The operation of a solid-state superconducting quantum computer based on clean Josephson junctions between two d-wave superconductors is considered. We show that freezing of passive qubits can be achieved using a dynamic global refocusing technique. Further, we demonstrate that a universal set of gates can be realized on this system, thereby proving its universality.

We present a strategy to engineer a simple cavity-QED two-bit universal quantum gate using mesoscopic distinct quantum superposition states. The dissipative effect on the decoherence and the amplitude damping of the quantum bits are analyzed and the critical parameters are presented.

Most natural Hamiltonians do not couple specific pairs of quantum bits and spurious couplings occur along with the intended one. We present an efficient scheme that couples any designated pair of spins in heteronuclear spin systems. The scheme is based on the existence of Hadamard matrices. For a system of n spins with pairwise coupling, the scheme concatenates $\mathrm{cn}$ intervals of system evolution and uses at most ${\mathrm{cn}}^2$ pulses where $c\approx 1.$ Our results demonstrate that, in many systems, selective recoupling is possible with linear overhead, contrary to common speculation that exponential effort is always required.

I present a variety of results on the theory of quantum secret sharing. I show that any mixed state quantum secret sharing scheme can be derived by discarding a share from a pure state scheme, and that the size of each share in a quantum secret sharing scheme must be at least as large as the size of the secret. I show that the only constraints on the existence of quantum secret sharing schemes with general access structures are monotonicity (if a set is authorized, so are larger sets) and the no-cloning theorem. I also discuss some aspects of sharing classical secrets using quantum states. In this situation, the size of each share can sometimes be half the size of the classical secret.

Excited states of the hydrogen molecule that are subject to a homogeneous magnetic field are investigated for the parallel configuration in the complete regime of field strengths $B=0\mathrm{}–100\hspace{0.5em}\mathrm{a}.\mathrm{u}.\mathrm{}$ Up to seven excitations are studied for gerade as well as ungerade spin singlet states of $\Sigma$ symmetry with a high accuracy. The evolution of the potential-energy curves for the individual states with increasing field strength, as well as the overall behavior of the spectrum, are discussed in detail. A variety of phenomena such as the sequence of changes for the dissociation channels of excited states and the resulting formation of outer wells are encountered. Possible applications of the obtained data to the analysis of magnetic white dwarfs are outlined.

A quantum fluid-dynamic (QFD) control formulation is presented for optimally manipulating atomic and molecular systems. In QFD the control quantum system is expressed in terms of the probability density ρ and the quantum current j. This choice of variables is motivated by the generally expected slowly varying spatial-temporal dependence of the fluid-dynamical variables. The QFD approach is illustrated for manipulation of the ground electronic state dynamics of HCl induced by an external electric field.

We present a detailed Monte Carlo study of the many-particle density correlations in an interacting two-dimensional (2D) Bose gas, which are responsible for the recombination and spin relaxation rates in real experiments. We confirm that a $1/m!$ effect for local correlators does exist in 2D and can be used for the study of phase transitions. In agreement with a recent experiment [A. I. Safonov et al., Phys. Rev. Lett. 81, 4545 (1998)], the appearance and growth of quasicondensate fluctuations changes local correlations well before the Kosterlitz-Thouless transition point. Even a moderate interaction considerably reduces the amplitude of the $1/m!$ effect. This conclusion indicates that the enhancement of the $1/m!$ effect observed in the experiment by Safonov et al. cannot be ascribed to finite-interaction effects in a purely 2D system.

Spontaneous emission from a V-type three-level atom in a photonic crystal is investigated. Quantum interference between the two atomic transitions affects the constructor of dressed states, and leads to an interesting behavior of the populations in the two upper levels: antitrapping, periodic oscillation, and no population inversion. Those properties depend strongly on the relative position of the upper levels from the forbidden gap and the initial state of the atom, and differ from that of a two-level atom in a photonic crystal. The emitted field, which is composed of localized mode(s) and propagating mode(s), is also studied. Quantum interference can enhance or reduce the energy of the localized field.

We suggest a self-testing teleportation configuration for photon qubits based on a Mach-Zehnder interferometer. That is, Bob can tell how well the input state has been teleported without knowing what that input state was. One could imagine building a “locked” teleporter based on this configuration. The analysis is performed for continuous variable teleportation but the arrangement could be applied equally to discrete manipulations.

We demonstrate experimentally the usefulness of selective pulses in NMR to perform quantum computation. Three different techniques based on selective pulse excitations have been proposed to prepare a spin system in a pseudopure state. We describe the design of “portmanteau” gates using the selective manipulation of level populations. A selective pulse implementation of the Deutsch-Jozsa algorithm for two and three-qubit quantum computers is demonstrated.

We simulate Shor’s algorithm using the Ising spin quantum computer. The influence of nonresonant effects is analyzed in detail. It is shown that the method developed earlier in our papers successfully suppresses nonresonant effects even for relatively large values of the Rabi frequency.

Secondary-electron yields were investigated for 28-MeV protons, 126-MeV oxygen-ions, and 182-MeV gold ions incident on 304 stainless-steel surfaces. The dependence on the incidence angle was studied in detail, and a system was developed which allows accurate measurements to be performed over a wide angular range extending to nearly grazing collisions. Electron yield estimates of interest for future accelerator applications are developed for 1-GeV protons, and the possible mitigation of deleterious effects by using serrated rather than flat surfaces is analyzed.

We investigate the two-time intensity correlation functions of the fluorescence field emitted from a $V$-type three-level atom. We are particularly interested in the manner in which the atom emits photons in the presence of quantum interference. We show that under strong-field excitation quantum interference leads to anticorrelations of photons emitted from the atomic excited levels which can exist for extremely long times. This indicates that the excited atomic levels are not the preferred radiative states. We find that the atom spends most of its time in a superposition of the excited atomic levels from which it emits strongly correlated photons. The strong correlations are present only for a nonzero splitting between the excited levels, and for degenerate levels the correlations reduce to that of a two-level atom. Moreover, we find that the transition from the ground level to the symmetric superposition of the excited levels does not saturate even for a strong driving field. We also calculate the correlation functions for a weak driving field, and find that in this case the photon correlations are not significantly affected by quantum interference, but the atom can emit a strongly correlated pair of photons produced by a three-wave mixing process. Under appropriate conditions, with near-maximal quantum interference, it is possible to make the maximum value of the correlation function extremely large, in marked contrast with the corresponding case with no quantum interference.

The standard multimode rate equation for semiconductor lasers is based on the approximation that modal field shapes (if they change at all) depend on the instantaneous value of the time-dependent dielectric function. This is known as the adiabatic approximation. It will break down if the inverse of the modulation frequency approaches the photon round-trip time in the cavity. In this paper, we derive a criterion for the validity of the adiabatic approximation and find that it also involves the fractional modulation of the dielectric function, not just the photon round-trip time and the modulation frequency. Recognizing that present laser designs are starting to approach the limits of validity as set by our criterion, we study what can be done to get past that limit. We obtain corrections to the equations presently used by rederiving these equations from the time-dependent wave equation without discarding the time derivative of the dielectric function, an approximation made in every other derivation we have encountered. Retaining this time derivative introduces new terms into the usual equations. These terms correctly account for propagation delay-time effects and for nonadiabatic couplings between the modes. Surprisingly, the new terms alter only the source of photons to each mode. The usual rate equations are driven by a spontaneous emission term $R_{\nu }^{\mathrm{sp}}\mathrm{}\left(t\right),$ which represents the rate at which photons are emitted spontaneously into the mode ν. In the rate equation derived here, the spontaneous emission term $R_{\nu }^{\mathrm{sp}}\mathrm{}\left(t\right)\mathrm{}$ is augmented by a term ${\Xi }_{\nu }\mathrm{}\left(t\right)\mathrm{}$ which counts photons that were earlier emitted spontaneously into other modes μ, accumulated and perhaps amplified there, and are now, because of the breakdown of the adiabatic approximation, leaking into the $\nu \mathrm{th}$ mode. Although casting the equations into this form makes sense from a physical point of view, it leads to great computational difficulties in solving the equations because ${\Xi }_{\nu }\mathrm{}\left(t\right)\mathrm{}$ refers explicitly to the past history of the laser. To overcome this practical problem, we provide an efficient and accurate algorithm for stepping the laser forward in time without having to retain history prior to the start of the present time step. Our method allows the equations to be solved with substantially the same computational effort as is normally expended in solving the conventional rate equations, and, moreover, provides error estimates at each step of the way.

We propose a method of generating skyrmion vortices in a pair of Bose-Einstein condensates occupying two internal states of the same atom. We show that a variety of different periodic arrays of vortices may be prepared with an appropriate superposition of orthogonal standing electromagnetic waves inducing a coherent coupling between the two condensates.

We study ground-state properties of trapped atomic condensates with electric-field-induced dipole-dipole interactions. A rigorous method for constructing the pseudopotential in the spirit of ladder approximation is developed for general nonspherical (polarized) particles interacting anisotropically. We discuss interesting features not previously considered for currently available alkali-metal condensates. In addition, to provide a quantitative assessment for controlling atomic interactions with electric fields, our investigation may also shed a different light into the macroscopic coherence properties of the Bose-Einstein condensation (BEC) of dilute interacting atoms.

There have been studies on the formation of quantum-nonlocal states in two spatially separate cavities. We suggest a nonlocal test for a field prepared in two cavities. We couple classical driving fields with cavities where a nonlocal state is prepared. Two independent two-level atoms are then sent through respective cavities to interact off-resonantly with the cavity fields. The atomic states are measured after the interaction. Bell’s inequality can be tested by the joint probabilities of the two-level atoms being in their excited or ground states. We find that a violation of Bell’s inequality can also be tested using a single atom sequentially interacting with the two cavities. Potential experimental errors are also considered. We show that with the present experimental condition of 5% error in the atomic velocity distribution, the violation of Bell’s inequality can be measured.

In the framework of the two-step model of Auger decay the alignment of the xenon ${4d}^{-1}$ primary hole states has been derived as a function of photon energy. This quantity was obtained from angular distribution measurements of the $N_{4,5}{O}_{2,3}{O}_{2,3}$ Auger lines after ionization of free atoms by monochromatized synchrotron radiation of 80–250 eV photon energy. The data show a minimum of the alignment parameter in the Cooper minimum of the $4d$ photoionization cross section, which proves the vanishing of the outgoing $\epsilon f$ electron wave. Furthermore, the intrinsic parameters for all lines in this Auger group were determined and are compared with previous experiments and calculations.

The spectrum of bremsstrahlung due to photoelectrons ejected by incident photons of energy 59.5 keV was measured on an absolute scale. A coincidence experimental setup with two high-purity germanium detectors was used. One detector served as the target as well as the detector of ejected electrons and another (second) detector served as the detector of bremsstrahlung radiation. The applied experimental method gives a very clean bremsstrahlung spectrum which can be reliably determined on an absolute scale in the low- and mid-energy range. Detailed processing of the data is presented, including calculations of all other processes that have also been recorded in the experiment. A simple theoretical model for the bremsstrahlung of photoelectrons in an infinitely thick target is applied and the results are compared with experimental values. Good agreement between the theoretical and experimental results has been obtained.

When an ideal Bose gas is enclosed in two boxes with different volumes connected by a narrow tube, and the temperature is below the transition temperature for Bose-Einstein condensation, it is expected that the particles will be localized in the larger box, because the ground state is localized here. In this paper, we consider the effect of the interaction between particles on the localization using Gross-Pitaevskii theory. Our consideration is limited to the case of positive scattering length and zero temperature. We determine a condition for the localization with an approximate method. It is shown that the system must be extremely dilute or extremely small for localization to occur. The localization condition is confirmed by a numerical calculation.