We describe a simplified scheme for quantum logic with a collection of laser-cooled trapped atomic ions. Building on the scheme of Cirac and Zoller, we show how the fundamental controlled-NOT gate between a collective mode of ion motion and the internal states of a single ion can be reduced to a single laser pulse, and the need for a third auxiliary internal electronic state can be eliminated.

We present another version of the Bell inequality experiment employing four harmonic oscillators proposed by Yurke and Stoler [Phys. Rev. A 51, 3437 (1995)]. In the present gedanken experiment, the interaction time between two oscillators plays the same role as the polarization angle in the standard version of the experiment employing two photons. We obtain a correlation between two causally disconnected pairs of oscillators which violates Bell's inequalities.

The generic two-level model with time-dependent matrix elements becomes soluble in the limit that its diagonal and off-diagonal terms vary along a flat ellipse, encircling the diabolical singular point in the parameter space. The time evolution of the state vector is explicitly obtained, and the condition for its evolution to form a closed circuit in the projective Hilbert space of rays is given as a result of destructive interference at level crossing. The Aharonov-Anandan geometrical phase is shown to be related to the Stokes phase for the Landau-Zener model, which is a natural extension of Berry's phase to nonadiabatic evolutions.

The quantum Zeno effect of a single system is the effect of a series of measurements on the unitary time evolution of the system and on the ability to monitor this time evolution using the measurement results. This effect is shown to be equivalent to that of the indetermination of the quantum state of a single system [Phys. Rev. Lett. 74, 4106 (1995)], in which one considers the statistics of the results of a series of measurements performed on a single system, with no time evolution in between successive measurements.

Multichannel quantum-defect theory is extended to treat molecular dissociation processes simultaneously with ionization processes. Allowance is made for nuclear momentum coupling and for coupling with core excited channels. This theory combines the advantages of several less comprehensive approaches and is noniterative, nonperturbative, and applicable for arbitrary interaction strength. Several examples showing good agreement with experimental results relating to the ${\mathrm{H}}_2$ molecule are given.

Anisotropy in the photoinduced transient absorption change originating from two-dimensional transitions, i.e., transitions of twofold spatial degeneracy, is investigated. The polarization ratio of the transient absorption change is derived for randomly oriented molecules with two-dimensional transitions. It is shown that the ratio is either 4:3 or 8:1, depending on the nature of transitions involved, while in randomly oriented molecules with one-dimensional character it is always 3:1. This specific anisotropy is experimentally demonstrated with a solution of molecules with ${\mathrm{C}}_{4\mathrm{v}}$ symmetry, a vanadylphthalocyanine derivative, by means of subpicosecond pump-probe spectroscopy. A polarization ratio of 4:3 is observed both in bleaching and in excited-state absorption, reflecting the two-dimensional character of the molecule.

A computation of ${}_{}{}^{87}{}_{}{}^{}\mathrm{-}_{}^{87}{}_{}{}^{}$Rb spin-exchange rate constants as functions of the two-body singlet scattering length ${\mathrm{a}}_{\mathrm{s}}$ demonstrates that the inelastic collision rate is suppressed over a small range of the possible values for ${\mathrm{a}}_{\mathrm{s}}$. A two-channel model relates this inelastic suppression to an interference phenomenon, manifested in the near coincidence of the singlet and triplet scattering lengths. This mechanism explains the diminished rates measured in recent ``double-trap'' experiments. Combining information extracted from these rates and from previous scattering length measurements allows us to place bounds on the ${}_{}{}^{87}{}_{}{}^{}\mathrm{Rb}$ singlet scattering length (74–102 a.u.).

The observation of a heteronuclear reverse energy pooling process is reported. The cross section for the collision processes K(5D)+Na(3S)→K(4P)+Na(3P) has been measured and compared to calculations carried out using a multicrossing Landau-Zener model.

The energy spectrum of electrons emitted in the single-electron detachment process in collisions of 200-keV ${\mathrm{He}}^{\mathrm{-}}$ ions with He was measured. Taking advantage of the method of the 0° electron spectroscopy carried out with good angular and energy resolution, the lines from the decay of the (${1\mathrm{s}2\mathrm{p}2\mathrm{p}}^{|\mathrm{I}\mathrm{H}}$) ${}_{}{}^4{}_{}{}^{}\mathrm{P}_{}^{\mathrm{e}}{}_{}{}^{}$ shape resonance of ${\mathrm{He}}^{\mathrm{-}}$ were resolved, and the absence of the characteristic cusp peak in the double-differential cross section was established.

Very slow, highly charged ions impinging on metal surfaces are known to be accelerated by their image and to drop irremediably on the surfaces which they touch or slightly penetrate. We present experiments which demonstrate that above insulators or semiconductors, at normal incidence, the ions are backscattered at a certain distance from the surface and do not touch it. This finding is explained by the transient buildup of positive charges due to the removal of valence electrons, which overcome the acceleration of the ion by its own image. This effect is found to cancel out at grazing incidence.

We present a mean-field model of a weakly interacting Bose condensate in a harmonic potential. The effect of interactions is included as a level shift. We use the model to calculate the transition temperature, the condensate fraction, and the heat capacity. Our results show good agreement with recent experimental data on dilute alkali-metal vapors.

We consider the interaction between a single-mode atomic trap and a set of external modes that are decribed by a continuum of eigenstates. This theory is designed to model output coupling from current and proposed Bose-Einstein condensation experiments and proposed models for atom lasers. This work generalizes the optical input-output formalism [M. J. Collett and C. Gardiner, Phys. Rev. A 30, 1386 (1984)].

We have observed surprising detailed structure in the photoelectron kinetic-energy spectrum of xenon under high-intensity short pulse conditions. We show that most of the photoelectrons with kinetic energies from 0–50 eV result from resonant processes at intensities up to 1.9×${10}^{14}$nW/${\mathrm{cm}}^2$. In particular, we find that the high-energy photoelectron structure in above-threshold ionization is actually composed of narrow individual peaks whose energy positions do not shift with intensity. The amplitudes of the structures change rapidly with intensity and turn on at different specific intensities. While those structures appear to be due to resonances, they cannot be attributed to traditional Rydberg transient resonances.

We report on an experimental observation of correlated atom-photon pairs produced in an atom-optical experiment. The pairs are generated from an excited atom that spontaneously emit a single photon. Clear evidence for the pair identification as well as correlation measurements in the atomic motion are reported.

We report on experiments investigating the optimization of laser-ablated plasmas which are used to produce recombination-pumped, short-wavelength lasers. We evaluate the density of electrons and neutral atoms in laser ablated lithium and carbon plasmas as a function of time and distance away from the ablated target surface. We use an interferometric technique which can reveal information about the temperature of the plasma electrons. We find that the cold electrons which result in gain in recombination-pumped lithium lasers on the Lyman-α transition are produced by the high-intensity pump pulse rather than the lower intensity ablating pulse.

An upper limit is given to the amount of quantum information that can be transmitted reliably down a noisy, decoherent quantum channel using the high-probability states of quantum sources. A class of quantum error-correcting codes is presented that allows the information transmitted to attain this limit. The result is a quantum analog of Shannon's bound and code for the noisy classical channel [C. E. Shannon and W. Weaver, The Mathematical Theory of Communication (University of Illinois Press, Chicago, 1948)].

The electromagnetic field is quantized for normal transmission of incident waves through a parallel-sided dielectric slab. The dielectric material is dispersive and it acts as a linear amplifier over limited ranges of the frequency and as a linear attenuator at the remaining frequencies. The field operators derived for the three spatial regions within and on either side of the slab are shown to satisfy the canonical commutation relations. The noise fluxes emitted by the slab are evaluated and shown to satisfy the general requirements for the minimum noise associated with linear amplifiers and attenuators. The behavior of the amplifier gain profile on the approach to the lasing threshold of the slab is determined, but the results are restricted to the below-threshold state of the system. The spectra of the electric-field fluctuations are evaluated for the three spatial regions and for amplifying and attenuating frequencies.

A reduced dimensional control dynamics formulation is presented for an arbitrary number of degrees of freedom. The traditional control feedback partial differential equation now becomes a small set of ordinary differential equations amenable to efficient numerical solution. This approach is capable of finding optimal solutions with only a few feedback iterations, thereby dramatically reducing the effort to obtain control designs. As an example, the reduced formalism is applied to selective dissociation of a triatomic molecule.

A fully quantum-mechanical theory of multiwave mixing in a three-level cascade atomic system with two field modes is presented. The top and bottom levels are coupled with a strong external field. Certain conditions on the Rabi frequency and detuning of the driving field have been worked out for the violation of Bell's inequality and for when the system may behave as nondegenerate parametric oscillator.

The power spectral formula of Cerenkov radiation of the system with two equal charges is derived in the framework of the source theory. The distance between charges is supposed to be relativistically contracted, which manifests in the spectral formula. The knowledge of the spectral formula then can be used to verify the Lorentz contraction of the relativistic length. The radiative corrections to the Cerenkov effect are also considered. A feasible experiment for the verification of the Lorentz contraction is suggested.