We demonstrate that the recently observed damping of Rabi oscillations of a trapped ${}^9{\mathrm{Be}}^{+}$ ion initially cooled to its motional ground state [Meekhof et al., Phys. Rev. Lett. 76, 1796 (1996)] is caused by quantum jumps to an auxiliary electronic state. This mechanism also predicts an asymmetric decay of the electronic-state population that is found to be in good agreement with the measured data.

The quantum teleportation process is composed of a joint measurement performed upon two uncorrelated subsystems A and B, followed by a unitary transformation (parameters of which depend on the outcome of the measurement) performed upon a third subsystem C (EPR correlated with system $B).\mathrm{}$ The information about the outcome of the measurement is transferred by classical means. It is shown that this measurement process, plus possible measurements on subsystem C (with the classical channel switched off), cannot be described by a local realistic theory.

A quantum computer that gives us the result of a single quantum computation has been constructed. The quantum register was realized by modes and polarization of photons, and the unitary transformation was implemented with linear optics. For each quantum computation, the answer to the Deutsch Jozsa problem for any four-bit digit is given by a single-photon detection signal with a small error rate of less than 4%.

Collision cross sections between oxygen molecules and helium atoms are computed at translational energies between 0.1 K and 10.0 K, motivated by the recently demonstrated cooling of molecules by a helium buffer gas. Detailed calculations based on a rigid-rotor model demonstrate the differences among various isotopic combinations of oxygen atoms and also serve to illustrate resonant features unique to cold molecules. Comparisons of elastic versus spin-changing inelastic collision rates suggest that buffer-gas cooling is likely to be a widely applicable tool for producing cold molecular samples.

Quasiclassical trajectory calculations are used to study the dynamics of endohedral ${\mathrm{Li}}^{+}$ ion migration along the inner wall of the ${\mathrm{C}}_{60}$ cage. The migration involves ion hopping (isomerization) from ring to ring via minimum-energy paths. The thermal rate coefficients for five- to six-member ring isomerization, $k_5,$ and for six- to five- and six- to six-member ring isomerization, $k_6,$ are given by the expressions $k_5=(4.0\mathrm{}\times {10}^{14})\exp \left[\right(-7800{\mathrm{c}\mathrm{a}\mathrm{l})/\mathrm{R}\mathrm{T}]\mathrm{}\mathrm{}\mathrm{s}}^{\mathrm{-}1}$ and $k_6=(5.6\mathrm{}\times {10}^{14})\exp \left[\right(-10000\mathrm{c}\mathrm{a}\mathrm{l})/\mathrm{R}\mathrm{T}]{\mathrm{s}}^{\mathrm{-}1}.$ The present work establishes that the ion is not static but moves around along the walls of the molecular cage.

We analyze a system of two coupled cavities, where one of them is leaky, namely, is interacting with the environment. We derive the master equation for the system, and show that the relaxation term, in the case of strongly coupled cavities, is not simply the standard one, which is obtained by neglecting the interaction between the two cavities, when calculating the effect of the bath. The relaxation term is calculated here by first diagonalizing the Hamiltonian of the coupled modes of the cavities, and then it is properly evaluated by eliminating the degrees of freedom of the reservoir, maintaining only the energy-conserving terms. Only by following this prescription one can guarantee that the coupled system approaches thermal equilibrium. We study both the steady-state solutions of the master equation, and the time evolution of the system from given initial conditions.

We have performed an absolute optical frequency measurement of the cesium $D_2$ line at 352 THz (852 nm). This frequency is an important scaling parameter in atomic interferometry. The $D_2$ line has been compared with the fourth harmonic of a methane stabilized He-Ne laser at $88.4\hspace{0.5em}\mathrm{THz}$ $\left(3.39\mathrm{}\hspace{0.5em}\mu \mathrm{m}\right).\mathrm{}$ A frequency mismatch of $1.78\hspace{0.5em}\mathrm{THz}$ between $4\times 88.4\hspace{0.5em}\mathrm{THz}=354\mathrm{}\hspace{0.5em}\mathrm{THz}$ and the $D_2$ line was bridged with an optical frequency comb generator. We find $f_{D2\mathrm{}}=351\mathrm{}725718.50\left(11\right)\mathrm{}\hspace{0.5em}\mathrm{MHz}$ for the hyperfine centroid, improving previous results by almost two orders of magnitude.

The absolute photoionization cross sections of ${\mathrm{I}}^{+}$ and ${\mathrm{I}}^{2+}$ ions have been measured from 45 to 140 eV, covering the region of $4d$ ionization where a large maximum, also known as a “giant resonance,” occurs in the cross section. For both ions, the maximum cross section appears near 90 eV and is measured to be 23(3) Mb and 24(4) Mb, respectively. This is significantly larger than that previously reported for atomic iodine, and only slightly smaller than the values calculated using the random-phase approximation with exchange.

Calculations of the vacuum-polarization screening corrections to the low-lying energy levels of He-like highly charged ions are presented. The calculations are carried out for extended nuclei in the range $Z=20\mathrm{}–100.$

We present an approach to studying the Casimir effects by means of the effective theory. An essential point of our approach is replacing the mirror separation into the size of space $S^1$ in the adiabatic approximation. It is natural to identify the size of space $S^1$ with the scale factor of the Robertson-Walker-type metric. This replacement simplifies the construction of a class of effective models to study the Casimir effects. To check the validity of this replacement we construct a model for a scalar field coupling to the two-dimensional gravity and calculate the Casimir effects by the effective action for the variable scale factor. Our effective action consists of the classical kinetic term of the mirror separation and the quantum correction derived by the path-integral method. The quantum correction naturally contains both the Casimir energy term and the back-reaction term of the dynamical Casimir effect, the latter of which is expressed by the conformal anomaly. The resultant effective action describes the dynamical vacuum pressure, i.e., the dynamical Casimir force. We confirm that the force depends on the relative velocity of the mirrors, and that it is always attractive and stronger than the static Casimir force within the adiabatic approximation.

We study propagation of the decohering influence caused by a local measurement performed on a distributed quantum system. As an example, the gas of bosons forming a Bose-Einstein condensate is considered. We demonstrate that the local decohering perturbation exerted on the measured region propagates over the system in the form of a decoherence wave, whose dynamics is governed by elementary excitations of the system. We argue that the post-measurement evolution of the system (determined by elementary excitations) is of importance for transfer of decoherence, while the initial collapse of the wave function has negligible impact on the regions that are not directly affected by the measurement.

Given the result of a measurement on the output of a quantum optical communication channel, we show how to calculate a retrodictive state at the input. This state can be used by the receiver to determine the probability that any one of a given set of states was selected by the transmitter. We establish the remarkably simple result that retrodicting the prepared input signal for an attenuating (amplifying) channel corresponds to predicting the measured output signal for an amplifying (attenuating) channel.

Light emission induced by inner-shell excitation of ${\mathrm{N}}_2$ molecules has been investigated in the wavelength region $250<~{\lambda }_{\mathrm{fluo}}<~1000\hspace{0.5em}\mathrm{nm}.$ Synchrotron radiation from the SuperACO storage ring in Orsay (France) has been used for the photoexcitation and a high-resolution secondary monochromator $(\Delta {\lambda }_{\mathrm{fluo}}=0.2\mathrm{}\mathrm{nm})$ for the spectral analysis of the fluorescence light. Vibrationally resolved molecular ${\mathrm{N}}_2^{+}$ $(B{}^2{\Sigma }_u^{+}\to X{}^2{\Sigma }_g^{+})$ transitions show clearly the coupling between the $\left(1\mathrm{}\overrightarrow{s}{\pi }^{*}\right)$ core excitation and the ionization of the $\left(2\mathrm{}{\sigma }_u\right)$ valence electron. The main contribution to the fluorescence spectra stems from excited neutral and excited singly charged atoms produced by the dissociation of highly excited molecular states. The singly ionized ${\mathrm{N}}^{+}$ atoms are produced upon resonant $\left(1\mathrm{}\overrightarrow{s}{\pi }^{*}\right)$ excitation as well as upon excitation to the ${(1s}^{-1}-{\sigma }^{*})$ continuum resonance. Fluorescence transitions of the type ${\mathrm{N}}^{+}$ ${1s}^2{2s}^{2}2p4\overrightarrow{f}{1s}^2{2s}^{2}2p3d$ as well as $2p3\overrightarrow{d}2p3p$ and $2p3\overrightarrow{p}2p3s$ have been analyzed. The neutral N fragment is found nearly exclusively after excitation of the ${(1s}^{-1}-{\pi }^{*})$ resonance, and mainly radiative decay of the N ${1s}^2{2s}^2{2p}^{2}3p$ excited states has been observed. The obtained experimental results represent additional and very detailed information on the final states of the molecular dissociation and provide a necessary basis for a theoretical treatment of the relaxation processes of inner-shell excited molecules.

The motion of circular wave packets for one electron in a central Coulomb field with a high atomic number Z is calculated. The wave packet is defined in terms of solutions of the Dirac equation in order to take into account all possible relevant effects, in particular the spin-orbit potential. A time scale is defined within which spin dynamics must be taken into account, mainly in atoms with high Z. Within this time scale there exists a mechanism of collapses and revivals of the spin, already shown by the authors for harmonic oscillator potential and called the spin-orbit pendulum. Though this effect does not have the exact periodicity of the simpler model, the wave packet spatial motion is nevertheless quite similar.

A revised simplified relativistic configuration-interaction method is used to study the dielectronic recombination processes of He-like carbon ions. Every resonance of type $1s2Lnl$ has been determined for $n=2,3.\mathrm{}$ The total integrated cross sections of the dielectronic recombination are in agreement with the experimental measurements within 10%. It is found that the strong resonance peak around 283 eV consists of three important doubly excited resonance states with the dominant contribution due to two $\left(1s2p{}^{1}P\right)3d{}^2{F}_{5/2,7/2}$ states, in contrast to the previous assignment of $\left(1s2p{}^{1}P\right)3p{}^2{S}_{1/2}$ as the dominant resonance state.

The single ionization of multielectron atoms by the impact of protons, antiprotons, and highly charged bare ions is studied. The targets are treated as one-electron atoms, where the interaction of the active electron with the rest of the target is represented by a model potential. The final electronic state is described by the product of two Coulomb wave functions that put the distortion due to the projectile and the target on equal footing. We calculate the total and differential ionization cross sections for helium- and lithium-atom targets. The results thus obtained are compared with existing experimental data and other available theoretical predictions.

In this work, we will provide the full details of the coupled-channels-optical method as applied to study positron scattering from atomic hydrogen at intermediate energies. An optical potential method for the continuum has been implemented within the close-coupling framework that includes both the positron-hydrogen and positronium-proton channels. Elastic, excitation, ionization, positronium formation, and total cross sections are reported and compared to other available theoretical and experimental data.

The dissociation of molecular ions $({\mathrm{N}}_2{)}^{q+}$ produced by 4.7 MeV/amu ${\mathrm{Bi}}^{57+,25+}$ and 5.9 MeV/amu ${\mathrm{Xe}}^{43+,18+}$ ion impact on ${\mathrm{N}}_2$ has been studied using the coincidence technique with a position- and time-sensitive multiparticle detector. The kinetic energy release distributions for all observed ion pairs ${\mathrm{N}}^{q_1+}+{\mathrm{N}}^{q_2+}$ are measured. Analysis of the distributions is made with emphasis on the highly charged-molecular ions, $q>\mathrm{}3.\mathrm{}$ In these cases the most probable total kinetic energy of the two fragments is quite well described by the Coulomb explosion model for point charges, while the measured width of the distributions is much larger than that predicted by this simplified model. We therefore suggest a more elaborate model of Coulomb fragmentation based on a statistical description of the individual potential energy curves of the molecular ion. This enables us to describe both the position and the width of the kinetic energy release distributions. We suggest that the main cause of the large width of the distributions for the high q values is the spread of the potential energy curves at the equilibrium internuclear distance; this is mainly due to the dispersion interaction arising from the mutual polarization of the atomic ions.

An entirely analytical method for computing Regge-pole trajectories for singular potentials (potentials diverging faster than $r^{-2}$ at the origin) is presented. Explicit results are presented for polarization ${(1/r}^4)$ and Lennard-Jones (6,12) potentials. Numerical precision of the calculations is fully controlled and often better than the state of the art numerical methods. The present method combines two different analytical approaches. The first one obtains a perturbation expansion of the Regge trajectories in terms of an appropriate parameter. Then the convergence of the resulting asymptotic series is improved through Padé approximations. Highly accurate results are then possible within a wide range of energies. For typical parameters used in the literature, the two-term expansion, for which explicit formulas are presented, gives at least five digits of accuracy, while higher approximations give up to 14 digits of accuracy.

We report a study of fine-structure mixing $\mathrm{Rb}\left(5\mathrm{}{}^2{D}_{5/2}\right)\to \mathrm{Rb}\left(5\mathrm{}{}^2{D}_{3/2}\right)$ in collisions with ground-state Rb atoms. In the experiment, two-photon cw laser excitation was applied to the Rb vapor cell. The measured cross section for the process was $(5.8\mathrm{}\pm 1.9)\times {10}^{-14}{\mathrm{cm}}^2.$ Theoretical calculations using nonadiabatic collision theory gave a value of $3.4\times {10}^{-14}{\mathrm{cm}}^2.$