The stochastic limit approximation method for “rapid” decay is presented, where the damping rate $\gamma$ is comparable to the system frequency $\Omega ,$ i.e., $\gamma \sim \Omega ,$ whereas the usual stochastic limit approximation is applied only to the weak damping situation $\gamma \ll \Omega .$ The key formulas for rapid decay are very similar to those for weak damping, but the dynamics are quite different. From a microscopic Hamiltonian, the spin-boson model, a Bloch equation containing two independent time scales is derived. This is a useful method to extract the minimal dissipative dynamics at high temperature $k_{B}T\gg ħ\Omega$ and the master equations obtained are of the Lindblad form unlike that of Caldeira and Leggett. The validity of the method is confirmed by comparing the master equation derived through this method with the exact one.

Classical communication through quantum channels may be enhanced by sharing entanglement. Superdense coding allows the encoding, and transmission, of up to two classical bits of information in a single qubit. In this paper, the maximum classical channel capacity for states that are not maximally entangled is derived. Particular schemes are then shown to attain this capacity, first for pairs of qubits, and second for pairs of qutrits.

Strong correlation in quadrupole photoionization amplitudes, along with strong correlation in dipole photoionization amplitudes, is found at keV photon energies. As a result, correlation is shown to profoundly influence the $E1\mathrm{}-E2\mathrm{}$ nondipole photoelectron angular distribution parameters, which themselves are found to be large. An approximate cancellation of correlation effects for measurements made using a certain geometry is demonstrated. Suggestions for future measurements to spotlight correlation in nondipole effects are indicated.

The cross sections for electron detachment of ${\mathrm{Si}}^{-}$ were measured for He, Ne, and Ar targets at relative velocities in the 0.25–1.4 a.u. range. Argon target cross sections were measured using ${\mathrm{Si}}^{-}$ ions from two different origins, the results agreeing well with each other and with literature values. The velocity dependence of the ${\mathrm{Si}}^{-}$ cross sections is strikingly similar to that of known ${\mathrm{H}}^{-}$ results, with the former being consistently larger than the latter and, at large velocities, differing by a multiplicative constant.

We discuss two models for electron attachment to molecules: the Vogt-Wannier model for capture into a polarization well and the resonance model for dissociative attachment. The Vogt-Wannier model is generalized for the case of a target with a permanent dipole moment, and results are presented for dissociative attachment to ${\mathrm{CH}}_3\mathrm{I}.$ It is shown that the resonance theory should incorporate in this case a weakly bound dipole-supported state of ${\mathrm{CH}}_3{\mathrm{I}}^{-},$ whereas the generalized Vogt-Wannier theory gives a reasonable estimate for the cross section in the meV and sub-meV region. The Vogt-Wannier model is also applied to the process of attachment to ${\mathrm{SF}}_6,$ ${\mathrm{CCl}}_4,$ and ${\mathrm{C}}_{60}.$ In the first case the s-wave capture model provides a satisfactory description of the experimental data for energies below the first vibrational excitation threshold, whereas for ${\mathrm{CCl}}_4$ it underestimates the attachment cross section by a factor of 2 in the sub-meV region. For ${\mathrm{C}}_{60}$ we suggest that electron attachment is dominated by s-wave capture in the region below 2 meV and by p-wave capture in the energy range above 4 meV. Our model reproduces data for Rydberg electron and free-electron attachment observed in beam experiments. It is, however, at variance with the strong rise of the attachment rate coefficients with electron temperature observed in flowing afterglow–Langmuir probe measurements.

We present a consistent variational approach to the vicinage effect on ion charges in a cluster made of partially stripped ions, based on the Brandt–Kitagawa theory for isolated ions, supplemented by dynamically screened interionic interactions. Interferences in cluster stopping power are shown to be strongly influenced by the ion-charge dependence on internuclear separations, and may give rise to energy loss enhancement or deenhancement, when compared to the case when the ion charges are frozen at the value corresponding to isolated ions at the same speed. In addition, the ion charge reduction is shown to be responsible for softening the interionic interactions responsible for Coulomb explosion. Calculations of the vicinage effect on ion charges are performed for a fast ${\mathrm{N}}_2$ molecule moving through a carbon target, and the roles of Coulomb explosion and multiple scattering in energy loss are discussed.

Maximum-likelihood estimation is applied to identification of an unknown quantum-mechanical process. In contrast to linear reconstruction schemes, the proposed approach always yields physically sensible results. Its feasibility is demonstrated by performing the Monte Carlo simulations for the two-level system (single qubit).

The quantum advantage arising in a simplified multiplayer quantum game is found to be a disadvantage when the game’s qubit source is corrupted by a noisy “demon.” Above a critical value of the corruption rate, or noise level, the coherent quantum effects impede the players to such an extent that the “optimal” choice of game changes from quantum to classical.

The formula for differentiation of functional $A\left[\rho \right]$ with respect to $\rho \left(\mathbf{r}\right)$ while keeping the normalization $\int \rho \left(\mathbf{r}\right)d\mathbf{r}$ of $\rho \left(\mathbf{r}\right)$ fixed is derived, and basic properties arising from it are concerned. The results are then generalized for time-dependent theories. One of the essential consequences of normalization conservation for functional differentiation, namely, that it ruins the symmetry in r of multiple derivatives, is shown to give the resolution of the causality paradox of response functions in time-dependent density-functional theory. The formula for the differentiation of functionals $A\left[\rho \right]$ that conserves the shape of $\rho \left(\mathbf{r}\right)$ is also presented.

A phase-shift analysis technique for two-channel scattering has been studied. The inputs to the study were the experimental differential cross sections and the unitarity condition was then used to extract the phase of the scattering amplitudes. A Newton iterative method based upon Frechet derivatives gave convergent results. The method was tested by using both simulated data and theoretical calculations for electron-helium scattering.

We have investigated the ionization dynamics of He-like, singly charged indium atoms using a short-pulsed Ti:sapphire laser at $\lambda =800\mathrm{}\mathrm{nm},$ in both linear and circular polarization. For linear polarization clear evidence of nonsequential double ionization of ${\mathrm{In}}^{+}$ was found in the intensity regime of ${10}^{14}–{10}^{15}{\mathrm{W}/\mathrm{c}\mathrm{m}}^2.$ The experimental data are excellently reproduced using an empirical multiple-ionization formula introduced previously. For circular polarization the ion yield data exhibit no distinct evidence of nonsequential double ionization.

Large nondipole effects are predicted in the angular distribution of photoelectrons from the $5s$ and $5p$ subshells of xenon for photon energies below 200 eV. The nondipole parameter ${\gamma }_{5s}$ exhibits a dispersion-curve variation near the first minimum of the $5s$ cross section at 35 eV, reaching a minimum value of $-0.8$ near 40 eV. Rapid variation of ${\gamma }_{5s}$ is also found near the second minimum of the $5s$ cross section at 150 eV, where ${\gamma }_{5s}$ reaches a maximum value of 1.2. Smaller, but significant, nondipole effects are also found in the parameter ${\zeta }_{5p}={\gamma }_{5p}+3\mathrm{}{\delta }_{5p},$ which has a maximum value of 0.15 near 50 eV, and a second maximum value of 0.18 near 160 eV. The higher energy maxima in ${\gamma }_{5s}$ and ${\zeta }_{5p}$ arise from correlation enhanced by shape resonances in the $4\overrightarrow{p}f$ quadrupole photoionization channels. These predictions are based on relativistic random-phase approximation calculations in which excitations from $5p,$ $5s,$ $4d,$ $4p,$ and $4s$ subshells are coupled.

The quadrupole and hexadecapole coupling constants for ${}^{127}\mathrm{I}$ in LiI were determined from relativistic Dirac-Fock electronic structure and self-consistent nuclear structure calculations. While the calculated quadrupole coupling constant agrees with the experimental value, the predicted hexadecapole coupling constant ranges between +6 and +20 mHz, which is of opposite sign and about three orders of magnitude smaller than the value deduced from recent high resolution radio-frequency molecular beam measurements [J. Cederberg, D. Olson, A. Nelson, D. Laine, P. Zimmer, M. Welge, M. Feig, T. Höft, and N. London, J. Chem. Phys. 110, 2431 (1999)].

We study the zero-temperature dynamics of Bose-Einstein condensates in driven high-quality optical cavities in the limit of large atom-field detuning in a one-dimensional model. We calculate the stationary ground state and the spectrum of coupled atom and field mode excitations for standing-wave cavities as well as for traveling-wave cavities. Finite cavity response times lead to damping or controlled amplification of these excitations. Analytic solutions in the Lamb-Dicke expansion are in good agreement with numerical results for the full problem and show that oscillation frequencies and the corresponding damping rates are qualitatively different for the two cases.

We analyze the atomic state obtained by photodissociation of a molecular condensate. The process is not amenable to the usual mean-field description, and we base our analysis on approximate equations for the second moments of the atomic field operators and on Monte Carlo simulations derived from a positive-P phase-space description of the system. Our calculations show that the dissociation process produces an atomic condensate in a squeezed state in analogy with the down-conversion process leading to squeezed light in quantum optics. Atomic interactions lead to a threshold similar to the one in optical down-conversion, and to an atomic “Kerr” effect, which ultimately deteriorates the degree of squeezing.

Multiphoton ionization profiles in xenon under three-photon excitation by azimuthally segmented conical laser beams have been studied and analyzed on the basis of the interference-based cooperative shift of the atomic resonance line. Segmentation of the conical wave front with various geometrical slits reduces the possible three-photon azimuthal sub-beam combinations in the third order process and a principal (or “homogeneous”) Lorentzian component can be selected from the overall excitation profile. This Lorentzian is identical to the pressure-broadened atomic line shifted by the cooperative shift. A possibility of controlling the nonlinear response by spatial selection of interacting waves in conical beams is discussed.

An alternative method of preparing pseudopure states for NMR quantum computing is presented. The method involves population transfer by the use of a multifrequency excitation pulse. The system studied was a spin-7/2 nucleus ${(}^{133}\mathrm{Cs})$ in a liquid-crystalline environment. It is shown that all eight pseudopure quantum states for this three-qubit system can be prepared by using only 1–3 pulses, while the intensities of the relevant peaks increase by a factor of up to 4 compared to the equilibrium NMR spectrum. A comparison of the intensity enhancement by population transfer for quadrupolar nuclei and that for coupled spin-1/2 systems is given.

We present a simple half-collision model that allows the approximate calculation of absolute cross sections for triple ionization as well as cross-section ratios ${\sigma }^{3+}/{\sigma }^{2+}$ of lithium by photon impact. The model decomposes the three-electron breakup process into a double ionization of the two inner electrons followed by electron-electron half-scattering of the receding electrons at the residual “spectator” $2s$ electron. We find surprisingly good agreement with recent experimental data.

We present direct spectroscopic measurement of the excited state angular-momentum-state distribution of ${\mathrm{Na}}_2^{*}(3\mathrm{}{}^2{S}_{1/2}+3\mathrm{}{}^2{P}_{3/2})$ molecules photoassociated by the trapping lasers in a sodium magneto-optical trap. We find that states with quantum numbers up to $J=22\mathrm{}$ are excited and that the measured population distribution can be well described using a local equilibrium model.

We demonstrate that the stochastic resonance phenomenon can occur in a pure quantum regime for a wider class of microscopic systems described by a dissipative anharmonic oscillator driven by two periodic forces. We apply a quantum-state diffusion method, and display the synchronization of quantum trajectories by the stochastic resonance process. The model proposed is accessible for experiments, and results in the formation of quantum states of the anharmonic oscillator, namely sub- and super-Poissonian statistics for time intervals exceeding the relaxation rate. We show that stochastic resonance phenomenon can be described in terms of the minimization of the quantum von Neuman entropy, and demonstrate the possibility of controlling both the dissipative dynamics and quantum statistics of anharmonic oscillator by two external periodic forces.