Excitation and ionization of hydrogenic projectiles in collisions with metal free-electron gas are analyzed in the high but nonrelativistic energy regime. Transition matrix elements are calculated in the first Born approximation, and the free-electron gas response is described by using the Mermin-Lindhard dielectric function. Projectile excitation and loss probabilities per unit length are found to be smaller than those corresponding to the collisions with a wave packet of single electrons of equivalent density. This behavior is explained in terms of the collective effect (shielding) of the free-electron gas and depends on the energy transferred to the atomic electron and on the impact velocity. By comparing results obtained using a binary collisional formalism and those using the dielectric formalism, we estimate the plasmon excitation contribution to the total probabilities.

Improvements in the characterization of II-VI compound-based solar cells, and the recent experimental characterization of small clusters and nanoparticles, make the study of small II-VI clusters very interesting. In this work, the ground states of small ${\mathrm{Zn}}_i{\mathrm{O}}_i$ clusters are studied, where $i=1\mathrm{}–9.$ Ringlike structures have been found to be the global minima for clusters as large as $i=7,$ and three-dimensional spheroid structures for larger ones, where $i=8\mathrm{}$ and 9. This is due to the stability of obtuse O-Zn-O angles in the first case, and to the stability gained from higher coordination in the second case. Three-dimensional structures may be envisioned as being built from ${\mathrm{Zn}}_2{\mathrm{O}}_2$ and ${\mathrm{Zn}}_3{\mathrm{O}}_3$ rings, as was the case for ZnS three-dimensional global minima and other ZnO calculations. Calculated natural orbital charges become larger as the cluster size increases, showing a tendency toward bulk charges.

In density-functional theory, the standard Perdew-Zunger method of self-interaction correction leads to exchange-correlation (xc) potentials that are orbital-dependent. We study various simplified implementations of self-interaction correction—derived in the framework of the so-called optimized effective potential method—that lead to a single, orbital-independent xc potential. The performance of the various self-interaction corrected functionals is tested for sodium clusters: we first consider a variety of ground-state properties (total energies, Kohn-Sham orbital eigenvalues, and ionic potential curves) and then study the dynamical regime of weak and strong electronic excitations. It turns out that for sodium clusters the simplest, global-averaging scheme gives a good description of static and dynamic properties, even in the regime of strong excitations.

We study the interplay between resonant multiphoton population transfer and strong field ionization in potassium atoms, by calculating their response to intense, midinfrared (MIR) radiation. We use wavelengths close to the three or five photon resonance between the ground $4s$ state and the first excited $4p$ state, and find that the amount of population transferred to the excited state as a function of wavelength and intensity shows clear signatures of these resonances. The ionization yield also shows moderate resonant enhancement for intermediate intensities. We can further distinguish between population transfer to and ionization of the $4p$ state by using a chirped laser pulse to temporally separate the two processes. We find that the amount of population remaining in the $4p$ state at the end of the pulse shows a pronounced sensitivity to the sign of the chirp whereas the total amount of ionization does not. From this we conclude that direct, nonresonant ionization of the ground state competes strongly with the resonant process.

We propose a single-photon interferometer which provides cryptographic quantum communication equivalent to that obtained using a pair of polarization entangled photon twins as a carrier [Phys. Rev. A 60, 157 (1999)]. Instead of manipulating and measuring the internal states of entangled photon twins, cryptographic quantum communication is possible by manipulating a single photon in an extended interferometer. The use of the single-photon interferometer offers significant experimental advantages compared to our previous approach.

A precision measurement of the Casimir force using metallic gold surfaces is reported. The force is measured between a large gold-coated sphere and flat plate using an atomic force microscope. The use of gold surfaces removes some theoretical uncertainties in the interpretation of the measurement. The forces are also measured at smaller surface separations. The complete dielectric spectrum of the metal is used in the comparison of theory to the experiment. The average statistical precision remains at the same 1% of the forces measured at the closest separation. These results should lead to the development of stronger constraints on hypothetical forces.

We present a unified approach for solving and classifying exactly solvable potentials. Our unified approach encompasses many well-known exactly solvable potentials. Moreover, our approach can be used to search systematically for a new class of solvable potentials.

Stochastic resonance shows that under some circumstances noise can enhance the response of a system to a periodic force. While this effect has been extensively investigated theoretically and demonstrated experimentally in classical systems, there is complete lack of experimental evidence within the purely quantum-mechanical domain. Here we demonstrate that stochastic resonance can be exhibited in a single ion and would be experimentally observable using well mastered experimental techniques. We discuss the use of this scheme for the detection of the frequency difference of two lasers to demonstrate that stochastic resonance may have applications in precision measurements at the quantum limit.

Recent advances in cooling and trapping of atomic particles has opened up the possibility of building microscopic networks of tubelike traps. As possible applications, we describe how a quantum controlled-NOT gate and a Bell-state analyzer could be implemented using atomic particles, which propagate through the system of channels and interact with the device potential and each other.

This paper discusses a discretization problem for quantum continuous channels. We show that the binary discretization asymptotically realizes the capacity and the maximum mutual information; the classical continuous channel does not have such an asymptotic property. We further clarify the significance of Gordon’s result from the aspect of binary discretization for the maximum mutual information.

Accurate results are reported for ground-state positronium formation in positron-hydrogen collisions using 12 correlated basis functions in a multichannel Schwinger’s principle in momentum space at intermediate energies 30–150 eV in an extension of our earlier work [Phys. Rev. A 59, 1913 (1999)]. The rich structure in the differential cross section is found to be present due to the existence of critical angles. Results for the total formation cross section are in excellent agreement with other variational and nonvariational calculations available in the literature. The observed data of Zhou et al. [Phys. Rev. A 55, 361 (1997)] are also in good accord with these predictions.

Total as well as partial cross sections for single-electron capture of ${\mathrm{H}}^{+}$ on Ar are calculated by the semiclassical impact parameter method, using a two-center atomic basis expansion, in the impact energy range 10–150 keV. The resulting total cross sections are in good agreement with previous experimental data. Partial cross sections agree qualitatively with measurements at high energies, while at low energies the experimental data are found to be approximately six times smaller than the present results.

Electron emission from ${\mathrm{H}}^0$ projectiles (1–20 keV) grazingly scattered off a LiF(001) single-crystal surface has been studied for impact angles from $0.5°$ to $1.8°.$ We observe that the threshold energy for electron emission increases with smaller projectile impact angle, whereas at higher impact energies smaller angles are more efficient for producing electrons. Our data are analyzed in terms of position- and velocity-dependent electron production rates that indicate that an electron promotion mechanism is responsible for electron emission.

We have studied the quantum-noise properties of a small slow-inversion laser experimentally and theoretically. As a function of pump rate, the average output intensity shows a sharp threshold, but the intensity fluctuations do not. Under quite generic conditions the intensity fluctuations of a sufficiently small slow-inversion laser show highly super-Poissonian statistics (even two times above threshold), due to the very weak damping of the spontaneous-emission-driven relaxation oscillation.

Off-resonant effects are a significant source of error in quantum computation. This paper presents a group-theoretic proof that off-resonant transitions to the higher levels of a multilevel qubit can be completely prevented in principle. This result can be generalized to prevent unwanted transitions due to qubit-qubit interactions. A simple scheme exploiting dynamic pulse control techniques is presented that can cancel transitions to higher states to arbitrary accuracy.

Quantum entanglement cannot be unlimitedly shared among an arbitrary number of qubits. The degree of bipartite entanglement decreases as the number of entangled pairs in an N-qubit system increases. We analyze a system of N qubits in which an arbitrary pair of particles is entangled. We show that the maximum degree of entanglement (measured in the concurrence) between any pair of qubits is $2/N.$ This tight bound can be achieved when the qubits are prepared in a pure symmetric (with respect to permutations) state with just one qubit in the basis state $|0〉$ and the others in the basis state $|1〉.$

We show how entangled qubits can be encoded as entangled coherent states of two-dimensional center-of-mass vibrational motion for two ions in an ion trap. The entangled qubit state is equivalent to the canonical Bell state, and we introduce a proposal for entanglement transfer from the two vibrational modes to the electronic states of the two ions in order for the Bell state to be detected by resonance fluorescence shelving methods.

A density matrix ρ may be represented in many different ways as a mixture of pure states, $\rho ={\sum }_i{p}_i|{\psi }_i〉〈{\psi }_i|.$ This paper characterizes the class of probability distributions ${(p}_i)$ that may appear in such a decomposition, for a fixed density matrix ρ. Several illustrative applications of this result to quantum mechanics and quantum information theory are given.

We develop a multivalued logic for quantum computing for use in multi-level quantum systems, and discuss the practical advantages of this approach for scaling up a quantum computer. Generalizing the methods of binary quantum logic, we establish that arbitrary unitary operations on any number of d-level systems $\mathrm{}(d>\mathrm{}2\mathrm{})\mathrm{}$ can be decomposed into logic gates that operate on only two systems at a time. We show that such multivalued logic gates are experimentally feasible in the context of the linear ion trap scheme for quantum computing. By using d levels in each ion in this scheme, we reduce the number of ions needed for a computation by a factor of ${\log }_{2}d.\mathrm{}$