We have searched for parity nonconserving (PNC) optical rotation in five optical transitions within the ${4f}^6{6s}^2$ ground configuration of atomic samarium using laser polarimetry. It had been suggested that these transitions might be favorable for studying electroweak effects because the upper levels have close-lying levels of opposite parity which might give rise to strong mixing by the PNC Hamiltonian $H_{\mathrm{PNC}}.$ In terms of the usual parameter $R[=\mathrm{Im}({\mathcal{E}}_{\mathrm{PNC}}/\mathcal{M}\left)\right],$ we obtain (in units of ${10}^{-8}$) $R_{662}=2.1\mathrm{}\left(1.8\right),$ $R_{628}=-7\left(16\right),$ $R_{611}=-6\left(8\right),$ $R_{569}=-17\left(30\right),$ and $R_{558}=-6\left(12\right),$ where the subscripts give the wavelengths of the transitions. The values of R are not significantly greater than those already reported for the heavy elements bismuth, lead, and thallium, and we conclude that the samarium transitions do not offer the prospect of a critical test of electroweak theory. Upper limits on the matrix elements of $H_{\mathrm{PNC}}$ between the upper levels of the transitions and their close-lying opposite-parity neighbors are derived from the results.

Thus far, all proposals for an optical evidence of macroscopic quantum interference have been frustrated by the low detector efficiencies. Two innovations permit such an evidence with currently available equipment. We refer to the generation scheme of entangled states by parametric amplification proposed by S. Song, C. M. Caves, and B. Yurke [Phys. Rev. A 41, 5261 (1990)]. Setting a low gain for the parametric amplifier introduces a drastic cutoff in the generated photon-number probabilities that compensates for the broadening of the detection probability in the first detector. Furthermore, we supplement the final homodyne detector with a degenerate type-I parametric amplifier which brings nearer the two previously distant states of the superposition. The availability of optical superposition states with large separation suggests a practical way of transforming them into entangled coherent states associated with different paths, thus providing a new convenient source for a “which path” experiment.

We extend the concept of distinguishability and likelihood introduced by Englert in Phys. Rev. Lett. 77, 2154 (1996) to encompass quantum erasure. We delineate the necessary and sufficient condition for obtaining optimal values of distinguishability and erasure. Finally we apply these measures on a gedanken welcher Weg experiment consisting of a coupled atom-photon interferometer.

The quantum state space S over a $d$-dimensional Hilbert space is represented as a convex subset of a $\left(D-1\right)$-dimensional sphere $S_{D-1}\subset {\mathbf{R}}^D,$ where ${D=d}^2-1.$ Quantum transformations ($\mathrm{CP}$ maps) are then associated with the affine transformations of ${\mathbf{R}}^D$ and $N\mapsto M$ cloners induce polynomial mappings. In this geometrical setting it is shown that an optimal cloner can be chosen to be covariant and induces a map between reduced density matrices given by a simple contraction of the associated $D$-dimensional Bloch vectors.

Collective decoherence is possible if the distance between quantum bits is smaller than the effective wavelength of the noise field. Collectivity in the decoherence helps us to devise more efficient quantum codes. We present a class of optimal quantum codes for preventing collective amplitude damping to a reservoir at zero temperature. It is shown that two quantum bits (qubits) are enough to protect one bit quantum information, and approximately $L+\frac{1}{2}{\ln }_2(\pi L/2)$ qubits are enough to protect L-qubit information when L is large. For preventing collective amplitude damping, these codes are much more efficient than the previously discovered quantum error correcting or avoiding codes.

We show that no source encoding is needed in the definition of the capacity of a quantum channel for carrying quantum information. This allows us to use the coherent information maximized over all sources and block sizes, but not encodings, to bound the quantum capacity. We perform an explicit calculation of this maximum coherent information for the quantum erasure channel and apply the bound in order find the erasure channel’s capacity without relying on an unproven assumption as in an earlier paper.

We report detailed results for positron-${\mathrm{H}}_2$ collisions obtained with the Schwinger multichannel method. Our calculations include annihilation parameter, differential, integral, and momentum transfer cross sections for energies below the positronium formation threshold. The calculations were carried out in the static-plus-polarization approximation with symmetry-resolved cross sections. Energy (temperature) dependence and symmetry-resolved contributions for the annihilation parameter $Z_{\mathrm{eff}}$ are also reported. Our ab initio integral cross sections are found to be in good agreement with the experimental data.

The amount of statistical information present in the one-particle density matrix and its diagonal, the charge density, is probed by comparison of the Jaynes and Shannon entropies for molecular systems. Numerical results show that these information entropies are not equal but that there exists a correspondence between them. The entropies are also compared among members of isoelectronic groups, and it is shown that these quantities are sensitive to the chemical environment and may be used as measures of the degree of electron delocalization or localization within a chemical system.

Wavelengths of the fine-structure transitions $2s{}^2{S}_{1/2}–2p{}^2{P}_{3/2}$ and $2s{}^2{S}_{1/2}–2p{}^2{P}_{1/2}$ in lithiumlike ${\mathrm{Ni}}^{25+}$ and ${\mathrm{Zn}}^{27+}$ have been measured using beam-foil excitation and grazing-incidence spectroscopy. The respective transition wavelengths of $142.461\pm 0.006\AA$ and of $216.061\pm 0.011\AA$ for ${\mathrm{Zn}}^{27+}$ have not been measured so far and present the most precise values in lithiumlike heavy ions measured by beam-foil spectroscopy. The ${\mathrm{Ni}}^{25+}$ results are in very good agreement with data from grazing-incidence spectroscopy at tokamaks. The measurement precision for both ions corresponds to less than or equal to 1% of the quantum electrodynamic contributions to the transition energies. This provides sensitivity to two-photon exchange processes for the screening of the electron self-energy.

Following Zhao, Morrison, and Parr [Q. Zhao, R. C. Morrison, and R. G. Parr. Phys. Rev. A 50, 2138 (1994)], we examine the effective homogeneity of the exact exchange-correlation energy functional for a set of open-shell atoms. By explicitly considering the influence of the integer discontinuity in the exact exchange-correlation potential, our results suggest that on the limiting electron deficient and electron abundant sides of the integer, the exact functional is poorly represented by any functional that is approximately homogeneous in the electron density. In contrast, a functional whose potential averages over the exact discontinuity must be approximately homogeneous of degree 4/3 in the density, and this is particularly so for the heavier atoms. This observation supports the view that continuum functionals, dominated by local density exchange, must exhibit such an average behavior. The asymptotic breakdown of this average behavior in conventional continuum functionals has important consequences for the computation of long-range properties.

Formation and decay of the $(p{\mu }^3{\mathrm{H}\mathrm{e})}^{*}$ and $(p{\mu }^4{\mathrm{H}\mathrm{e})}^{*}$ molecules have been studied in binary-gas mixtures of ${\mathrm{H}}_2{+}^3\mathrm{He}$ and ${\mathrm{H}}_2{+}^4\mathrm{He}$ at 30 K. The muon ground-state transfer rates from hydrogen to the two helium isotopes were extracted from the time distribution of the 7-keV decay x rays of the muonic hydrogen-helium molecules, measured with a Ge detector. The obtained transfer rates are ${\lambda }_{p^3\mathrm{He}}=(0.46\mathrm{}\pm 0.15)\times {10}^8{\mathrm{}\mathrm{s}}^{-1}$ and ${\lambda }_{p^4\mathrm{He}}=(0.42\mathrm{}\pm 0.07)\times {10}^8{\mathrm{}\mathrm{s}}^{-1}.$ The radiative branching ratios of the decay of the $(p{\mu }^3{\mathrm{H}\mathrm{e})}^{*}$ and $(p{\mu }^4{\mathrm{H}\mathrm{e})}^{*}$ molecules were determined by comparing the yields of the Lyman series of muonic hydrogen with the one of the 7-keV line with charge-coupled-device detectors. The ratios are in agreement with theoretical predictions. Muon transfer from excited states of muonic hydrogen to both helium isotopes was also observed.

Using a Si(Li) x-ray spectrometer, we have measured the respective fractions of $L_1,$ $L_2,$ and $L_3$ x rays in the L x-ray spectrum emitted in the ${}^{157}{\mathrm{T}\mathrm{b}-}^{157}\mathrm{Gd}$ electron-capture decay. Using, in addition, our previously measured value for the ratio of L and K x rays, we deduce values for the fluorescence and Coster-Kronig yields of the $L_1$ subshell. These are ${\omega }_1=0.101\mathrm{}\pm 0.005,$ $f_{12}=0.166\mathrm{}\pm 0.020,$ and $f_{13}=0.287\mathrm{}\pm \mathrm{0.014.}$ The $f_{12}$ and $f_{13}$ values are significantly below the predictions of the independent-particle model (IPM). The deduced $L_1$ level width of 3.7 eV is also significantly below the IPM width of 4.6 eV.

The $L\alpha$ and $L\beta$ spectra of tungsten were measured using a high-resolution single-crystal x-ray spectrometer. The observed spectra were fitted into Lorentzians. The fit residuals of the $L\alpha$ and $L\beta$ spectra indicate the presence of satellites in the vicinity of each spectrum, which are originated from Coster-Kronig transitions. Linewidths, energies, and intensities were estimated for each diagram line and compared with previous data. The $L{\beta }_2$ satellite line observed by Salgueiro et al. [J. Phys. (Paris), Colloq 48, C9-609 (1987)] was confirmed on the high energy side of the $L{\beta }_2$ line. The origin of this satellite is discussed experimentally and theoretically.

A Bethe-Salpeter based bound-state formalism is applied to the calculation of recoil contributions of order $m{\alpha }^6$ to hyperfine splitting in ground-state positronium. The calculation involves the numerical evaluation of two- and three-photon exchange diagrams along with derivative terms and many-potential contributions. After inclusion of all other contributions of the same order, a comparison with other calculations and experiment is made.

Closed-orbit theory gives a semiclassical formula for the photoabsorption oscillator strength density of atoms in external fields. The oscillator strength can be calculated from the properties of the classical orbits of the highly excited electron moving in the combined Coulomb and electric field. The deviation from the Coulomb potential due to an alkali-metal atom core causes scattering between the classical orbits when a closed orbit returns to the origin. Bifurcations of closed orbits happen when a focus moves through the origin and we present a theory that describes the scattering of the electron waves by the alkali-metal core for atoms in external electric fields in the presence of a bifurcation.

We calculate the atomic structure of hydrogen and helium atoms and molecules in an intense magnetic field analytically and numerically with a judiciously chosen basis.

Norm-conserving pseudopotentials are generated on the basis of the optimized potential method (OPM), combining the exact exchange with two different correlation functionals, i.e., the orbital-dependent form of Colle and Salvetti (CS) [Theor. Chim. Acta 37, 329 (1975)] and the local density approximation (LDA). Application of both types of OPM pseudopotentials to a number of diatomic molecules leads to substantially different predictions for spectroscopic constants, thus indicating the importance of a suitable choice for the correlation functional to be used in the OPM context. Moreover, while the CS values are closer to the experimental data, our results nevertheless indicate that a really adequate correlation functional for use with the exact exchange is not yet available. In any case, both variants of the OPM-based pseudopotentials show an improved transferability compared with standard LDA pseudopotentials.

The electron affinity of indium has been measured using the laser-photodetachment electron spectroscopy technique. Fine-structure-resolved photoelectron kinetic energy spectra of ${\mathrm{In}}^{-}$ were analyzed and the electron affinity of $\mathrm{In}{(}^2{P}_{1/2})$ was determined to be $0.404\pm 0.009\hspace{0.5em}\mathrm{eV}.$ The fine-structure splittings in the ground state of ${\mathrm{In}}^{-}{(}^3{P}_{0,1,2})$ were determined to be $0.076\pm 0.009\hspace{0.5em}\mathrm{eV}(J=\overrightarrow{0}J=1)$ and $0.175\pm 0.009\hspace{0.5em}\mathrm{eV}(J=\overrightarrow{0}J=2).\mathrm{}$ This measurement is compared to several recent calculations of the electron affinity of indium.

We have developed an approach to study doubly excited Rydberg states of atoms or ions with two optically active electrons outside an ionic closed-shell core. The interaction between the core and one valence electron is modeled by an accurate semiempirical pseudopotential. For any given parity and total angular momentum, single configuration Hartree-Fock calculations are performed to build a basis set of numerical two-electron wave functions. Configuration-interaction calculations then provide the energy positions of the correlated states and their compositions in terms of the single configuration basis set. Atomic properties, such as autoionization linewidths, can be derived. Results concerning neutral barium are reported to illustrate the approach. They are compared with available experimental and theoretical data, and discussed. Energy positions of the bound and autoionizing $J=0\mathrm{}$ even-parity and $J=1\mathrm{}$ odd-parity states of barium below the $5d{}^2{D}_{5/2}$ threshold are predicted as well as associated autoionization linewidths. The symmetrical ${\mathrm{ns}}^2\hspace{0.5em}(n=7\mathrm{}-11),$ ${7p}^2,$ and ${6d}^2$ configurations are also studied. The overall agreement is satisfactory and especially good for high-lying doubly excited Rydberg states.

Calculations of the principal Breit-Pauli relativistic corrections for the ground-state energy of the lithium atom have been carried out. The corrections have been determined using first-order perturbation theory. Only the non-fine-structure components of the Breit-Pauli Hamiltonian in the infinite nuclear mass approximation are considered. The wave function employed is an extensively optimized large-scale Hylleraas-type expansion. Comparisons are made with the few available relativistic corrections for the lithium atom that are available in the literature. A reevaluation of the first ionization potential for the ground state of Li is presented.