The discrepancy between theory and experiment for the orthopositronium (o-Ps) decay rate could be resolved by an exotic decay branch o-Ps→${\mathrm{X}}^0$γγ, where ${\mathrm{X}}^0$ is a low-mass C-odd boson. An experimental search for ${\mathrm{X}}^0$ is reported in which Compton-like interactions with electrons or decay γ rays (${\mathrm{X}}^0$→3γ) could be detected. None are observed and sensitive, but model-dependent limits are set (for two specific models). One model, testable by this method, allows the good agreement between theory and experiment for the parapositronium decay rate to be reconciled with the disagreement for the o-Ps decay rate.

Direct solution of the Schrödinger equation for the doubly excited resonant ${}_{}{}^1{}_{}{}^{}\mathrm{S}_{}^{\mathrm{e}}{}_{}{}^{}$ state of the helium atom is obtained with the help of the complex rotation method (which reduces the resonance problem to that of bound states with complex energy) and the correlation function hyperspherical harmonic (CFHH) method. In the CFHH method the solution is a product of a correlation function and a smooth factor expanded into hyperspherical harmonic functions. Given a proper correlation function, chosen from physical considerations, the method generates resonant wave functions, accurate in the whole range of interparticle distances. Since the method is nonvariational no stabilization procedure is needed. The calculated energy and width are shown to be strictly independent of the angle of complex rotation, hence there is no need for the angle optimization procedure as well. The results are compared with variational and other precise computations.

We report a measurement of the spectral distribution of photons from the two-photon decay of the ${1\mathrm{s}2\mathrm{s}}^1$${\mathrm{S}}_0$ level in heliumlike krypton. In the experiment, a beam of ${\mathrm{Kr}}^{34+}$ ions was excited using a thin carbon foil and the two-photon decays were observed as coincidences in an array of x-ray detectors. An important part of this work was the measurement of the efficiency of the detection system as a function of photon energy. A Monte Carlo model of the experiment was used to compare the data with theoretical calculations and demonstrated good agreement with the nonrelativistic calculations for Z=36.

I consider the theory of the quantum error correcting code (QECC), where each quantum particle has more than two possible eigenstates. In this higher spin system, I report an explicit QECC that is related to the symmetry group ${\mathrm{Z}}_2^{\otimes \left(\mathrm{N}\mathrm{-}1\right)}$⊗${\mathrm{S}}_{\mathrm{N}}$. This QECC, which generalizes Shor's simple majority vote code [Phys. Rev. A 52, 2493 (1995)], is able to correct errors arising from exactly one quantum particle. I also provide a simple encoding algorithm.

We have measured the cross sections for ionizating one-electron ${\mathrm{Au}}^{78+}$ and the total cross sections for electron capture by bare ${\mathrm{Au}}^{79+}$ at 10.8 GeV/nucleon in C, Al, Cu, Ag, and Au targets. We made the measurement by magnetically separating the charge states and measuring the fraction of ${\mathrm{Au}}^{78+}$ as a function of target thickness for each element. In contrast to the results reported by Westphal and He [Phys. Rev. Lett. 71, 1160 (1993)], our ionization measurements agree with the calculation of Anholt and Becker [Phys. Rev. A 36, 4628 (1987)]. Our capture cross-section measurements are in agreement with theory for those targets where radiative electron capture is the dominant capture process.

Fast alkali atoms and ions are scattered with keV energies under grazing incidence from the surface of the alkali halides LiF(100), KCl(100), KI(100), and the scattered beams are analyzed with respect to their charge fractions. From our experiments we find no evidence for occupied or unoccupied electronic surface states within the band gap of the insulator.

We propose a light-scattering scheme to measure the relative phase of atomic Bose-Einstein condensates and its diffusion rate. The proposal relies on the existence of two independent condensates coupled to a common excited state. To this end, we consider a two-well ground-state potential together with excited-state trap wave functions that extend over the whole region. When the first trap is driven by a weak monochromatic laser field, the light scattered from the second trap has a nonzero mean-electric field amplitude with a phase shift proportional to the difference of the condensate phases. When both condensates are driven, the phases of the two laser fields can be adjusted to cancel the scattering completely by a quantum interference. The particular value of the laser phase difference that gives zero scattering determines the relative phase of the two condensates.

Atoms confined in a harmonic potential show universal oscillations in two dimensions (2D). We point out the connection of these ``breathing'' modes to the presence of a hidden symmetry. The underlying symmetry SO(2,1), i.e., the two-dimensional Lorentz group, allows pulsating solutions to be constructed for the interacting quantum system and for the corresponding nonlinear Gross-Pitaevskii equation. We point out how this symmetry can be used as a probe for recently proposed experiments of trapped atoms in 2D.

We observe quantum tunneling of cold sodium atoms from an accelerating one-dimensional standing wave of light. Atoms are trapped in a far-detuned standing wave that is accelerated for a controlled duration. For sufficiently large values of the acceleration, we observe an exponential decay in the number of atoms that remain trapped as a function of the interaction time. We show that this loss is due to quantum tunneling, and compare the decay rates with Landau-Zener theory. We also observe oscillations in the tunneling rate as a function of the acceleration which are due to quantum interference effects.

A systematic measurement of ellipticity-dependent high-order harmonic generation has been made in the energy region around the ionization potential of He, Ne, Ar, and Kr, using a 150-fs, 785-nm Ti:sapphire laser. Anomalous ellipticity dependence having a maximum at a nonzero ellipticity was observed at the 11th and 13th harmonics for Ne and at the 7th and 9th for Ar and Kr. The experimental results strongly suggest that the anomalous dependence observed is due to the near-resonant multiwave mixing process an even number of linearly polarized photons with a single circularly polarized photon in the neutral atomic gases.

Wave packets comprising a coherent superposition of n≃390 Stark states have been created in potassium by rapid application of a dc field. Their properties are examined using a half-cycle probe pulse that is applied following a variable time delay and that ionizes a fraction of the excited atoms. The survival probability exhibits pronounced oscillations (quantum beats) that are associated with the time evolution of the wave packet. Interestingly, even in the present regime of complete overlap of different Stark manifolds, a single dominant beat frequency is observed, and this is explained quantum mechanically in terms of energy-level statistics. Similar behavior is predicted by classical trajectory Monte Carlo simulations that reproduce well the experimental observations demonstrating classical-quantum correspondence in high-n systems.

This paper is primarily concerned with the development and application of quantum bounds on mutual information, although some of the methods developed can be applied to any figure of merit indicating degree of correlation, such as coincidence rate. Three basic techniques for obtaining bounds are described: mappings between joint-measurement and communication correlation contexts; a duality relation for quantum ensembles and quantum measurements; and an information exclusion principle [M. J. W. Hall, Phys. Rev. Lett. 74, 3307 (1995)]. Results include a proof of Holevo's communication bound from a joint-measurement inequality; a measurement-dependent dual to Holevo's bound; lower bounds for mutual information under ensemble and measurement constraints; information exclusion relations for measurements described by probability-operator measures; a proof that Glauber coherent states are optimal signal states for quantum communication based on (noisy) optical heterodyne detection; and an information inequality for quantum eavesdropping. Relations between the three techniques are used to further obtain upper bounds for quantum information, and to extend the information exclusion principle to a joint-measurement context.

The construction of large, coherent quantum systems necessary for quantum computation remains an entreating but elusive goal, due to the ubiquitous nature of decoherence. Recent progress in quantum error correction schemes have given new hope to this field, but thus far, the codes presented in the literature assume a restricted number of errors and error free encoding, decoding, and measurement. We investigate a specific scenario without these assumptions; in particular, we evaluate a scheme to preserve a single quantum bit against phase damping using a three-qubit encoding based on Shor, and study the effect of decoherence not only during storage but also during processing. We derive an general upper limit on the allowable decoherence per time step. Physically, our results suggest the feasibility of engineering artificial metastable states through repeated error correction.

The n=9 F-to-G intervals in atomic lithium are measured to a precision of 1 ppm. These transitions are driven using microwaves in a Ramsey-separated-oscillatory-field configuration. The results are 1920.6767(22) and 1928.7221(22) MHz for the ${9\mathrm{F}}_{7\mathrm{/}2}$ -to-${9\mathrm{G}}_{9\mathrm{/}2}$ and ${9\mathrm{F}}_{5\mathrm{/}2}$ -to-${9\mathrm{G}}_{7\mathrm{/}2}$ intervals, respectively. These are the most accurate measurements of any fine-structure intervals in Rydberg states of the lithium atom.

Resonantly excited carbon and oxygen x-ray-emission spectra of gaseous carbon monoxide are presented. Emission spectra obtained with selective excitation to the ${\mathrm{\pi }}^{\mathrm{*}}$ valence orbital and to various Rydberg levels are compared to satellite-free nonresonant spectra. Screening effects caused by the excited electron, creating energy shifts and intensity variations in the resonant spectra compared to the nonresonant spectra, are observed, as well as an angular dependence of the resonantly excited spectra. The experimental spectra are compared to simulated spectra where the vibronic part is computed by means of a lifetime-vibrational interference formalism. The electronic intensities are analyzed by a separate-state, self-consistent-field method and a formalism for resonant inelastic x-ray scattering, focusing on screening and angular dependence.

A systematic study of vibrational band profiles in resonantly excited x-ray-emission spectra of free molecules is made. Carbon and oxygen K-emission spectra of CO following core excitations, using narrow-bandpass synchrotron radiation, to the 2π orbital are presented. Because of the short lifetime of the core-excited states, interference effects between close-lying coherently excited vibrational levels may occur. The importance of lifetime-vibrational interference in the carbon and oxygen x-ray-emission spectra is investigated by comparing the experimental band profiles with simulated profiles. It is shown that to properly describe the band profiles in the oxygen emission spectra it is necessary to include the interference contributions, while in the carbon spectra the interference contributions are important only when the excitation energy is tuned towards higher vibrational levels. For example, when the excitation energy is tuned to the v=2 vibrational level significant interference effects are observed. It is also demonstrated how emission spectra recorded with a detuning of the excitation energy from the resonance can be used to check the shape of the photon distribution of the synchrotron radiation.

An approximate method using iterative techniques developed for solving the extended Schrödinger equation is applied to principles of the first and second kind of balance theory and an example for (Zns${)}^2$ heliumlike ions is presented. The implementation of the equal-factors method presented here, which leads to a series of self-consistent equations, has advantages that provide the results for an entire series of energy levels and an entire isoelectronic sequence in a single calculation. Results for all two-electron ions for the ground-state energies for Z=1 to 10 are tabulated and compared with other results for these systems. Theoretical predictions of energy levels for (Z2s${)}^2$ to (Z11s${)}^2$ , excited autoionizing states of heliumlike ions with Z=1,10, are tabulated as well.

We have calculated energy levels of the ${2\mathrm{s}}^2$${\mathrm{}}^1$${\mathrm{S}}_0$ ground state and the 2s2p${\mathrm{}}^{1,3}$${\mathrm{P}}_1$ excited states of berylliumlike ions with Z=10–92 using the relativistic configuration-interaction method. These calculations are based on the relativistic no-pair Hamiltonian which includes Coulomb and retarded Breit interactions and employ finite B-spline basis functions. Quantum electrodynamic and mass polarization corrections are also calculated. Good agreement between theory and experiment for the ${2\mathrm{s}}^2$${\mathrm{}}^1$${\mathrm{S}}_0$-2s2p${\mathrm{}}^{1,3}$${\mathrm{P}}_1$ transition energies is found throughout the isoelectronic sequence.

Ghost states arise in applications of the hyperspherical close-coupling method to atoms with two valence electrons outside a closed-shell core. A rigorous formulation of ghost-state elimination is proposed on the basis of a projection operator. Its practical implementation exploits the R-matrix propagator. The procedure is tested for the Temkin-Poet ${\mathrm{s}}^2$ model by removing a target orbital regarded as an occupied core state. The partial success of the recently proposed scheme of Zhou and Lin [Phys. Rev. A 51, 1286 (1995)] is accounted for as due to an extreme insensitivity of the R-matrix states on the projection parameter.

A theoretical treatment, based on a tensorial expansion of the scattering matrix, is presented for collisional coupling between hyperfine and Stark components of molecular spectral lines. This treatment also describes the dependence of collisional broadening of well-resolved components on F and ${\mathrm{M}}_{\mathrm{F}}$ quantum numbers. The case of molecules with a large dipole moment is discussed in detail and simple selection rules are obtained for collisional coupling by using our treatment of the relaxation matrix within a perturbative expansion of the scattering matrix. The existing experimental results are explained.