This paper presents a scheme to realize the SWAP gate between flying and stationary qubits through cavity QED, which is a necessary condition for networkability of quantum computation. As application, the storage of quantum information and teleportation of atomic and ionic states are present.

It is shown that, under typical experimental conditions, the violations of unitarity associated with the Wigner-Weisskopf approximation are negligibly small.

The investigation of the coherent-population-trapping (CPT) resonance on the degenerate two-level system of the $(F_g=2\to F_e=1)$ transition of the ${}^{87}\mathrm{Rb}{D}_1$ line by means of a Hanle effect configuration in an uncoated vacuum cell [Opt. Lett. 28, 1817 (2003)] has shown that the measured in fluorescence resonance has a complex shape—a very narrow (about 1 mG) structure superimposed on a broader one (about few tens of mG). In this work, the dependence of the width and amplitude of the CPT resonance structures are measured at different laser power densities when the registration is in fluorescence and transmission. While the narrow resonance width does not change within the limits of the accuracy of our measurements, the wide resonance width dependence is complex and is different in fluorescence and transmission. The origins of the observed resonance width narrowing of the CPT resonance structures registered in fluorescence and transmission are discussed.

We show that the method of maximum likelihood (MML) provides us with an efficient scheme for the reconstruction of quantum channels from incomplete measurement data. By construction this scheme always results in estimations of channels that are completely positive. Using this property we use the MML for a derivation of physical approximations of unphysical operations. In particular, we analyze the optimal approximation of the universal NOT gate as well as the physical approximation of a quantum nonlinear polarization rotation.

Two observers, who share a pair of particles in an entangled mixed state, can use it to perform some nonbilocal measurements over another bipartite system. In particular, one can construct a specific game played by the observers against a coordinator, in which they can score better than a pair of observers who only share a classical communication channel. The existence of such a game is an operational implication of an entanglement witness.

The author proposes an approach to adaptive homodyne detection of digitally modulated quantum optical pulses in which the phase of the local oscillator is chosen to maximize the average information gain, i.e., the mutual information, at each step of the measurement. The properties of this adaptive detection scheme are studied by considering the problem of classical information content of ensembles of coherent states. Simulations of quantum trajectories and visualizations of corresponding measurement operators demonstrate that the proposed measurement scheme adapts itself to the features of each ensemble. For all considered ensembles of coherent states, it consistently outperforms heterodyne detection and Wiseman’s adaptive scheme for phase measurements [H.M. Wiseman, Phys. Rev. Lett. 75, 4587 (1995)].

We analyze and study the effects of locality on the fault-tolerance threshold for quantum computation. We analytically estimate how the threshold will depend on a scale parameter $r$ which characterizes the scale-up in the size of the circuit due to encoding. We carry out a detailed seminumerical threshold analysis for concatenated coding using the seven-qubit CSS code in the local and the ‘nonlocal’ setting. First, we find that the threshold in the local model for the ⟦7,1,3⟧ code has a $1∕r$ dependence, which is in correspondence with our analytical estimate. Second, the threshold, beyond the $1∕r$ dependence, does not depend too strongly on the noise levels for transporting qubits. Beyond these results, we find that it is important to look at more than one level of concatenation in order to estimate the threshold and that it may be beneficial in certain places, like in the transportation of qubits, to do error correction only infrequently.

We promote use of the geometric entropy formula derived by Holzhey et al. from conformal field theory, $S_{\ell }\sim (c∕3)\log (\sin \pi \ell ∕N)$, to identify critical regions in zero temperature 1D quantum systems. The method is demonstrated on a class of one-dimensional $XY$ and $XYZ$ spin-$\frac{1}{2}$ chains, where the critical regions and their correponding central charges can be reproduced with quite modest computational efforts.

A time-dependent close-coupling method for three-electron atomic systems is formulated to calculate the double autoionization of hollow-atom states. Initial excited states are obtained by relaxation of the Schrödinger equation in imaginary time, while autoionization rates are obtained by propagation in real time. A 12-coupled-channels nonperturbative calculation on a three-dimensional radial lattice yields a double-autoionization rate for the $\mathrm{Li}\left(2s^{2}2p\right)\to {\mathrm{Li}}^{2+}\left(1s\right)+2e^{-}$ transition that that is somewhat smaller than earlier many-body perturbation theory calculations and in reasonable agreement with rates extracted from resonance profiles found in more recent $\gamma +\mathrm{Li}$ experiments.

Translational energy spectroscopy has been used to study one-electron capture by ${\mathrm{O}}^{6+}$ ions in collisions with ${\mathrm{H}}_2\mathrm{O}$, $\mathrm{C}{\mathrm{O}}_2$, and $\mathrm{C}{\mathrm{H}}_4$, and ${\mathrm{C}}^{4+}$ ions in collision with $\mathrm{C}{\mathrm{H}}_4$ within the energy range $200–1500\mathrm{eV}{\mathrm{amu}}^{-1}$. The relative importance of dissociative and nondissociative electron-capture mechanisms has been determined, and the main ${\mathrm{O}}^{5+}(n.l)$ and ${\mathrm{C}}^{3+}(n.l)$ excited product channels have been identified. In all cases, the measurements confirm the highly selective nature of the electron-capture process in spite of the large number of possible nondissociative and dissociative product channels associated with a wide spread in energy defects $\Delta E$. Collisions of ${\mathrm{O}}^{6+}$ in ${\mathrm{H}}_2\mathrm{O}$, $\mathrm{C}{\mathrm{O}}_2$, and $\mathrm{C}{\mathrm{H}}_4$ exhibit very similar patterns of behavior with features also observed in previous studies of the simpler ${\mathrm{O}}^{6+}{\mathrm{H}}_2$ system. Electron capture for ${\mathrm{C}}^{4+}$ in $\mathrm{C}{\mathrm{H}}_4$, while dominated by selective nondissociative ${\mathrm{C}}^{3+}(n=3)$ formation, also includes significant contributions from highly endothermic dissociative capture channels leading to ${\mathrm{C}}^{3+}(n=4)$.

We give a Floquet analysis of photoemission of electrons and high-order harmonic generation from a solid surface interacting with intense laser fields. A general theoretical algorithm, within a three-dimensional quasi-free-electron model, is presented. It takes account of the effective masses of the electron both within and outside the solid, as well as treating both reflected and refracted laser fields. The steady-state photocurrent is obtained from the diagonal photon components of the expectation value of the time-dependent current operator outside the surface, while the off-diagonal terms of the expectation value of the current operator from both inside and outside the surface provide the probability of coherent emission of high-order harmonic radiation. It is found, among other things, that electrons can be emitted from the surface requiring no net absorption of photons or even accompanied by emission of extra photons. The origin of the effect lies in the difference in the ponderomotive energy inside and outside the surface that can arise from the small ratio of the effective mass of the electron inside the solid (e.g., GaAs) and the mass of the free electron. Another result of qualitative significance is the formation of a plateau, well known in the atomic case, in both the kinetic energy distribution of the photocurrent and the spectrum of the high-order harmonic radiation. They are shown to arise in the present case from the evanescent part of the electron wave function at the surface.

We analyze the dynamics of dark matter-wave solitons on a Thomas-Fermi cloud described by the Gross-Pitaevskii equation with radial symmetry. One-dimensional, ring, and spherical dark solitons are considered, and the evolution of their amplitudes, velocities, and centers is investigated by means of a Lagrangian approach. In the case of large-amplitude oscillations, higher-order corrections to the corresponding equations of motion for the soliton characteristics are shown to be important in order to accurately describe its dynamics. The numerical results are found to be in very good agreement with the analytical predictions.

Motivated by recent experiments involving the nondestructive imaging of magnetization of a spin-1 ${}^{87}\mathrm{Rb}$ Bose gas [J. M. Higbie et al., Phys. Rev. Lett., 95, 050401 (2005)], we address the question of how the spontaneous magnetization of a ferromagnetic Bose-Einstein condensate occurs in a spin-conserving system. Due to competition between the ferromagnetic interaction and the total spin conservation, various spin structures such as staggered magnetic domains and helical and concentric ring structures are formed, depending on the geometry of the trapping potential.

We discuss the quantum-jump operation in an open system and show that jump superoperators related to a system under measurement can be derived from the interaction of that system with a quantum measurement apparatus. We give two examples for the interaction of a monochromatic electromagnetic field in a cavity (the system) with two-level atoms and with a harmonic oscillator (representing two different kinds of detectors). We show that the derived quantum-jump superoperators have a “nonlinear” form $J\rho =\gamma \mathrm{diag}\left[F\left(\widehat{n}\right)a\rho a^{†}F\left(\widehat{n}\right)\right]$, where the concrete form of the function $F\left(\widehat{n}\right)$ depends on assumptions made about the interaction between the system and detector. Under certain conditions the asymptotical power-law dependence $F\left(\widehat{n}\right)={(\widehat{n}+1)}^{-\beta }$ is obtained. A continuous transition to the standard Srinivas-Davies form of the quantum-jump superoperator (corresponding to $\beta =0$) is shown.

We have measured the natural decay lifetime of the $5{\mathrm{P}}_{1∕2}$ state in potassium by pulsed excitation followed by nonresonant photoionization to monitor the state population. The result for the lifetime is $\tau =137.6\pm 1.3\mathrm{ns}$, an improvement compared to previous results in the literature.

Consider a system with an arbitrary constraint on its electron density (e.g., that there are $N$ charges on a certain group of atoms). We show that in Kohn-Sham density functional theory, the minimum energy state consistent with the constraint is actually a maximum with respect to the constraint potential, and that this solution is unique. This leads us to an efficient algorithm for calculations on constrained systems. Illustrative studies are shown for charge transfer in the zincbacteriochlorin-bacteriochlorin complex, polyene and alkane chains.

The realistic application of geometric quantum computation is crucially dependent on an unproved robustness conjecture, claiming that geometric quantum gates are more resilient against random noise than dynamic gates. We propose a suitable model that allows a direct and fair comparison between geometrical and dynamical operations. In the presence of stochastic control errors we find that the maximum of gate fidelity corresponds to quantum gates with a vanishing dynamical phase. This is a clear evidence for the robustness of nonadiabatic geometric quantum computation. The predictions here presented can be experimentally tested in almost all of the already existing quantum computer candidates.

We have applied the recently developed $B$-spline $R$-matrix method with nonorthogonal orbitals to treat electron collisions with ${\mathrm{Fe}}^{+}$. The flexibility of the code allowed us to generate a target description of unprecedented accuracy for collision calculations. Partial-wave contributions and the total collision strength for the spin-forbidden $\left(3d^{6}4s\right)a{}^{6}D–\left(3d^7\right)a{}^{4}F$ transition are presented for incident electron energies from threshold to $10\mathrm{Ry}$, together with several effective collision strengths for electron temperatures up to $80000\mathrm{K}$. The agreement of our results with those from recent calculations by Ramsbottom et al. [J. Phys. B 35, 3451 (2002); 37, 3609 (2004)] is generally good. However, a few significant differences are revealed in the low-energy regime, which is dominated by resonance structure.

The effect of a finite control beam on the transverse spatial profile of the slow light propagation in an electromagnetically induced transparency medium is studied. We arrive at a general criterion in terms of eigenequation, and demonstrate the existence of a set of localized, stationary transverse modes for the negative detuning of the probe signal field. Each of these diffraction-free transverse modes has its own characteristic group velocity, smaller than the conventional theoretical result without considering the transverse spatial effect.

We investigate the utility of Einstein-Podolsky-Rosen correlations of the position and momentum of photon pairs from parametric down-conversion in the implementation of a secure quantum key distribution protocol. We show that security is guaranteed by the entanglement between down-converted pairs, and can be checked by either direct comparison of Alice and Bob’s measurement results or evaluation of an inequality of the sort proposed by Mancini et al. [Phys. Rev. Lett. 88, 120401 (2002)].