We construct a Mach-Zehnder interferometer using Bose-Einstein condensed rubidium atoms and optical Bragg diffraction. In contrast to interferometers based on normal diffraction, where only a small percentage of the atoms contribute to the signal, our Bragg diffraction interferometer uses all the condensate atoms. The condensate coherence properties and high phase-space density result in an interference pattern of nearly 100% contrast. The two arms of the interferometer may be completely separated in space, making it an ideal tool that can be used to detect vortices or other topological condensate phases.