In this study we evaluate the cross section for forming the nonrelativistically bound state of a positron with the helium triplet atom by radiative attachment. Although this cross-section was expected to be small compared with a competing rearrangement process involving positronium and ground-state helium, this calculation is a first step which should be useful for experimental production of this interesting system

Profiles of the ${\mathrm{Ly}}_{\alpha }$ line of $\mathrm{O}\mathrm{VIII}$ ions in ${\mathrm{N}}_2\mathrm{O}$ clusters irradiated by fs laser pulses (laser flux up to $4\times {10}^{17}\mathrm{W}∕{\mathrm{cm}}^2$) were recorded. The features (peaks and dips) in the ${\mathrm{Ly}}_{\alpha }$ $\mathrm{O}\mathrm{VIII}$ line profiles were observed. The experimental spectra were simulated theoretically assuming that strong oscillating electric fields are generated in the cluster plasma. We showed that allowing for oscillating electric fields of the amplitude $E_0\sim (0.5–1.0)\times {10}^9\mathrm{V}∕\mathrm{cm}$ and frequency $\omega \sim (0.7–2.4)\times {10}^{15}{\mathrm{s}}^{-1}$ leads to spectral features in the theoretical profiles of the ${\mathrm{Ly}}_{\alpha }$ $\mathrm{O}\mathrm{VIII}$ line that are similar to the features observed experimentally. Possible mechanisms responsible for the generation of strong oscillating electric fields in the cluster plasma are discussed.

We report on measurements of the branching ratios of emission of small ${\mathrm{C}}_n^{+}$ fragments in asymmetrical fission of highly charged ${\mathrm{C}}_{60}^{r+}$ ions $(r=4–6)$. For the channels corresponding to the emission of one fragment, only small fragments with an even number of carbon atoms are observed. For the channels with the emission of two fragments, successive emission of small fragments with an odd number of carbon atoms has been observed with a surprisingly high branching ratio (30%). In order to reproduce the experimental branching ratios in the framework of a statistical evaporation model, the height of fission barriers for the emission of one odd numbered fragment has to be reduced at higher temperature in order to allow the opening of these channels that are forbidden at lower temperature

We investigate the dynamics of a Bose-Einstein condensate in the presence of a random potential created by optical speckles. We first consider the effect of a weak disorder on the dipole and quadrupole collective oscillations, finding uncorrelated frequency shifts of the two modes with respect to the pure harmonic case. This behavior, predicted by a sum-rules approach, is confirmed by the numerical solution of the Gross-Pitaevskii equation. Then we analyze the role of disorder on the one-dimensional expansion in an optical guide, discussing possible localization effects. Our theoretical analysis provides a useful insight into the recent experiments performed at LENS [J. E. Lye, L. Fallani, M. Modugno, D. S. Wiersma, C. Fort, and M. Inguscio, Phys. Rev. Lett. 95, 070401 (2005); C. Fort, L. Fallani, V. Guarrera, J. E. Lye, M. Modugno, D. S. Wiersma, and M. Inguscio, Phys. Rev. Lett. 95, 170410 (2005)].

First and second sound speeds are calculated for a uniform superfluid gas of Fermi atoms as a function of temperature, density, and interaction strength. The second sound speed is of particular interest as it is a clear signal of a superfluid component and it determines the critical temperature. The sound modes and their dependence on density, scattering length, and temperature are calculated in the BCS, molecular Bose-Einstein condensate (BEC), and unitarity limits and a smooth crossover is extrapolated. It is found that first and second sounds undergo avoided crossing on the BEC side due to mixing. Consequently, they are detectable at crossover both as density and thermal waves in traps.

We present a generalized two-level scheme for an “atom diode,” namely, a laser device that lets a two-level ground-state atom pass in one direction, say from left to right, but not in the opposite direction. The laser field is composed of two lateral state-selective mirror regions and a central pumping region. We demonstrate the robustness of the scheme and propose a physical realization. It is shown that the inclusion of a counterintuitive laser field blocking the excited atoms on the left side of the device is essential for a perfect diode effect. The reason for this, the diodic behavior, and the robustness may be understood with an adiabatic approximation. The conditions to break down the approximation, which imply also the diode failure, are analyzed.

We derive a lower limit to the amount of absorptive loss present in passive linear optical devices such as a beam splitter. We choose a particularly simple beam splitter geometry, a single planar slab surrounded by vacuum, which already reveals the important features of the theory. It is shown that, using general causality requirements and statistical arguments, the lower bound depends on the frequency of the incident light and the transverse resonance frequency of a suitably chosen single-resonance model only. For symmetric beam splitters and reasonable assumptions on the resonance frequency ${\omega }_T$, the lower absorption bound is $p_{\min }\approx 2\times {10}^{-6}{(\omega ∕{\omega }_T)}^4$.

We study the influence of delayed optical feedback from a short external cavity (EC) on the emission dynamics of vertical-cavity surface-emitting semiconductor lasers (VCSELs). We find the emergence of pulse packages (PP) exhibiting characteristics which originate from the interplay of time-delay induced dynamics and polarization mode (PM) competition. We provide detailed analysis of the polarization dynamics applying complementary cross-correlation and spectral analysis techniques. The analysis reveals an interplay of the dynamics linked to the hierarchy of time scales present in the laser system: the fast time scale of the EC delay, the slower time scale of the PP, and the slow time scale of the polarization mode competition. For an analysis of the dynamics we provide a toolbox of methods adapted to the different relevant time scales and temporal variations of the dynamics. This complementary view unveils distinct changes in the relative oscillation phase of the two PM for increasing the injection current on both the fast and the slow time scales. Our results emphasize the significance of the polarization mode competition for PP dynamics in VCSELs, contrasting the observed behavior to what was reported for edge emitters.

The lifetimes of the lowest ${}^{3}P_1$ states of Ba and Ra were determined to be $1345\pm 14\mathrm{ns}$ and $422\pm 20\mathrm{ns}$, respectively, by measuring the exponential decay of fluorescence after illuminating a thermal atomic beam with pulses of laser light. In addition, the ${}^{1}S_0(F=1∕2)\text{−}{}^{3}P_1(F=3∕2)$ transition frequency in ${}^{225}\mathrm{Ra}$ was measured to be $13999.269\pm 0.001{\mathrm{cm}}^{-1}$ by referencing a nearby ${\mathrm{I}}_2$ transition.

We have calculated above-threshold-ionization (ATI) spectra for argon in a strong, few-cycle 400 nm laser pulse with an ab initio solution of the time-dependent Schrödinger equation and by using a model based on the strong field approximation. We find additional peaks in the resulting ATI spectra which are not present for longer pulses. This substructure is due to the rapidly changing ponderomotive potential in the short laser pulse. It is sensitive to the pulse duration and the pulse envelope and is a general feature of ionization spectra originating from a short, intense laser pulse.

We report experimental observations of interference between three-photon and one-photon excitations, and phase control of light attenuation and/or transmission in a four-level system. Either constructive or destructive interferences can be obtained by varying the phase and/or frequency of a weak control laser. The interference enables absorptive switching of one field by another field at different frequencies and ultralow light levels.

The Schrödinger equation for a spinless electron near an azimuthally symmetric curved surface $\Sigma$ in the presence of an arbitrary uniform magnetic field $\mathbf{B}$ is developed. A thin-layer quantization procedure is implemented to bring the particle onto $\Sigma$, leading to the well-known geometric potential $V_C\propto h^2-k$ and a second potential that couples $A_N$, the component of $\mathbf{A}$ normal to $\Sigma$ to mean surface curvature, as well as a term dependent on the normal derivative of $A_N$ evaluated on $\Sigma$. Numerical results in the form of ground-state energies as a function of the applied field in several orientations are presented for a toroidal model.

We propose an efficient scheme for constructing arbitrary two-dimensional (2D) cluster states using probabilistic entangling quantum gates. In our scheme, the 2D cluster state is constructed with starlike basic units generated from 1D cluster chains. By applying parallel operations, the process of generating 2D (or higher-dimensional) cluster states is significantly accelerated, which provides an efficient way to implement realistic one-way quantum computers.

We introduce an architecture for robust and scalable quantum computation using both stationary qubits (e.g., single photon sources made out of trapped atoms, molecules, ions, quantum dots, or defect centers in solids) and flying qubits (e.g., photons). Our scheme solves some of the most pressing problems in existing nonhybrid proposals, which include the difficulty of scaling conventional stationary qubit approaches, and the lack of practical means for storing single photons in linear optics setups. We combine elements of two previous proposals for distributed quantum computing, namely the efficient photon-loss tolerant build up of cluster states by Barrett and Kok [Phys. Rev. A 71, 060310(R) (2005)] with the idea of repeat-until-success (RUS) quantum computing by Lim et al. [Phys. Rev. Lett. 95, 030505 (2005)]. This idea can be used to perform eventually deterministic two qubit logic gates on spatially separated stationary qubits via photon pair measurements. Under nonideal conditions, where photon loss is a possibility, the resulting gates can still be used to build graph states for one-way quantum computing. In this paper, we describe the RUS method, present possible experimental realizations, and analyze the generation of graph states.

We study the exact evolution of two noninteracting qubits, initially in a Bell state, in the presence of an environment, modeled by a kicked Ising spin chain. Dynamics of this model range from integrable to chaotic and we can handle numerics for a large number of qubits. We find that the entanglement (as measured by concurrence) of the two qubits has a close relation to the purity of the pair, and closely follows an analytic relation derived for Werner states. As a collateral result we find that an integrable environment causes quadratic decay of concurrence as well as of purity, while a chaotic environment causes linear decay. Both quantities display recurrences in an integrable environment.

Three different methods have been discussed to verify continuous variable entanglement of intense light beams. We demonstrate all three methods using the same setup to facilitate the comparison. The nonlinearity used to generate entanglement is the Kerr effect in optical fibers. Due to the brightness of the entangled pulses, standard homodyne detection is not an appropriate tool for the verification. However, we show that by using large asymmetric interferometers on each beam individually, two noncommuting variables can be accessed and the presence of entanglement verified via joint measurements on the two beams. Alternatively, we witness entanglement by combining the two beams on a beam splitter that yields certain linear combinations of quadrature amplitudes which suffice to prove the presence of entanglement.

We describe a quantum algorithm to prepare an arbitrary pure state of a register of a quantum computer with fidelity arbitrarily close to 1. Our algorithm is based on Grover’s quantum search algorithm. For sequences of states with suitably bounded amplitudes, the algorithm requires resources that are polynomial in the number of qubits. Such sequences of states occur naturally in the problem of encoding a classical probability distribution in a quantum register.

We report on theoretical and computational studies of electron and nuclear energies in the Coulomb explosion of ${\left(D_2\right)}_{n∕2}$ clusters ($n=250–33000$, cluster radius $R_0=11\mathrm{\AA }–55\mathrm{\AA }$) coupled to ultraintense Gaussian laser fields (laser peak intensities $I_M={10}^{15}–{10}^{18}\mathrm{W}{\mathrm{cm}}^{-2}$, pulse widths $\tau =25–50\mathrm{fs}$, and frequency $\nu =0.35{\mathrm{fs}}^{-1}$). Molecular dynamics simulations were fit by semiempirical relations for the average $E_{\mathrm{av}}$ and maximal $E_M$ ion energies and for their dependence on the cluster radius $\left(R_0\right)$ and on the laser parameters. This revealed two kinds of Coulomb explosion domains separated by the border radius $R_0^{\left(\mathrm{I}\right)}$, which marks complete cluster outer ionization and which depends on $I_M$ and $\tau$, (i) the cluster vertical ionization (CVI) domain $(R_0<R_0^{\left(\mathrm{I}\right)})$, where $E_{\mathrm{av}}$, $E_M\propto R_0^2$ is independent of the laser parameters, (ii) the non-CVI domain $(R_0>R_0^{\left(\mathrm{I}\right)})$, which prevails at lower laser intensities over a broad region of cluster sizes (i.e., at $I_M={10}^{15}\mathrm{W}{\mathrm{cm}}^{-2}$, $R_0>R_0^{\left(\mathrm{I}\right)}=6.2\mathrm{\AA }$ for $\tau =25\mathrm{fs}$, and $R_0>R_0^{\left(\mathrm{I}\right)}=9.5\mathrm{\AA }$ for $\tau =50\mathrm{fs}$). The effects of the persistent nanoplasma on Coulomb explosion in the non-CVI domain are manifested by a distinct cluster size dependence, i.e., $E_{\mathrm{av}}\propto R_0^{\eta }$ $(\eta =0)$ and $E_M\propto R_0^{\eta }$ $(\eta =1)$ for $R_0>(2.0–2.5)R_0^{\left(\mathrm{I}\right)}$, and by a bimodal distribution of the ion kinetic energies. The energetics of Coulomb explosion in the non-CVI domain and its dependence (or independence) on the cluster size and laser parameters was semiquantitatively described by a cold nanoplasma model, which is based on a lychee configuration of the cluster charge, and which induces Coulomb explosion in the presence of the persistent nanoplasma. The results of our analyses are general for the cluster size dependence of the energetics of Coulomb explosion of multicharged elemental and molecular ${\mathrm{Xe}}_n$, ${\left(\mathrm{C}{\mathrm{H}}_4\right)}_n$, and ${\left(\mathit{DI}\right)}_n$ clusters in the non-CVI domain.

It is proven that in molecular interferometry a which-path information can be recorded using detection of spontaneously emitted photons of long wavelengths in the optical near field of the apertures. This does not, however, contradict Heisenberg’s uncertainty principle: It is shown that even if the momentum transfer in this experiment is negligible, the entanglement between the detector molecule and photon is responsible for the loss of coherence.