We investigate deuterium atoms adsorbed on the surface of liquid helium in equilibrium with a vapor of atoms of the same species. These atoms are studied by a sensitive optical method based on spectroscopy at a wavelength of 122 nm, exciting the 1S-2P transition. We present a direct measurement of the adsorption energy of deuterium atoms on helium and show evidence for the existence of resonantly enhanced recombination of atoms residing on the surface to molecules.

This paper presents a series solution to the image charge fields of a single ion in a Penning trap. The calculation of these fields and resulting frequency shifts will be important for advances in a variety of high precision Penning trap studies, particularly for work with highly charged ions. The simple technique is applicable to cylindrically symmetric traps of otherwise arbitrary geometry and provides an efficient alternative to finite grid relaxation techniques. The present calculation is in agreement with previous measurements in a hyperbolic trap, and systematic frequency shifts for recent atomic mass measurements using multiply charged ions are given.

We investigate macroscopic quantum tunneling of a Bose-Einstein condensate, and how it is affected by the interatomic interaction. We study the dynamics of a condensate falling under gravity and scattering on a Gaussian potential barrier that models a mirror formed by a far-detuned sheet of light. We observe bouncing, interference, and quantum tunneling of the condensate. We find that the tunneling rate is very sensitive to the interatomic interaction and to the shape of the condensate. Under many conditions the tunneling rate is strongly enhanced by the interaction as achieved, for instance, by increasing the number of condensed particles. In a quasi one-dimensional situation the tunneling pulse displays two peaks. The quantum tunneling can be quasiperiodic, and in this way one could generate coherent Bose-Einstein condensed atomic pulses.

Ionization of atomic hydrogen by antiproton impact is studied by a two-center atomic-orbital close-coupling method supplementing projectile orbitals to the one-center expansion. Two-center effects are not negligible at intermediate and low energies, though antiprotons do not have bound states of electrons. The one-center expansion fails to represent the electron distribution near the antiproton where the electron tends to be evacuated by the repulsive interaction from the negatively charged nucleus. The integrated ionization cross sections are less sensitive to the details of the electron distribution, but below 1 keV the one-center expansion underestimates the ionization cross sections due to its inability to represent the expanding distribution of ionized electrons.

We study the properties of a Rb magneto-optic trap loaded from a commercial getter source. The source provides a large flux of atoms for the trap along with the capability of rapid turn off necessary for obtaining long trap lifetimes. We have studied the trap loading at two different values of background pressure to determine the cross section for $\mathrm{Rb}-{\mathrm{N}}_2$ collisions to be $3.5\left(4\right)\mathrm{}\times {10}^{-14}{\mathrm{cm}}^2$ and that for Rb-Rb collisions to be of order $3\times {10}^{-13}{\mathrm{cm}}^2$. At a background pressure of $1.3\times {10}^{-9}$ torr, we load more than ${10}^8$ atoms into the trap with a time constant of 3.3 s. The $1/e$ lifetime of trapped atoms is 13 s limited only by background collisions.

We report experimental observation of antiphase mode dynamics in the high-frequency chaotic state of a multimode semiconductor laser with optical feedback and provide a detailed investigation of its frequency dependence. Comparison of the power spectral density for total intensity with the incoherent sum of power spectral densities for individual mode intensities shows that the oscillations around the relaxation oscillation frequency are in-phase in all the modes, while the oscillations at lower frequencies exhibit partial antiphase behavior. We demonstrate that synchronized phase dynamics with similar frequency dependence can be induced in a second laser by unidirectional optical injection.

A quantum key distribution protocol based on quantum encryption is presented in this Brief Report. In this protocol, the previously shared Einstein-Podolsky-Rosen pairs act as the quantum key to encode and decode the classical cryptography key. The quantum key is reusable and the eavesdropper cannot elicit any information from the particle Alice sends to Bob. The concept of quantum encryption is also discussed.

Capture of slow antiprotons by atomic hydrogen and positronium is simulated by the classical trajectory Monte Carlo method. Statistically accurate cross sections for protonium and antihydrogen formation are obtained and the energy dependence of the process is established. The results agree very well with experimental data for proton capture by positronium. Antihydrogen formation from antiproton-positronium collisions in the presence of a laser is simulated and the effects of laser polarization, frequency, and intensity are studied. Enhancements of the antihydrogen formation cross section are observed. For example, an increase of 70% is found for light of intensity $1.4\times {10}^{13}\hspace{0.5em}{\mathrm{W}\mathrm{}\mathrm{}\mathrm{cm}}^{-2}$ and wavelength $\lambda =248\mathrm{}\hspace{0.5em}\mathrm{nm},$ for an antiproton collision energy of 1 keV.

Recently, Zhang, Li, and Guo have proposed a particular eavesdropping attack [Phys. Rev. A 63, 036301 (2001)] that shows that my quantum key distribution protocol based on entanglement swapping [Phys. Rev. A 61, 052312 (2000)] is insecure. However, security against this attack can be attained with a simple modification. In addition, a simpler version of the protocol using four qubits is introduced.

A determination of the relative partial photoionization cross sections of the $3d$ and $4s$ main lines and associated satellite lines of atomic scandium in the region of the $3\overrightarrow{p}nd,ms$ excitations has been carried out using electron spectrometry in conjunction with monochromatized synchrotron radiation. The superior resolution inherent in these measurements has revealed additional structure not previously seen in earlier experimental work, highlighting the rich and complex structure of this relatively simple open-shell atom. Relative intensities and binding energies of the $3p$ photolines are also reported. A comparison with recent many-body perturbation theory calculations in LS coupling shows only marginal agreement. Discrepancies between theory and experiment are thought to arise from the use of LS coupling and neglect of satellite channels in the calculation.

Density-matrix equations for electrons in laser-coupled quantum wells are derived in second quantization, including an off-diagonal radiative-decay coupling between a pair of electron transitions. Calculations of spontaneous photoluminescence and time-resolved optical absorption for the probe field are formulated. The zero absorption of the pump-laser field within an overlapping region between two absorption peaks is found in a resonant asymmetric double-quantum-well system and explained as the quantum interference between two nearly degenerate electron transitions. Quantum interference is clearly demonstrated through phase cancellation between the two statistically averaged transition dipole moments. The laser frequency for zero absorption can be tuned within a tunneling gap by applying a small dc bias field. The $k_{\Vert }$-dependent energy-level separation is found to be a crucial factor for destroying quantum interference. The optical gain of the probe field is seen as a hole in the weak absorption peak for the resonant asymmetric double quantum wells selectively coupled by a laser field and shown to be a result of the partial inversion of the electron occupation probabilities in momentum space after laser excitation. The probe-field gain increases with the strength of the pump laser. The effects of transition blocking, induced quantum coherence, and off-diagonal radiative-decay coupling are quantitatively analyzed for this gain.

An exact analytical solution to the time-dependent Schrödinger equation with cutoff wave initial conditions is used to investigate the fast tunneling response of a rectangular potential barrier. We find that just across the tunneling region, the probability density exhibits at short times a transient behavior that may be characterized by a peak $t_p$ and a width $\Delta t.\mathrm{}$ We show that $t_p$ provides the earliest tunneling response of the system and that the top-barrier S-matrix poles play an important role in the process. As a function of the barrier width, $t_p$ exhibits two regimes. Along the first regime, $t_p$ remains almost a constant; as the barrier width increases, a second regime appears where $t_p$ grows linearly with the barrier width.

We consider mixed states of two qubits and show under which global unitary operations their entanglement is maximized. This leads to a class of states that is a generalization of the Bell states. Three measures of entanglement are considered: entanglement of formation, negativity, and relative entropy of entanglement. Surprisingly all states that maximize one measure also maximize the others. We give a complete characterization of these generalized Bell states and prove that these states for fixed eigenvalues are all equivalent under local unitary transformations. Furthermore we characterize all nearly entangled states closest to the maximally mixed state and derive a lower bound on the volume of separable mixed states.

We provide several applications of a previously introduced isomorphism between physical operations acting on two systems and entangled states [Phys. Rev. Lett. 86, 544 (2001)]. We show (i) how to implement (weakly) nonlocal two-qubit unitary operations with a small amount of entanglement; (ii) that a known, noisy, nonlocal unitary operation as well as an unknown, noisy, local unitary operation can be purified; (iii) how to perform the tomography of arbitrary, unknown, nonlocal operations; (iv) that a set of local unitary operations as well as a set of nonlocal unitary operations can be stored and compressed; and (v) how to implement probabilistically two-qubit gates for photons. We also show how to compress a set of bipartite entangled states locally, as well as how to implement certain nonlocal measurements using a small amount of entanglement. Finally, we generalize some of our results to multiparty systems.

Excitation energies for $2l-{3l}^{\prime }$ hole-particle states of Ne-like ions are determined to second order in relativistic many-body perturbation theory (MBPT). Reduced matrix elements, line strengths, and transition rates are calculated for electric-dipole $\mathrm{}\left(E1\mathrm{}\right),$ magnetic-quadrupole $\mathrm{}\left(E2\mathrm{}\right),$ magnetic-dipole $\mathrm{}\left(M1\mathrm{}\right),$ and magnetic-quadrupole $\mathrm{}\left(M2\mathrm{}\right)\mathrm{}$ transitions in Ne-like ions with nuclear charges ranging from $Z=11\mathrm{}$ to 100. The calculations start from a ${1s}^2{2s}^2{2p}^6$ closed-shell Dirac-Fock potential and include second-order Coulomb and Breit-Coulomb interactions. First-order many-body perturbation theory (MBPT) is used to obtain intermediate-coupling coefficients, and second-order MBPT is used to determine the matrix elements. Contributions from negative-energy states are included in the second-order $E1,$ $M1,$ $E2,$ and $M2\mathrm{}$ matrix elements. The resulting transition energies are compared with experimental values and with results from other recent calculations. Trends of $E1,$ $E2,$ $M1,$ and $M2\mathrm{}$ transition rates as functions of nuclear charge Z are shown graphically for all transitions to the ground state.

To support efforts on cooling and trapping of alkaline-earth-metal atoms and designs of atomic clocks, we performed ab initio relativistic many-body calculations of electric-dipole transition amplitudes between low-lying states of Mg, Ca, and Sr. In particular, we report amplitudes for ${}^1{\overrightarrow{P_1^o}}^1{S}_0{,}^3{S}_1{,}^1{D}_2,$ for ${}^3{\overrightarrow{P_1^o}}^1{S}_0{,}^1{D}_2,$ and for ${}^3{\overrightarrow{P_2^o}}^1{D}_2$ transitions. For Ca, the reduced matrix element $〈{4s4p}^1{P}_1^o||D||{4s}^{21}{S}_0〉$ is in good agreement with a high-precision experimental value deduced from photoassociation spectroscopy [Zinner et al., Phys. Rev. Lett. 85, 2292 (2000)]. An estimated uncertainty of the calculated lifetime of the ${3s3p}^1{P}_1^o$ state of Mg is a factor of 3 smaller than that of the most accurate experiment. Calculated binding energies reproduce experimental values within $0.1-0.2\%.\mathrm{}$

The photoabsorption spectrum of C II, from the ground and ${}^{4}P$ first excited levels, has been studied with the dual laser plasma technique. Both the discrete spectrum due to bound-bound transitions and the photoionization continua have been observed. Accurate wavelength measurements have been performed and new lines have been identified. Furthermore, relative oscillator strength values have been derived. The photoionization cross sections have been measured in relation to the oscillator strengths of the discrete transitions, and several resonances, showing characteristic Fano profiles, have been observed too. Experimental data have been compared with theoretical calculations.

We report new measurements of the decay of the core-excited O $1s–1{\pi }_g{}^3{\Pi }_u$ state in molecular oxygen under Auger resonant Raman conditions. The spectral features are interpreted with the aid of ab initio calculations using standard lifetime-vibrational interference, and in some cases, employing a model in which the bond-length dependence of the Auger transition rates is taken into account. By analyzing a series of Auger-decay spectra using different excitation energies, several instances of fixed kinetic energy are pointed out in the spectra; the nondispersive behavior arises from the decay between two potentials that are essentially parallel.

We have measured the radiation from medium-charge-state ions of tungsten in the spectral range from 45 to 70 Å. The ions were produced in an electron beam ion trap (EBIT), and the radiation was observed using a 2-m grazing-incidence spectrometer. Operating EBIT with beam energies between 500 eV and 4 keV allowed us to sample charge states ranging from I-like ${\mathrm{W}}^{21+}$ to Cu-like ${\mathrm{W}}^{45+}.$ Lines of Sr-like through Cu-like tungsten were observed; for the Sr-like to As-like ions new lines were identified. Analysis of the spectra is based on ab initio calculations using the relativistic HULLAC code. For charge states lower than Sr-like $W^{36+},$ the spectrum lines fuse into a bright emission band situated around $50\hspace{0.5em}\mathrm{\AA }.$ The band extends over an interval of approximately $2\hspace{0.5em}\mathrm{\AA }\mathrm{}$ and moves smoothly towards shorter wavelengths with decreasing ion-charge state. The shift and narrowing of the band emission cannot be interpreted in the framework of the standard unresolved transition array formalism assuming statistical population of the excited levels. Instead, the observations are explained by detailed calculations based on a collisional-radiative model. It is shown that the variation of the experimental spectrum with charge state is the signature of the low electron density in EBIT.

The state-selective charge transfer for ${\mathrm{H}}^{+}$ colliding with the resonantly excited alkali-metal targets (Na and K), along with their dependence on the initial alignment of the electron-charge cloud, has been reported in the low-energy region employing a semiclassical, impact-parameter, close-coupling approach based on the molecular-state expansion augmented with the plane-wave electron translation factor. Although these colliding pairs possess a pseudo-one-electron behavior, they exhibit quite different characteristics. A systematic change in various parameters is observed as we move along the target in the isoelectronic series. The anisotropy parameters $A\left(2\right)\mathrm{}$ and $A\left(3\right)\mathrm{}$ for both Na and K atoms are presented. We also report the anisotropy parameters for populating the excited states of Na and K target. For Ly-α emission, in the case of ${\mathrm{H}}^{+}-\mathrm{N}\mathrm{a}\mathrm{}\left(3p\right)\mathrm{}$ collisions, our low-energy results agree closely with the quantal calculations [H. Croft and A. S. Dickinson, J. Phys. B 29, 57 (1996)], but disagree with the measured values [V. S. Kushawaha, Z. Phys. A 313, 155 (1983)]. Comparison is also made for electron-capture cross sections into various nl levels of the target with the experimental and other theoretical results, whenever possible. The low-energy behavior of the excitation cross sections for both of the targets is also discussed.