A dipolar gate alternative to the exchange-gate-based Kane quantum computer is proposed where the qubits are electron spins of shallow group V donors in silicon. Residual exchange coupling is treated as gate error amenable to quantum error correction, removing the stringent requirements on donor positioning characteristic of all silicon exchange-based implementations [Koiller et al., Phys. Rev. Lett. 88, 027903 (2002)]. Contrary to common speculation, such a scheme is scalable with no overhead in gating time even though it is based on long-range dipolar interqubit coupling.

The exact solution of the Schrödinger equation for the Coulomb potential is used within the scope of both stationary and time-dependent scattering theories in order to find the parameters which determine the regularization of the Rutherford cross section when the scattering angle tends to zero but the distance $r$ from the center remains finite. The angular distribution of the particles scattered in the Coulomb field is studied on rather a large but finite distance $r$ from the center. It is shown that the standard asymptotic representation of the wave functions is inapplicable in the case when small scattering angles are considered. The unitary property of the scattering matrix is analyzed and the “optical” theorem for this case is discussed. The total and transport cross sections for scattering the particle by the Coulomb center proved to be finite values and are calculated in the analytical form. It is shown that the effects under consideration can be important for the observed characteristics of the transport processes in semiconductors which are determined by the electron and hole scattering by the field of charged impurity centers.

Electron-atom scattering in a strong laser field is analyzed using the strong-field approximation and modeling elastic scattering of electrons by atoms with a realistic analytical potential derived from an independent-particle model. The results that include both direct scattering and scattering with a repeated scattering (rescattering) are presented. In the latter case, in the intermediate step of the process, the electron can absorb the energy from the laser field and additional plateau structures appear. The features of these plateaus and their cutoffs are analyzed for various incident electron energies and scattering angles, for different laser intensities, and for various atomic gases. The boundaries of these plateaus are compared with classical estimates.

We discuss how a three-level system can be used to change the frequency-dependent magnetic permeability of an atomic gas to be significantly different from 1. We derive the conditions for such a scheme to be successful and briefly discuss the resulting macroscopic electrodynamics. We find that it may be possible to obtain left-handed electrodynamics for an atomic gas using three atomic levels.

We present a fully quantum mechanical treatment of the nondegenerate optical parametric oscillator both below and near threshold. This is a nonequilibrium quantum system with a critical point phase transition, that is also known to exhibit strong yet easily observed squeezing and quantum entanglement. Our treatment makes use of the positive $P$ representation and goes beyond the usual linearized theory. We compare our analytical results with numerical simulations and find excellent agreement. We also carry out a detailed comparison of our results with those obtained from stochastic electrodynamics, a theory obtained by truncating the equation of motion for the Wigner function, with a view to locating regions of agreement and disagreement between the two. We calculate commonly used measures of quantum behavior including entanglement, squeezing, and Einstein-Podolsky-Rosen (EPR) correlations as well as higher order tripartite correlations, and show how these are modified as the critical point is approached. These results are compared with those obtained using two degenerate parametric oscillators, and we find that in the near-critical region the nondegenerate oscillator has stronger EPR correlations. In general, the critical fluctuations represent an ultimate limit to the possible entanglement that can be achieved in a nondegenerate parametric oscillator.

We investigate the entanglement properties of a one-dimensional chain of qubits coupled via nearest-neighbor spin-spin interactions. The entanglement measure used is the -concurrence, which is distinct from other measures on spin chains such as bipartite entanglement in that it can quantify “global” entanglement across the spin chain. Specifically, it computes the overlap of a quantum state with its time-reversed state. As such, this measure is well suited to study ground states of spin-chain Hamiltonians that are intrinsically time-reversal-symmetric. We study the robustness of -concurrence of ground states when the interaction is subject to a time-reversal antisymmetric magnetic field perturbation. The -concurrence in the ground state of the isotropic model is computed and it is shown that there is a critical magnetic field strength at which the entanglement experiences a jump discontinuity from the maximum value to zero. The -concurrence for thermal mixed states is derived and a threshold temperature is computed below which the system has nonzero entanglement.

The double-charge-transfer (DCT) spectrum of $\mathrm{O}_2{}^{2+}$ has been recorded at $140\mathrm{meV}$ resolution using $1.5\mathrm{keV}$ ${\mathrm{H}}^{+}$ projectiles. Electronic bands of the low-lying triplet states, namely $A{}^3\Sigma _u^{+}$, $W{}^3\Delta _u$, $B{}^3\Pi _g$, and $B^{\prime }{}^3\Sigma _u^{-}$, have been resolved. Assuming that the observed bands directly reflect the Franck-Condon (FC) profiles, potential-energy curves of the $A{}^3\Sigma _u^{+}$, $W{}^3\Delta _u$, and $B^{\prime }{}^3\Sigma _u^{-}$ states within the FC region are determined based on the reflection approximation. For a quantitative check of the results, complete active space self-consistent-field and multireference configuration-interaction calculations have been performed to predict the potential-energy curves and the FC profiles. The calculated energy curves agree well with the experimentally derived ones. The theoretical FC profiles reproduced well the band shapes observed by both present DCT and previous Doppler-free kinetic-energy release methods. However, our results cast doubt on the previous assignment of luminescence to the $\mathrm{O}_2{}^{2+}$ $B{}^3\Pi _g\to A{}^3\Sigma _u^{+}$ transition and the observed bound level of the $A{}^3\Sigma _u^{+}$ state by the previous threshold photoelectron coincidence experiment.

Multiconfiguration Dirac-Fock calculations with the inclusion of the transverse (Breit) interaction and QED corrections have been carried out for Pd, Sn, Tb, Ta, Pb, and Th in order to obtain positions and intensities of various electric dipole, electric quadrupole, and magnetic dipole $K$ x-ray diagram lines and of their $K{L}^1$ satellites. Theoretically constructed stick spectra have been presented together with synthesized spectra (the sum of the Lorentzian natural line shapes) for each studied element. Taking into account the existence of an $L$-shell hole in the $2s$ or $2p$ subshell, the effect of additional $L$-shell ionization on the shapes and structure of the $K$ x-ray spectra has been examined. It has been observed that generally with increasing atomic number $Z$ the shapes of particular satellite line groups tend to become smoother and to differ less from the shapes of appropriate diagram lines. Relations between the values of energy shifts of various satellite lines for each element and the changes of these relations with $Z$ have also been studied. Additionally, the relations between the intensities of different diagram lines for each element have been systematically analyzed, likewise the changes with $Z$ of the role of particular diagram lines. This study can be helpful in reliable and quantitative interpretation of many experimental $K$ x-ray spectra of Pd, Sn, Tb, Ta, Pb, and Th induced in collisions with various projectiles.

We investigate local and global properties of the one-dimensional Bose-Hubbard model with an external confining potential, describing an atomic condensate in an optical lattice. Using quantum Monte Carlo techniques we demonstrate that a local-density approximation, which relates the unconfined and the confined model, yields quantitatively correct results in most of the interesting parameter range. We also examine claims of universal behavior in the confined system, and demonstrate the origin of a previously calculated fine structure in the experimentally accessible momentum distribution.

We consider a two-component Fermi gas interacting via a Feshbach molecular state. It is shown that only for broad resonances where the Feshbach state is strongly coupled to the free particle continuum can the gas be expected to enter a universal state in the unitarity limit on the atomic side of the resonance where there are no molecules present. The universal state is distinct from the molecular gas state on the other side of the resonance. We furthermore calculate the energy of the gas for this universal state and our results are related to current experiments on ${}^6\mathrm{Li}$ and ${}^{40}\mathrm{K}$.

In a time-orbiting-potential magnetic trap the neutral atoms are confined by means of an inhomogeneous magnetic field superimposed onto an uniform rotating one. We perform an analytic study of the atomic motion by taking into account the nonadiabatic effects arising from the spin dynamics about the local magnetic field. Geometric like magnetic fields determined by Berry’s phase appear within the quantum description. Application of the time-dependent variational principle on the original quantum equation leads to a set of dynamical evolution equations for the quantum average value of the position operator and spin variables. Within this approximation we derive the quantum-mechanical ground-state configuration matching the classical adiabatic solution and perform some numerical simulations.

We discuss the use of electromagnetically modified absorption to achieve selective excitation in atoms: that is, the laser excitation of one transition while avoiding simultaneously exciting another transition whose frequency is the same as or close to that of the first. The selectivity which can be achieved in the presence of coherent population trapping (CPT) is limited by the decoherence rate of the dark state. We present exact analytical expressions for this effect, and also physical models and approximate expressions which give useful insights into the phenomena. When the laser frequencies are near-resonant with the single-photon atomic transitions, CPT is essential for achieving discrimination. When the laser frequencies are far detuned, the “bright” two-photon Raman resonance is important for achieving selective excitation, while the “dark” resonance (CPT) need not be. The application to laser cooling of a trapped atom is also discussed.

Atomic data (energy levels, radiative rates, electron collision strengths, and excitation rate coefficients) of 107 fine-structure levels of the $\left(1s^{2}2s^{2}2p^6\right)3s^{2}3p^{6}3d^{10}$, $3s^{2}3p^{6}3d^{9}4l$, $3s^{2}3p^{5}3d^{10}4l$, and $3s3p^{6}3d^{10}4l(l=s,p,d,f)$ configurations of nickel-like Ta ions are calculated using the distorted-wave approximation. Coupled with these atomic data, a hydrodynamic code MED103 is used to optimize the nickel-like Ta x-ray laser at $4.48\mathrm{nm}$. Using the optimized drive-pulse configuration, an experiment is conducted to generate the nickel-like Ta x-ray laser at $4.48\mathrm{nm}$.

We investigate the scattering threshold and cavity-enhanced gain in nonlinear spheres with second- or third-order permeability. Pairs of pump-driven idler and signal modes are considered, satisfying morphology-dependent resonance conditions. The thresholds and gain coefficients of amplified and stimulated Raman scattering, parametric downconversion, and analogous parametric processes in microspheres are derived and evaluated under typical conditions. Applications may include the measurement of chemical impurity concentrations or the creation of low-threshold optical parametric amplifiers using microspheres.

Owing to the frequency dependence of the vacuum density of states, the spontaneous emission decay rate varies as the cube of a frequency. The actual frequency that enters into the decay rate depends to some extent on the physical system under investigation. We show that, in the weak-field limit and the rotating-wave approximation, the decay parameters associated with Ralyleigh scattering are evaluated at the field frequency, while the transient response following the turn-on of the field has a decay rate evaluated at the atomic frequency. The optical Bloch equations are modified to include additional coupling between populations and coherences, both in bare and dressed-state pictures.

The energy and angular distributions of backscattered electrons produced under impact of $8.0\text{−}\mathrm{keV}$ electrons with a thick tungsten target are measured. The energy range of backscattered electrons is considered between 70 and $1700\mathrm{eV}$. The angle of incidence $\alpha$ and the takeoff angle $\theta$ are chosen to have values $\alpha =0°$, 10°, and 20° and $\theta =110°$, 120°, and 130°, respectively. The energy distribution function exhibits two sharp peaks, which are found to appear at 216 and $548\mathrm{eV}$. They are identified as Auger peaks of tungsten arising due to electron transitions $4d\text{−}6s6p$ and $4s\text{−}6s6p$, respectively. The measured energy spectra are compared with two different theoretical models. The theoretical predictions are found to yield a good agreement with the experiment in the considered energy range of the backscattered electrons.

In a recent experiment at the vuv free-electron laser facility at DESY in Hamburg, the generation of multiply charged ions in a gas of atomic xenon was observed. This paper develops a theoretical description of the multiphoton ionization of xenon and its ions. The numerical results lend support to the view that the experimental observation may be interpreted in terms of the nonlinear absorption of several vuv photons. The method rests on the Hartree-Fock-Slater independent-particle model. The multiphoton physics is treated within a Floquet scheme. The continuum problem of the photoelectron is solved using a complex absorbing potential. Rate equations for the ionic populations are integrated to take into account the temporal structure of the individual vuv laser pulses. The effect of the spatial profile of the free-electron laser beam on the distribution of xenon charge states is included. An Auger-type many-electron mechanism may play a role in the vuv multiphoton ionization of xenon ions.

Orthogonality of two-photon polarization states belonging to a single frequency and spatial mode is demonstrated experimentally, in a generalization of the well-known anticorrelation “dip” experiment.