We investigate decoherence effects due to motional degrees of freedom of trapped electronically coded atomic or ionic qubits. For universal single bit rotations implemented with dipole coupling from a near resonant running wave laser, the achievable fidelity is found to depend only on a single parameter characterized by the initial motional state. For two-qubit conditional logic operations inside a high Q optical cavity, we find the popular dark state structure survives even when atomic wave packets are not localized along transverse directions. Our quantitative results provide a useful realistic view for current experimental efforts in quantum logic and computing.

Threshold-photoelectron coincidence spectra and threshold-photoelectron spectra are measured for Kr and Xe in the energy regions 105–127 and 75–100 eV, respectively. ${\mathrm{Kr}}^{2+}{3d}^{-1}{4p}^{-1}$ and ${\mathrm{Xe}}^{2+}{4d}^{-1}{5p}^{-1}$ doubly charged ion states are observed, and also ${\mathrm{Kr}}^{+}{3d}^{-1}{4p}^{-1}\mathrm{nl}$ and Xe ${4d}^{-1}{5p}^{-1}\mathrm{nl}$ satellite states. To our knowledge, this is the first direct observation in electron spectroscopy of doubly charged ion states where one hole occurs in an inner shell and another in a valence shell. The results are compared with theoretical calculations performed with an algebraic Green’s-function method, and also with previous experimental information from Auger electron spectroscopy.

Resonant ion-pair formation from the collisions of ${\mathrm{NO}}^{+}$ ions with electrons was studied using the heavy-ion storage ring CRYRING at the Manne Siegbahn Laboratory of Stockholm University. The total cross section is measured for the formation of ${\mathrm{N}}^{+}{+\mathrm{O}}^{\mathrm{-}}$ for electron energies 8–18 eV, and the results are compared with ion-pair formation in photoionization work. A peak in the cross section is observed at 12.5 eV, with a magnitude of $8.5\times {10}^{-19}{\mathrm{cm}}^2.$ An attempt to extract the cross section for the reverse process of associative ionization is made.

The interaction of ${\mathrm{He}}^{+}$ with a typical metal surface (Al or Pd) is described, analyzing in detail the different mechanisms that contribute to the neutralization of the projectile when backscattered from the surface. Auger and resonant neutralization processes are considered and analyzed including a detailed quantum-mechanical description of the He-metal interaction, for projectile energies between 100 eV and 3 keV. We show that the promotion of the $\mathrm{H}\mathrm{e}-1s$ level, due to its interaction with the metal-atom-core orbitals, is the crucial mechanism making resonant processes operative. We find, however, that resonant processes are much more important for Al than for Pd. In Al, both Auger and resonant processes are equally important for neutralization of the ion, while for Pd we find that Auger is the dominant mechanism, making the He/Pd system the ideal case for which Hagstrum’s exponential law appears to be practically valid for all velocities. We also find qualitative agreement with experimental data, which we consider a satisfactory result in view of the fact that our theory is a complex ab initio calculation free of adjustable parameters.

Traditional schemes for multistep resonance photoionization of atoms let every employed laser beam interact with the atoms simultaneously. In such a situation, analyses via time-dependent Schrödinger equation show that high ionization probability requires all the laser beams must be intense enough. In order to decrease laser intensity, we proposed a scheme that the laser beam used to pump the excited atoms (in a higher bound state) into an autoionization state does not interact with the atoms until all the population is transferred by the other lasers from a ground state to the bound state. As an interesting example, we examined three-step photoionization of ${}^{235}\mathrm{U}$ with our scheme, showing that the intensity of two laser beams can be lowered by two orders of magnitude without losing high ionization probability.

We study a system composed of N identical charged bosons confined in a harmonic trap. Upper- and lower-energy bounds are given. It is shown in the large-N limit that the ground-state energy is determined within an accuracy of ±8% and that the mean-field theory provides a reasonable result with a relative error of less than 16% for the binding energy.

We study semiclassical slow light propagation in trapped two-level atomic quantum gases. The temperature-dependent behaviors of both group velocity and transmissions are compared for low-temperature Bose, Fermi, and Boltzman gases within the local-density approximation for their spatial density profile.

We study the dynamics of a two-mode Bose-Einstein condensate in the vicinity of a mean-field dynamical instability. Convergence to mean-field theory (MFT), with increasing total number of particles N, is shown to be logarithmically slow. Using a density-matrix formalism rather than the conventional wave-function methods, we derive an improved set of equations of motion for the mean-field plus the fluctuations, which goes beyond MFT and provides accurate predictions for the leading quantum corrections and the quantum break time. We show that the leading quantum corrections appear as decoherence of the reduced single-particle quantum state; we also compare this phenomenon to the effects of thermal noise. Using the rapid dephasing near an instability, we propose a method for the direct measurement of scattering lengths.

Spontaneous decay of an excited atom near a dispersing and absorbing microsphere of given complex permittivity that satisfies the Kramers-Kronig relations is studied, with special emphasis on a Drude-Lorentz permittivity. Both the whispering gallery field resonances below the band gap (for a dielectric sphere) and the surface-guided field resonances inside the gap (for a dielectric or a metallic sphere) are considered. Since the decay rate mimics the spectral density of the sphere-assisted ground-state fluctuation of the radiation field, the strengths and widths of the field resonances essentially determine the feasible enhancement of spontaneous decay. In particular, strong enhancement can be observed for transition frequencies within the interval in which the surface-guided field resonances strongly overlap. When material absorption becomes significant, then the highly structured emission pattern that can be observed when radiative losses dominate, reduces to that of a strongly absorbing mirror. Accordingly, nonradiative decay becomes dominant. In particular, if the distance between the atom and the surface of the microsphere is small enough, the decay becomes purely nonradiative.

The Möbius inversion pair potential has been employed, in combination with genetic algorithms, to predict the lowest-energy structures of the rare-earth metallic clusters ${\mathrm{La}}_N,$ ${\mathrm{Ce}}_N,$ and ${\mathrm{Pr}}_N$ $(N=3\mathrm{}–20).$ Results are given for the symmetries, binding energies, nearest-neighbor distances of these clusters, and lowest-energy configurations of ${\mathrm{Ce}}_N$ clusters. Also, the calculated second finite difference of the total energy shows that for three species elements, the 13-atom clusters with $I_d$ symmetry are particularly stable. Some minor peaks are also found at size $N=4,$ 6, 11, and 15, which indicates that the corresponding clusters are relatively stable in structure. Present work points out a route to studying clusters.

We describe a procedure for creating pairs of condensates with appreciable relative number squeezing. We show that our procedure is relatively robust against the effects of loss and may therefore prove to be a practical way of generating such states. We use a quantum simulation for small numbers that enables us to validate a semiclassical model. This is used to predict the scaling for large numbers.

We demonstrate that a photon blockade effect exists in the intracavity coherent photoassociation of an atomic Bose-Einstein condensate and that the dynamics of the coupled atomic and molecular condensates can only be successfully described by a quantum treatment of all the interacting fields. We show that the usual mean-field calculational approaches give answers that are qualitatively wrong, even for the mean fields. The quantization of the fields gives a degree of freedom that is not present in analogous nonlinear optical processes. The difference between the semiclassical and quantum predictions can actually increase as the three fields increase in size so that there is no obvious classical limit for this process.

We study the rotational properties of a Bose-Einstein condensate confined in a rotating harmonic trap for different trap anisotropies. With simple arguments, we obtain the velocity field of the quantum fluid for condensates with or without a centered vortex. While the bosons describe open spiraling trajectories, on the frame of reference of the rotating trap, the fluid moves against the trap’s rotation. We also find expressions for the angular momentum of linear and Thomas-Fermi solutions for a vortexless state. In these two limits we find the same analytic relation between the shape of the cloud and the rotation speed. Our predictions are supported by numerical simulations of the mean-field Gross-Pitaevskii model and by current experiments with rotating Bose-Einstein condensates (K. W. Madison, F. Chevy, V. Bretin, and J. Dalibard, e-print cond-mat/0101051).

We study the spontaneous emission, absorption, and dispersion properties of a Λ-type atom where one transition interacts near resonantly with a double-band photonic crystal. Assuming an isotropic dispersion relation near the band edges, we show that two distinct coherent phenomena can occur. First the spontaneous emission spectrum of the adjacent free-space transition obtains “dark lines” (zeros in the spectrum). Second, the atom can become transparent to a probe laser field coupling to the adjacent free-space transition.

The absence of hyper-Raman lines in the observed spectrum emitted by an atom in the presence of a strong laser field is explained by evaluating the spectrum emitted by a unidimensional atom. It is found that the lines are emitted during a short time interval by the atom and that they therefore are small when compared with harmonical lines that are instead emitted during the laser pulse.

Using three phase-locked lasers, we have observed in a three-level system the phase shift of the frequency fixed coupling laser in dependence of the probe laser frequency relative to an off-resonant reference laser. In the vicinity of the two-photon resonance, the system exhibits electomagnetically induced transparency and we show that the dispersion of coupling and probe field shows different spectra if the probe field frequency is tuned. The parametric dispersion of the coupling field is measured on a cesium atomic beam with a heterodyne interferometer over several decades of coupling and probe laser power. Even with coupling powers in the nW range, the interferometer gives a good signal-to-noise ratio. The results are in good agreement with a numerical simulation of a semiclassical model. Analytical expressions for the dispersion are given.

We investigate Bell inequalities for neutral kaon systems from $\Phi$ resonance decay to test local realism versus quantum mechanics. We emphasize the unitary time evolution of the states, which means we also include all decay product states, in contrast to other authors. Only this guarantees the use of the complete Hilbert space. We develop a general formalism for Bell inequalities including both arbitrary “quasispin” states and different times; finally, we analyze Wigner-type inequalities. They contain an additional term, a correction function h, as compared to the spin $1/2$ or photon case, which changes considerably the possibility of quantum mechanics to violate the Bell inequality. Examples for special “quasispin” states are given, especially those that are sensitive to the $\mathrm{CP}$ parameters $\varepsilon$ and ${\varepsilon }^{\prime }.$

We consider a general unitary operator acting on two qubits in a product state. We find the conditions such that the state of the qubits after the action is as entangled as possible. We also consider the possibility of using ancilla qubits to increase the entanglement.

We have investigated ionization mechanisms in fast ion-atom collisions by measuring the low-energy electron emission cross sections in a pure three-body collision involving bare carbon ions $(v=6.35\mathrm{}\mathrm{a}.\mathrm{u}.\mathrm{})$ colliding with atomic hydrogen targets. The measurements have also been extended to molecular hydrogen and helium targets. In this paper we provide the energy and angular distributions of double differential cross sections of low-energy electron emission for atomic hydrogen targets. The Slevin rf source with a high degree of dissociation was used to produce the atomic H target. It is found that the two-center effect has a major influence on the observed large forward-backward angular asymmetry. A detailed comparison is presented with calculations based on the continuum distorted-wave (CDW) and CDW-EIS (eikonal initial-state) approximations. Both the continuum distorted-wave calculations provide a very good understanding of the data, whereas the first Born calculation predicts almost symmetric forward-backward distributions that do not agree with the data. The two-center effect is slightly better represented by the CDW calculations compared to the CDW-EIS calculation. The total cross sections are, however, in good agreement with the theories used. The results for molecular hydrogen and helium will be discussed in the following paper.