We investigate a class of entangled states, which we call superentangled, that have EPR correlations analogous to those in the vacuum state of a relativistic quantum field. We show that whenever superentangled states exist in any quantum theory, they are dense in its state space. We also give prescriptions for constructing superentangled states that involve an arbitrarily large collection of systems.

We consider a channel coding for sending classical information through a quantum channel with a given ensemble of quantum states (letter states). As is well known, it is generically possible in a quantum channel that the transmittable information in block coding of length $n$ can exceed $n$ times the maximum amount that can be sent without any coding scheme. This so-called superadditivity in classical capacity of a quantum channel is a distinct feature that cannot be found in a classical memoryless channel. In this paper, a practical model of channel coding that shows this property is presented. It consists of a simple code-word selection and the optimum decoding of the code words minimizing the average error probability. At first, optimization of decoding strategy is discussed. Then the channel coding that shows the superadditivity in classical capacity is demonstrated.

A physical model for the optimum collective decoding that attains the minimum average error probability in distinguishing code-word states is presented. This model is based on a cavity QED technique which is available at present. It will open a possibility for a quantum decoder that realizes the superadditivity in classical capacity of quantum channel which was demonstrated in M. Sasaki, preceeding paper, Phys. Rev. A 58, 146 (1998).

A formulation for calculating the one-photon self-energy of an electron in an external field completely in coordinate space, described in an earlier paper, is applied to the calculation of the self-energy of an electron in a Coulomb field by two different numerical methods. In one numerical approach, a high-order asymptotic expansion of the Coulomb Green function is employed to accelerate the convergence of the summation over angular momentum. In the other, a term-by-term subtraction of the divergent terms in the summation over angular momentum is employed. The latter method is found to be preferable from the point of view of both simplicity and precision. Numerical results are compared to the results of previous calculations.

We present precise results for the binding energies and sizes of the ${\mathrm{He}}^{+}$ ion moving across a strong pulsar-type magnetic field. Similarly to the previously studied case of hydrogen, the ion is strongly deformed by the action of the motion-induced Stark forces. Unlike the case of the neutral hydrogen atom whose transverse motion gives rise—for every discrete state of the nonmoving atom—to a continuum of displaced energy states with changing transverse momentum, transverse motion of the ${\mathrm{He}}^{+}$ ion gives rise to a discretely spaced energy spectrum. A quantitative understanding of this problem and related opacities is central in modeling neutron star atmospheres, and should help in the interpretation of thermal emission from radio pulsars.

The techniques of Rydberg electronic wave packets are used to explore the classical limit states of helium. A class of shape-preserving orbits is studied under the classical adiabatic approximation that separates the dynamics of the two electrons. These states form the classical basis for two-electron wave packet states whose hydrogenic counterparts are shown to be elliptic states in the presence of a rotating electric field.

In a weak magnetic field coherent dark resonances in cesium vapor are split into up to 15 resolved components, depending on field direction and laser polarizations. We find that the selection rules are different for vapor cells with and without buffer gas due to a change in multipolarity of the two-photon coupling. At low laser intensities or sufficiently high buffer-gas pressure optical pumping between different dark resonances can be neglected so that a simple model allows one to calculate the relative line strengths, giving complete agreement with the experimental spectra.

Auger rates are calculated for some unrealistically low-lying states of antiprotonic helium. Two methods are used. One method, which makes use of hydrogenic functions and the golden rule, should give accurate rates. The other method treats the Auger process as inelastic scattering in one dimension. It makes use of a two-state approximation and the modern Born-Oppenheimer approximation, and it relates the transition rate to a ratio of probability currents for radial motion on two coupled one-dimensional adiabatic potential surfaces with an avoided crossing. In most instances this method leads to good agreement with the golden rule if only one traversal of the crossing is taken into account, and it demonstrates that the transition can be thought of as occurring at a rather well-defined radial separation between the $\overline{p}$ and the nucleus, in accord with the Franck-Condon principle. It is found that taking into account many traversals of the crossing would lead to fairly good agreement with the golden rule if the phases of the separate contributions to the transition amplitude from these traversals are effectively random. It is argued that radial and (perhaps) angular motion would ultimately produce the requisite phases if the finite width of the initial state were to be taken into account. The calculations for antiprotonic helium are then used as the basis for a conjecture that a phase difference associated with the angular motion of a stopping ${\mu }^{-}$, ${\pi }^{-}$, or $K^{-}$ might largely account for the very striking, shell-dependent regularities that have been observed in x-ray yields from exotic atoms formed in heavier elements.

Using a configuration-interaction approach, we study statistics of the dipole matrix elements $(E1\mathrm{}$ amplitudes) between the 14 lower states with $J^{\pi }{=4\mathrm{}}^{-}$ and 21st to 100th even states with $J=4\mathrm{}$ in the Ce atom (1120 lines). We show that the distribution of the matrix elements is close to Gaussian, although the width of the Gaussian distribution, i.e., the root-mean-square matrix element, changes with the excitation energy. The corresponding line strengths are distributed according to the Porter-Thomas law which describes statistics of transition strengths between chaotic states in compound nuclei. We also show how to use a statistical theory to calculate mean-squared values of the matrix elements or transition amplitudes between chaotic many-body states. We draw some support for our conclusions from the analysis of the 228 experimental line strengths in Ce [J. Opt. Soc. Am. 8, 1545 (1991)], although direct comparison with the calculations is impeded by incompleteness of the experimental data. Nevertheless, the statistics observed give evidence that highly excited many-electron states in atoms are indeed chaotic.

The lifetime of the $1s2s{}^3{S}_1$ level of the He-like ${\mathrm{O}}^{6+}$ ion has been measured using the Electron Beam Ion Trap in the magnetic trapping mode. A value of ${956}_{-4}^{+5\mathrm{}}$ $\mu \mathrm{s}$ is found, which corresponds to a radiative transition rate of ${1046}_{-5}^{+4\mathrm{}}$ ${\mathrm{s}}^{-1}$ for the magnetic dipole transition to the ${1s}^2{}^1{S}_0$ ground state. This value is in excellent agreement with recent theoretical predictions and distinguishes among different treatments of negative energy states and correlation in multiconfiguration Dirac-Fock calculations.

We investigate Ps-H scattering at medium energies using the six-state $\left[\mathrm{Ps}\mathrm{}\right(1s,2s,2p\left)\mathrm{}\mathrm{H}\mathrm{}\right(1s,2s,2p\left)\mathrm{}\right]$ close-coupling approximation (CCA). Target inelastic cross sections are reported using CCA as well as the first Born approximation. The effect of the excitation of the target in addition to the excitation of the projectile Ps atom enhances the target elastic cross sections appreciably. van der Waals interaction is responsible for this enhancement.

A renormalization of the $D$-dimensional Hamiltonian is developed to ensure that the large-$D$ limit corresponds to a single well at any value of the internuclear distance $R.\mathrm{}$ This avoids convergence problems caused by a symmetry-breaking transition that is otherwise expected to occur when $R$ is approximately equal to the equilibrium bond distance $R_{\mathrm{eq}},$ with larger $R$ giving a double well. This symmetry breaking has restricted the applicability of large-order perturbation theory in $1/D$ to cases where $R$ is significantly less than $R_{\mathrm{eq}}.$ The renormalization greatly extends the range of $R$ for which the large-order expansion can be summed. A numerical demonstration is presented for H${}_2^{+}.$ The $1/D$ expansions are summed using Padé-Borel approximants with modifications that explicitly model known singularity structure.

This paper presents a combined experimental and theoretical investigation of the binding energy of the metastable ${\mathrm{Ar}}^{-}{(3p}^{5}4s4p{}^4{S}_{3/2}^e)$ ion. Utilizing the laser-photodetachment threshold technique combined with state-selective detection of neutral atoms by resonant ionization spectroscopy, we find a binding energy of $32.5\pm 1.0$ meV with respect to the ${3p}^{5}4s{}^3{P}_2^o$ state of neutral argon. Our calculations, based on many-body perturbation theory, yield a binding energy of 24.9 meV. As a test of the present computational method, we also include the results of a comparative study of ${\mathrm{Ca}}^{-}$.

Collisional relaxation and fine-structure mixing rate constants have been measured for the metastable $D$ doublet ${(3D}_{3/2},$ ${3D}_{5/2})$ of ${\mathrm{Ca}}^{+}$ ions in the presence of different neutral gases completing a previous work. Experimental results are given for He, Ne, Ar, ${\mathrm{H}}_2,$ ${\mathrm{N}}_2,$ and ${\mathrm{CH}}_4.$ These data have been obtained with a cloud of approximately ${10}^5$ ${\mathrm{Ca}}^{+}$ ions stored in a Paul trap at kinetic energies of around 1 eV. The investigated levels were populated by direct excitation of the corresponding $4S-3D$ transitions. The involved time constants were determined from the temporal evolution of the fluorescence of one of the resonance transitions, following the optical pumping of the $P$ level from the $D$ component. Comparison of our data with other similar experimental and theoretical work is finally presented.

The combination of recoil ion and ionized electron momentum spectroscopy provides an unparalleled method to investigate the details of ion-atom collision dynamics in kinematically complete experiments. To predict single ionization scattering behavior at the level now realized by experiment, the classical trajectory three-body Monte Carlo method has been used to obtain complete momenta information for the ionized electron, recoil ion, and projectile in the collision plane defined by the incident projectile and outgoing recoil ion. Strongly coupled systems were considered where the charge state of the projectile divided by the speed of the collision $q/v$ is greater than unity. Illustrated are 3.6-MeV/nucleon ${\mathrm{Se}}^{28+}$ and 9.5-MeV/nucleon ${\mathrm{Ni}}^{26+}$ collisions on He where experimental data are available. The theoretical results are in good agreement with these data and calculations have been performed for 165-keV/nucleon and 506-keV/nucleon ${\mathrm{C}}^{6+}+\mathrm{He}$ to compare results for the same $q/v$ perturbation strengths, but at much lower velocities. In all cases the ejected electrons are found to be preferentially emitted opposite to the recoil ion in the projectile-recoil collision plane. The 165-keV/nucleon ${\mathrm{C}}^{6+}$ spectra are especially rich in that electron capture strongly contributes to the overall electron loss process. Here, the electron capture to the continuum (ECC) spectrum is observed to have not only the known asymmetry in the longitudinal direction, but also has an almost complete asymmetry in the collision plane opposite to the recoil ion. Collision plane spectra differential in the transverse momenta of the recoil ion depict the transition from soft electrons for low transverse recoil momenta, to two-center, and ECC electrons for increasing transverse recoil ion momenta.

We present scattering from many-body systems in a new light. In place of the usual van Hove treatment, (applicable to a wide range of scattering processes using both photons and massive particles) based on plane waves, we calculate the scattering amplitude as a space-time integral over the scattering sample for an incident wave characterized by its correlation function, which results from the shaping of the wave field by the apparatus. Instrument resolution effects, seen as due to the loss of correlation caused by the path differences in the different arms of the instrument, are automatically included and analytic forms of the resolution function for different instruments are obtained. Each element of the apparatus is associated with a correlation length (or time). These correlation lengths, determined by the dimensions of the apparatus, are generally much smaller than these dimensions and larger than the wavelength. As is well known, these are the conditions for the validity of geometrical optics so that the conventional treatment, where the scattering is calculated by the van Hove plane-wave approach and the trajectories through the instrument are treated classically, is usually valid. In the present approach analytic expressions for the correlation functions are obtained. The intersection of the moving correlation volumes (those regions where the correlation functions are significant) associated with the different elements of the apparatus determines the maximum correlation lengths (times) that can be observed in a sample, and hence, the momentum (energy) resolution of the measurement. This geometrical picture of moving correlation volumes derived by our technique shows how the interaction of the scatterer with the wave field shaped by the apparatus proceeds in space and time. Matching of the correlation volumes so as to maximize the intersection region yields a transparent, graphical method of instrument design.

The process of electron capture to the continuum (ECC) has been studied at non-0° electron emission angles in a type of experiment in which the electrons ejected in a given direction are detected in coincidence with the outgoing projectiles scattered on the target nucleus in the same direction. Triply differential cross sections (differential with respect to the energy and angle of the ejected electron and the angle of the scattered projectile) have been determined for 75-keV proton on Ar collisions at scattering angles 3.7°, 5.0°, and 8.1°. The occurrence of the cusp peak in the coincidence spectra gives an evidence for the existence of the ECC process at non-0° emission angles. The experimental data are compared with results of first-order distorted-wave Born and classical trajectory Monte Carlo (CTMC) calculations.

Absolute cross sections are reported for the electron-impact dissociative excitation of the deuterated hydrocarbon ions ${\mathrm{CD}}_n^{+}$ $(n=2\mathrm{}-5)$ from threshold to 70 eV using a crossed beams technique. The method focuses on detecting and measuring light dissociation-product ions from a heavy target. The ion targets are typical of those formed in the discharge ion source, and some may be in excited electronic and/or vibrational states. A common feature of the studied targets was a dominant ${\mathrm{D}}^{+}$ formation cross section with almost the same absolute value of $2\times {10}^{-16}$ ${\mathrm{cm}}^2$ for all $n$ from 25 to 70 eV. The present measurements are compared with the available data, and for ${\mathrm{D}}^{+}$ from ${\mathrm{CD}}_2^{+}$ and ${\mathrm{CD}}_5^{+}$ the data agree well with recent results obtained with the storage-ring technique.

We describe complex Kohn calculations for elastic $e^{-}-{\mathrm{CF}}_4$ scattering at incident electron energies of 0.04–20 eV. An accurate representation of the electronic correlation effects involving target polarization is critical in the explicit demonstration of a Ramsauer-Townsend minimum. We use a set of polarized virtual orbitals to form a compact representation of closed channels in the trial scattering function to include these effects. These are the first ab initio calculations to verify the Ramsauer-Townsend minimum in ${\mathrm{CF}}_4.$ We find excellent comparison between the complex Kohn results and measured differential and integral cross sections.

Core-excited states of the ${\mathrm{CS}}_2$ molecule have been studied in high-resolution angular-resolved experiment. The total ion yield and the angular resolved electron-ion coincidence (PEPICO) spectra of the $\mathrm{S}\mathrm{}\left(2p\right)\mathrm{}$ and $\mathrm{C}\mathrm{}\left(1s\right)\mathrm{}$ excited molecule have been measured. Bending vibrations are excited at the $\mathrm{S}{(2p}^{-1}){\pi }_J^{*}$ state, while the linear geometry is preserved at the $\mathrm{S}{(2p}^{-1}){\sigma }_J^{*}$ state and at the $\mathrm{S}{(2p}^{-1})nl$ Rydberg states. The occurrence of strong vibrational coupling is observed at the $\mathrm{S}\mathrm{}\left(2p\right)\mathrm{}$ excited Rydberg states, which gives rise to excitations of asymmetric stretching modes. Additionally, we show that transitions to the valence $\mathrm{S}{(2p}^{-1}){\pi }_J^{*}$ and $\mathrm{S}{(2p}^{-1}){\sigma }_J^{*}$ states exhibit a strong atomic character, while transitions to the Rydberg states are of molecular type. At the carbon edge the measurements reveal excitations of the nonzero symmetric stretching vibrations at the $\mathrm{C}{(1s}^{-1}){\pi }^{*}$ and the Rydberg levels. Above the $\mathrm{C}{(1s}^{-1})$ threshold doubly excited states have been observed and their assignment is proposed.