Single- and double-electron-capture cross sections for ${\mathrm{Si}}^{q+}$ $(q=3,$ 4, and 5) ions in collisions with He atoms have been measured at collision energies of a few hundred to a few thousand eV. The observed cross sections for single-electron capture are found to be of the order of ${10}^{-15}{\mathrm{cm}}^2,$ relatively independent of the collision energy, and are generally in agreement with recent quantal calculations for ${\mathrm{Si}}^{3+}$ and ${\mathrm{Si}}^{4+}$ ions and with the present Landau-Zener calculations for ${\mathrm{Si}}^{5+}$ ions. It is found that the measured cross sections for double-electron-capture processes, for which calculations are available only for ${\mathrm{Si}}^{4+}$ ions, are roughly one order of magnitude smaller than those for single-electron capture and tend to increase as the collision energy increases.

We present an experimental and theoretical study of efficient optical guiding of trapped cold atoms by a blue-detuned hollow laser beam. We have measured the atomic guiding efficiency with respect to the propagation direction of the hollow beam, which is in good agreement with numerical analysis. In particular, using a copropagating guide laser, we have obtained 20-fold enhancement of atom guiding with a guiding efficiency of 50%. Simulation shows that the efficiency can be increased to 80% with an improved hollow beam mode. The possibility of coherent atom guiding is also discussed.

A theoretical model of nuclear spin conversion in molecules controlled by an external electromagnetic radiation resonant to a rotational transition is developed. It is shown that one can produce an enrichment of spin isomers and influence their conversion rates in two ways, through coherences and through level population changes induced by radiation. The effect on conversion ranges from significant speed-up to almost complete inhibition of the process by proper choice of frequency and intensity of the external field.

The diffusion Monte Carlo method is applied to describe a trapped atomic Bose-Einstein condensate at zero temperature, fully quantum mechanically and nonperturbatively. For low densities, ${n\left(0\right)a}^3<~2\times {10}^{-3}$ $\left[n\right(0),$ peak density; a, s-wave scattering length], our calculations confirm that the exact ground-state energy for a sum of two-body interactions depends to a good approximation on only the atomic-physics parameter a, and no other details of the two-body model potential. Corrections to the mean-field Gross-Pitaevskii energy range from being essentially negligible to about 20% for $N=2\mathrm{}–50$ particles in the trap with positive s-wave scattering length $a=100\mathrm{}–10000\mathrm{a}.\mathrm{u}.\mathrm{}$ Our numerical calculations confirm that inclusion of an additional effective potential term in the mean-field equation, which accounts for quantum fluctuations [see, e.g., E. Braaten and A. Nieto, Phys. Rev. B 56, 14 745 (1997)], leads to a greatly improved description of trapped Bose gases.

We explore the role played by the quantum relative phase in a well-known model of atom-field interaction, namely, the Dicke model. We introduce an appropriate polar decomposition of the atom-field relative amplitudes that leads to a truly Hermitian relative-phase operator, whose eigenstates correctly describe the phase properties, as we demonstrate by studying the positive operator-valued measure derived from it. We find the probability distribution for this relative phase and, by resorting to a numerical procedure, we study its time evolution.

We present an efficient scheme to generate two-mode SU(2) macroscopic quantum superposition states, entangled number states, and entangled coherent states for the vibrational motion of an ion trapped in a two-dimensional harmonic potential well. We also show that the same scheme can be used to realize a Fredkin gate operation.

We consider an atomic Bose-Einstein condensate held within an optical cavity and interacting with laser fields. We show how the interaction of the cavity mode with the condensate can cause energy due to excitations to be coupled to a lossy cavity mode, which then decays, thus damping the condensate. We show how to choose parameters for damping specific excitations, and how to target a range of different excitations to potentially produce extremely cold condensates.

A new scheme to measure parity-nonconserving effects in atomic ytterbium is proposed. This method entails measuring parity-nonconserving optical rotation on the $1.28\hspace{0.5em}\mu \mathrm{m}\hspace{0.5em}6s6p{\hspace{0.5em}}^3{P}_0\to 6s6p{}^1{P}_1$ transition, for which optical rotation per unit absorption length is predicted to be an order of magnitude larger than that for transitions experimentally studied in thallium, lead, and bismuth. In the proposed experiment using a vapor cell filled with buffer gas, the lifetime of the lower metastable $6s6p{\hspace{0.5em}}^3{P}_0$ state is limited by collisional de-excitation. Atoms are excited to the lower metastable state by, e.g., short $\left(\sim 10\hspace{0.5em}\mathrm{ns}\right)$ near-resonant light pulses and spurious optical rotation is subtracted on a pulse by pulse basis. The potential sensitivity of this technique may enable precise measurements of electroweak parameters.

Time evolution is considered in phase space in terms of evolution kernels for various phase-space quasi-distributions. The propagators for the Wigner function, the standard-ordered function, the Kirkwood (antistandard-ordered) function, the Glauber P and Q functions, and the Husimi function are explicitly written as bilinear transforms of the evolution operator. Free propagation, propagation in dispersive media, and scattering, are studied, and manifestations of causality and interference are analyzed. It is shown that free propagation and scattering in the Husimi, Glauber, and Kirkwood representations with the underlying dynamics of the Schrödinger equation involve divergent evolution kernels connecting distant phase-space points at all times. The time evolution is much simpler in the Wigner representation where (i) free propagation is a simple classical translation involving no interference, and (ii) analytical properties of the scattering matrix restrict the velocities of propagation so that no information can travel due to scattering faster than free motion. As an example, a correlation is found between the coordinate and momentum of particles detected after they are released from a box. Propagators with relativistic dispersion relations of free photons or Klein-Gordon particles are briefly discussed in an Appendix.

It is known that quantum error correction via concatenated codes can be done with exponentially small failure rate if the error rate for physical qubits is below a certain accuracy threshold. Other, unconcatenated codes with their own attractive features such as improved accuracy threshold, local operations, have also been studied. By iteratively distilling a certain two-qubit entangled state it is shown how to perform an encoded Toffoli gate, important for universal computation, on codes whose C-NOT operation is bitwise. For certain codes, over a very large range of block sizes, this technique requires at most one concatenation.

Quantum-dot cellular automata (QCA), arrays of coupled quantum-dot devices, are proposed for quantum computing. The notion of coherent QCA (CQCA) is introduced in order to distinguish QCA applied to quantum computing from classical digital QCA. Information is encoded in the spatial state of the electrons in the multidot system. A line of CQCA cells can work as a quantum register. The basic single- and multi-qubit operations can be realized by pulses given to the cell electrodes.

Absolute measurements of the photoionization cross sections of ${\mathrm{Al}}^{+}$ ions ranging from 20 to 160 eV and of ${\mathrm{Al}}^{2+}$ ions from 70 to 105 eV have been performed utilizing a merged-photon–ion-beam setup with synchrotron radiation from an undulator. The continuum cross sections are in good agreement with published theoretical predictions. Structures resulting from excitation of valence or inner-shell electrons are resolved, identified, and compared with assignments from earlier spectral measurements. The analysis of the spectra is performed with the support of multiconfigurational atomic structure calculations.

Eightfold and fivefold differential cross sections (8DCS and 5DCS, respectively) for the $\mathrm{}(e,2e)\mathrm{}$ process on a water molecule are calculated within the framework of the first Born approximation, and the role of the molecular orientation on the collision process is studied and analyzed. The molecular wave functions are described by the linear combination of atomic orbitals (LCAO) (self-consistent-field LCAO molecular orbitals method). We present results for electron impinging at an intermediate energy of 250 eV and in an asymmetric coplanar geometry and kinematics. When particular molecule orientations are chosen, the 8DCS exhibit large differencies, in shape as well as in magnitude. A comparison of these results with an averaged-space calculation (integration of the 8DCS over Euler angles) confirms the necessity to take properly into account the molecular orientation in the calculations of the ionization cross sections of polyatomic molecules.

Finiteness of the computational resources is a hindrance to representing the time evolution of an infinitely extended system. Several numerical techniques are available for mimicking the nonboundedness of the system despite the restricted Hilbert space of the employed expansion basis set. We present a formulation based on the outgoing-wave Siegert pseudostates. A harmonic oscillator exposed to a periodic train of impulsive pulses (“kicks”) demonstrate the efficiency of the Siegert method.

Partial and total cross sections for the radiative association process $\mathrm{H}+\mathrm{H}\overrightarrow{¯}\mathrm{H}\mathrm{H}¯+h\nu$ in the sub-Kelvin temperature regime are reported. We calculate the emission spectra for this process and suggest its utility as a diagnostic. The cross section for radiative association is found to have the value $\sigma =6.85\mathrm{}\times {10}^{-22}\hspace{0.5em}{\mathrm{cm}}^2/\sqrt{T}$, where T is an effective temperature. It is significantly smaller than the in-flight annihilation and fragmentation cross sections. We discuss the role that radiative association plays in the formation of a quasibound system of the $\mathrm{H}\mathrm{H}¯$ molecule.

Combining the first-principles electronic structure theory and time-dependent technique, we evaluate the vibrational structure in the photoelectron spectrum of ${\mathrm{C}}_{10}^{-}.$ This combination approach is exact within the Condon approximation and the harmonic-vibration model, i.e., it includes displacement, frequency change, and mode mixing (Duchinsky effect) of the vibrations between the initial and final charge states. The evaluation shows a broad asymmetric band shape, which is consistent with an experimentally measured spectrum. This asymmetric shape, which is common to the Franck-Condon spectra for quasistrong vibronic (vibration-electron) coupling, is found to be well fitted to a skewed Gaussian function. And, it has also been found that the asymmetry is increased as the temperature becomes high. On the contrary, the vibrational model taking into account only displacement, which is widely accepted in analyzing Franck-Condon spectra, always predicts a symmetric shape at high temperatures; therefore the temperature induced increase of asymmetry in the ${\mathrm{C}}_{10}^{-}$ system is, to our knowledge, novel.

We present a reliable scheme for engineering arbitrary motional ionic states through an adaptation of the projection synthesis technique for trapped-ion phenomena. Starting from a prepared coherent motional state, the Wigner function of the desired state is thus sculpted from a Gaussian distribution. The engineering process has also been developed to take into account the errors arising from intensity fluctuations in the exciting-laser pulses required for manipulating the electronic and vibrational states of the trapped ion. To this end, a recently developed phenomenological-operator approach that allows for the influence of noise will be applied. This approach furnishes a straightforward technique to estimate the fidelity of the prepared state in the presence of errors, precluding the usual extensive ab initio calculations. The results obtained here by the phenomenological approach, to account for the effects of noise in our engineering scheme, can be directly applied to any other process involving trapped-ion phenomena.

The optical Bloch equations exhibit symmetries that are directly related to the probabilities of the processes of absorption and stimulated emission. As a matter of fact, a picture similar to that holding for a two-level system can be recovered also in multilevel systems and, in particular, for any multiphoton process tuned between a given couple of levels. Thus, new quantities, such as the generalized Einstein B coefficients, directly related to the probabilities of the involved process, and atomic level indicators, whose inversion signals the existence of gain for that given process, can be defined. It is shown how these lead to valuable conceptual simplifications and to the disclosure of new insights into some popular problems of quantum optics. In particular, the issue of an efficient gain process to be realized within a multilevel atom-field configuration, a problem closely related to the widely investigated problem of lasing configurations without population inversion, can be viewed entirely as a rate problem and therefore analyzed with increased confidence.

Collisional relaxation of highly rotating hydrogen molecules is investigated as a function of energy. Calculations demonstrate that inelastic collisions are dramatically suppressed for specific rotational levels of the molecule as the energy is lowered due to the closing of quasiresonant rotation-vibration channels. It is predicted that a ${}^3\mathrm{He}$ buffer gas may be used to load these highly excited molecules into a trap without a significant loss of population. It is further predicted that evaporative cooling may be used to cool the “super rotors” to even lower temperatures.

An approximate local-density functional, quite similar to the well-known Colle-Salvetti functional and complying with a basic requirement of the exchange-correlation hole, is derived and represented as a simple polynomial. This expression gives an essentially correct behavior for the Coulomb hole. Test calculations on some two-electron atoms lead to energy values closer than 0.05% to the experimental ones. The expression can be further improved by taking into account the effects of the density gradient.