The transmissivity calculated as a function of the frequency of oscillation for a particle meeting an oscillating barrier presents a resonant structure, as shown by Hagmann [Appl. Phys. Lett. 66, 789 (1995)]. The origin of this structure is explained with the help of the expression giving the scattering amplitudes in the Floquet picture applied to a periodic Hamiltonian. It is possible to interpret it as due to the poles of the scattering amplitudes in the complex-frequency domain. For a moderate modulation amplitude the analysis makes use of the data obtained for the static barrier. The existence of these poles is not limited to the present model.

We analyze a quantum-computer design based on nuclear spin qubits in a quasi-one-dimensional chain of non-Kramers doublet atoms. We explore the use of spatial symmetry breaking to obtain control over the local dynamics of a qubit. We also study the decoherence mechanisms at the single qubit level and the interactions mediated by the magnetic media. The design can be realized in ${\mathrm{PrBr}}_{3-x}{\mathrm{F}}_x$ with nuclear magnetic resonance techniques.

We consider quantum error correction against correlated noise using simple and concatenated Calderbank-Shor-Steane codes as well as $n$-qubit repetition codes. We characterize the performance of various codes by means of the fidelity following the error correction of a single logical qubit in a quantum register. For concatenated codes we find a threshold in the single-qubit error rate below which the encoded qubit is perfectly protected. The threshold depends on the correlation strength of the noise and goes to zero for perfect correlation. Finally, we concatenate the traditional error correcting codes with a decoherence free subspace and evaluate the performance over the whole range from uncorrelated noise to perfectly correlated noise.

Antiferromagnetic spin chains play an important role in condensed matter and statistical mechanics. Recently $\mathit{XXX}$ spin chain was discussed in relation to information theory. Here we consider localizable entanglement. It is how much entanglement can be localized on two spins by performing local measurements on other individual spins (in a system of many interacting spins). We consider the ground state of antiferromagnetic spin chain. We study localizable entanglement [represented by concurrence] between two spins. It is a function of the distance. We start with isotropic spin chain. Then we study effects of anisotropy and magnetic field. We conclude that anisotropy increases the localizable entanglement. We discovered high sensitivity to a magnetic field in cases of high symmetry. We also evaluated concurrence of these two spins before the measurement to illustrate that the measurement raises the concurrence.

The double-to-single photoionization ratio of atomic lithium has been measured for photon energies ranging from $120\mathrm{eV}$ to $910\mathrm{eV}$. Through extensive use of various filters we were able to significantly extend the previous range of measurements [M.-T. Huang et al., Phys. Rev. A 59, 3397 (1999)]. We find that our data are in agreement with the predicted high-energy limit of $3.4\%$. By applying simple model curves to our data, we believe that sequential processes contribute substantially to the double-photoionization cross-section ratio as predicted by theory.

We report new measurements of ${\mathrm{Kr}}^{q+}$ photo-ions, coincident with $K\alpha$ or $K\beta$ fluorescence as incident-photon energy is swept through the Kr $K$-shell threshold. From the branching ratios just above threshold, we obtain measurements of the ion charge-state probabilities for decay from the Kr $\left[2p\right]$ and Kr $\left[3p\right]$ states. In the threshold region, we observe both resonant enhancement and depletion of the branching ratios. By analyzing this behavior in light of theory, we extract sticking probabilities, which we feel are a useful set of parameters for investigating the general relationship between cascade decay from resonant and nonresonant hole states. A simplified theoretical model is employed to calculate these probabilities for the ${\mathrm{Kr}}^{2+}$ and ${\mathrm{Kr}}^{3+}$ cases.

We present an ab initio study of elastic scattering and vibrational excitation of $\mathrm{NO}$ by low-energy $\left(0–2.0\mathrm{eV}\right)$ electron impact. The low-energy scattering cross sections are dominated by shape resonance contributions associated with the ${}^3\Sigma ^{-}$, ${}^1\Delta$, and ${}^1\Sigma ^{+}$ states of ${\mathrm{NO}}^{-}$. Resonance parameters for the three anion states were extracted from an analysis of fixed-nuclei variational (complex Kohn) calculations that employed elaborate trial wave functions. Independent estimates of the resonance parameters were obtained by analytically continuing the results of large-scale coupled-cluster calculations into the plane of complex momentum. The local complex potential model was used to calculate vibrational excitation cross sections, as well as the resonant portion of the vibrationally elastic cross sections. These results were combined with background contributions from the fixed-nuclei calculations to compute elastic and grand total cross sections. Our results capture the essential features of recent measurements of the cross sections, but suggest the need at lower energies for a more sophisticated, nonlocal treatment of nuclear dynamics.

We experimentally study the group time delay for a light pulse propagating through hot ${}^{87}\mathrm{Rb}$ vapor in the presence of a strong coupling field in a $\Lambda$ configuration. We demonstrate that the ultraslow pulse propagation is transformed into superluminal propagation as the one-photon detuning of the light increases due to the change in the transmission resonance line shape. Negative group velocity as low as $-c∕{10}^6=-80\mathrm{m}∕\mathrm{s}$ is recorded. We also find that the advance time in the regime of the superluminal propagation grows linearly with increasing laser field power.

The propagation in two dimensions of “optical” pulses in electromagnetically induced transparency media is analyzed. Results are presented for coupled Maxwell-Bloch equations with slowly varying envelope approximation, for both adiabatic and nonadiabatic situations. The possibility of changing the direction of the pulse by a switch of control beam direction is investigated in detail.

We propose a scheme for approximately and conditionally teleporting an unknown atomic state in cavity $\mathrm{QED}$. Our scheme does not involve the Bell-state measurement and thus an additional atom is unnecessary. Only two atoms and one single-mode cavity are required. The scheme may be used to teleport the state of a cavity mode to another mode using a single atom. The idea may also be used to teleport the state of a trapped ion.

Highly precise variational calculations of nonrelativistic energies of the $\left(2p^2\right){}^{3}P^e$ state of the helium atom are presented. We get an upper bound energy $E=-0.7105\mathrm{001\hspace{0.17em}556}5678\mathrm{a.u.}$, the lowest yet obtained.

The coherent excitation of overlapping atomic states by ion projectiles has a strong effect on the angular distribution of Auger electrons. Therefore, the measurement of the angular anisotropy of outgoing Auger electrons is suitable for the quantitative studies of the influence of the coherence. In our experiment the Ne target was bombarded with $700–2000\text{keV}$ ${\mathrm{H}}^{+}$ and $2000\text{keV}$ ${\mathrm{He}}^{+}$ projectiles. The angular distributions of ${KL}_{2,3}{\text{−}LLL}_{2,3}$ Auger electrons emitted from the decay of the coherently excited $1s2p$ doubly vacancy states of Ne were measured. The anisotropy parameter of the Auger decay was deduced from the measurements and compared with theoretical results calculated using the multiconfiguration Dirac-Fock method. The results indicate that one should go beyond the $LS$ coupling scheme even at this relatively low atomic number element, but also suggest that only partially overlapping states need to be taken into account.

We investigate the effect of solitons and vortices, with a range of different topologies, on the dynamics of Bose-Einstein condensates in a one-dimensional optical lattice and a three-dimensional harmonic trap. The solitons and vortices are imposed on the initial state of the atom cloud, which is set in oscillatory motion by a sudden displacement of the harmonic trap. We study this motion using both quantum-mechanical calculations, based on the nonlinear Schrödinger equation, and a semiclassical model for transport through the lowest energy band of the optical lattice. We demonstrate that the geometry of the vortices and solitons in the initial state plays a crucial role in the evolution of the atom cloud. In particular, we find that the center-of-mass motion of a condensate containing a vortex is only weakly damped if the vortex core lies parallel to the optical lattice direction, but is strongly damped when the core is orientated perpendicular to the lattice direction. Severe damping is observed for a condensate containing a soliton whose nodal plane is perpendicular to the optical lattice.

A representation of the $\mathrm{SO}\left(3\right)$ group is mapped into a maximally entangled two qubit state according to the literature. To show the evolution of the entangled state, a model is set up on an maximally entangled electron pair, two electrons of which pass independently through a rotating magnetic field. It is found that the evolution path of the entangled state in the $\mathrm{SO}\left(3\right)$ sphere breaks an odd or even number of times, corresponding to the double connectedness of the $\mathrm{SO}\left(3\right)$ group. An odd number of breaks leads to an additional $\pi$ phase to the entangled state, but an even number of breaks does not. A scheme to trace the evolution of the entangled state is proposed by means of entangled photon pairs and Kerr medium, allowing observation of the additional $\pi$ phase.

The interelectronic-interaction effect on the transition probabilities in high-$Z$ He-like ions is investigated within a systematic quantum electrodynamic approach. The calculation formulas for the interelectronic-interaction corrections of first order in $1∕Z$ are derived using the two-time Green-function method. These formulas are employed for numerical evaluations of the magnetic transition probabilities in heliumlike ions. The results of the calculations are compared with experimental values and previous calculations.

Triple-differential cross sections (TDCS) have been calculated for the $K$-shell ionization of carbon atom by fast electron impact for highly asymmetric kinematics. In the present calculation the three Coulomb two-body interaction, the dynamic screening modification of these interactions and the correlation in the final channel, the screening effect of the multielectron target, and the distortion of Coulomb force in the initial channel have been taken into account. Results are compared with the relative measurement performed on the ${\mathrm{C}}_2{\mathrm{H}}_2$ molecule as target. The present TDCS is found to be in better accord with experiment.

We consider formation of dissipationless shock waves in Bose-Einstein condensates with repulsive interaction between atoms. It is shown that for big enough initial inhomogeneity of density, interplay of nonlinear and dispersion effects leads to wave breaking phenomenon followed by generation of a train of dark solitons. Analytical theory is confirmed by numerical simulations.

We examine a scheme for the optical creation of a superfluid vortex in a trapped Bose-Einstein condensate, using the stimulated Raman adiabatic passage technique (STIRAP). By exposing an oblate, axis-symmetric condensate to two copropagating laser pulses, one can transfer external angular momentum from the light field to the matter wave, if one of the beams is the fundamental Gaussian mode and the other is a Gauss-Laguerre mode of angular momentum $1\hslash$. We demonstrate the complete transfer efficiency by numerical integration of the multicomponent Gross-Pitaevskii equation and explain the results with an intuitive and accurate approximation within the Thomas-Fermi limit. In addition, we discuss residual excitations (breathing modes) which occur in the two-dimensional regime and present the Bogoliubov excitation spectrum.

A model for a one-dimensional quantum particle interacting with a point detector is developed and analyzed. The analysis leads to any simple, intuitively pleasing expression for the final density operator of the particle in terms of the freely propagating particle density operator, a projection operator representing the detector volume, and a probability density for the time at which the detector fires. This probability density is found explicitly to be the presence-time density given by ${\mid {\psi }_f(0,t)\mid }^2∕\int {\mid {\psi }_f(0,t)\mid }^{2}dt$ where ${\psi }_f(0,t)$ is the free particle wave function evaluated at the detector position $(x=0)$. Moreover, it is shown that the resultant density operator transformation is a nonlinear transformation, not describable as a positive operator valued measure.