The scattering of positrons from atomic hydrogen is studied by using the momentum space coupled channels optical method at intermediate energy (15–100 eV). Ionization continuum and positronium formation channels are included in the coupled channels calculation via a complex equivalent-local optical potential.

Photoionization and electron-impact ionization of ${\mathrm{Xe}}^{3+}$ ions in the region of $4d$ inner-shell excitation-autoionization have been studied using interacting-beam spectroscopic techniques. Absolute cross sections for photoionization of ${\mathrm{Xe}}^{3+}$ have been obtained and the electron-impact spectrum has been normalized to previously published absolute data. The ${\mathrm{Xe}}^{3+}$ spectra show large contributions from $4d\to nf$ excitation-autoionization processes compared to the lower charge states of Xe due to the significant collapse of the $nf$ wave functions into the ion core as the initial charge state of the ion is increased. In addition, a tentative value for the ionization potential of ${\mathrm{Xe}}^{3+}$ of $42.2\pm 0.2\mathrm{eV}$ was obtained.

In this paper, we report the measurement of molecular formation rate constant of ${\mathrm{Rb}}_2$ directly in a magneto-optical trap (MOT). The ground state molecules are detected by two-photon ionization, resonantly enhanced through the intermediate $a^3{\Sigma }_u^{+}\to 2^3{\Pi }_g$ molecular band. We have measured the rate constant as a function of atomic density to conclude that the molecules ${\mathrm{Rb}}_2$ are formed in the MOT by a photoassociation process caused by the trapping laser beams. We also measured the rate constant as a function of the trapping laser intensity. The results here presented are of importance to future experiments involving trapping of cold molecules.

The cross sections (both differential and total) for ${\mathrm{H}}^{-}$ formation in the process $\mathrm{Ps}\left(1s\right)+\mathrm{H}\left(1s\right)\to e^{+}+{\mathrm{H}}^{-}$ in the energy range $10–500\mathrm{eV}$ have been studied. The calculations have been performed in the framework of the Coulomb-modified eikonal approximation and take account of the long-range Coulomb attraction between the outgoing $e^{+}$ and ${\mathrm{H}}^{-}$ ion in the final channel.

The analysis of the paper by J.-L. Chen et al. [Phys. Rev. A 67, 012101 (2003)] suffers from a conceptual error. Its results contradict some well-established mathematical facts and are incorrect.

The van der Waals coefficients, from $C_{11}$ through to $C_{16}$ resulting from second-, third-, and fourth-order perturbation theory are estimated for the alkali-metal (Li, Na, K, and Rb) atoms. The dispersion coefficients are also computed for all possible combinations of the alkali-metal atoms and hydrogen. The parameters are determined from sum rules after diagonalizing a semiempirical fixed core Hamiltonian in a large basis. Comparisons of the radial dependence of the $C_n∕r^n$ potentials give guidance as to the radial regions in which the various higher-order terms can be neglected. It is seen that including terms up to $C_{10}∕r^{10}$ results in a dispersion interaction that is accurate to better than 1% whenever the inter-nuclear spacing is larger than $20a_0$. This level of accuracy is mainly achieved due to the fortuitous cancellation between the repulsive $(C_{11},C_{13},C_{15})$ and attractive $(C_{12},C_{14},C_{16})$ dispersion forces.

The $B$-spline $R$-matrix method is used to investigate the electron-impact excitation of neutral carbon over an energy range from threshold to $60\mathrm{eV}$. A multiconfiguration Hartree-Fock method with nonorthogonal orbitals is employed to generate an accurate representation of the target wavefunctions. The present close-coupling expansion includes the bound and autoionizing states of neutral carbon derived from the $1s^{2}2s^{2}2p^2$, $1s^{2}2s^{2}2p3\ell (\ell =0,1,2)$, $1s^{2}2s^{2}2p4s$, $1s^{2}2s2p^3$, and $1s^{2}2p^4$ configurations, plus eight pseudostates to fully account for the polarizability of the ground state of atomic carbon. Cross sections and effective collision strengths are presented for important transitions from the ground state $2p^2{}^{3}P$, and the metastable $2p^2{}^{1}D$ and ${}^{1}S$ states. Our predictions for the cross sections show significant discrepancies with those from previous calculations carried out with the standard $R$-matrix approach in a similar scattering model [Dunseath et al., J. Phys. B 30, 277 (1997)]. These discrepancies are mostly due to the different target descriptions, with the present one giving overall superior agreement with experiment for energy levels and oscillator strengths. The excitation cross sections exhibit prominent resonance structures in the low-energy region. The energy positions, widths, and classifications for the detected resonances are presented.

We investigate the nonclassicality of a photon-subtracted Gaussian field, which was produced in a recent experiment, using negativity of the Wigner function and the nonexistence of well-behaved positive $P$ function. We obtain the condition to see negativity of the Wigner function for the case including the mixed Gaussian incoming field, the threshold photodetection and the inefficient homodyne measurement. We show how similar the photon-subtracted state is to a superposition of coherent states.

For a general entangling probe attacking the Bennett-Brassard 1984 protocol in quantum key distribution, I calculate three classes of optimized unitary transformations, all yielding the same maximum information to the probe. The simplest one corresponds to a probe having a two-dimensional Hilbert space of states, and is uniquely determined by the error rate induced by the probe in the legitimate receiver. The second class corresponds to a probe having a four-dimensional Hilbert space of states, and is determined by the error rate and two continuous angle parameters which are mutually constrained by the error rate. The third class corresponds to a probe having a four-dimensional Hilbert space, and is determined by the error rate and two continuous angle parameters, one of which is constrained by the error rate. Furthermore, I show that the simplest quantum circuit representing the optimal entangling probe consists of a single controlled-NOT gate in which the control qubit consists of two polarization-basis states of the signal, the target qubit consists of two probe-basis states, and the initial state of the probe is set by the error rate. A method is determined for measuring the appropriate correlated state of the probe. Finally, a possible implementation of the entangling probe is described.

The 1%-accurate calculations of the van der Waals interaction between an atom and a cavity wall are performed in the separation region from $3\mathrm{nm}\text{to}150\mathrm{nm}$. The cases of metastable ${\mathrm{He}}^{*}$ and Na atoms near metal, semiconductor, and dielectric walls are considered. Different approximations to the description of wall material and atomic dynamic polarizability are carefully compared. The smooth transition to the Casimir-Polder interaction is verified. It is shown that to obtain accurate results for the atom-wall van der Waals interaction at short separations with an error less than 1% one should use the complete optical-tabulated data for the complex refractive index of the wall material and the accurate dynamic polarizability of an atom. The obtained results may be useful for the theoretical interpretation of recent experiments on quantum reflection and Bose-Einstein condensation of ultracold atoms on or near surfaces of different kinds.

In a recent experiment (Wang et al., e-print cond-mat/0407689), it was observed that a sequence of two standing-wave square pulses can split a Bose-Einstein Condensate at rest into $\pm 2\hslash \mathbf{k}$ diffraction orders with almost 100% efficiency. By truncating the Raman-Nath equations to a two-state model, we provide an intuitive picture that explains this double-square-pulse beam-splitter scheme. We further show it is possible to optimize a standing-wave multiple-square-pulse sequence to efficiently diffract an atom at rest to a symmetric superposition of $\pm 2n\hslash \mathbf{k}$ diffraction orders with $n>1$. The approach is considered to be qualitatively different from the traditional light-pulse schemes in the Bragg or the Raman-Nath region, and can be extended to more complex atomic optical elements that produce various tailored output momentum states from a cold atom source.

It is shown how simulated annealing can be used to determine the accessible information and the capacity of a quantum channel. The method is applied to a specific example of a symmetric $N$-state quantum channel that frequently appears in the analysis of eavesdropping in quantum key distribution protocols. The numerical results for the accessible information of this quantum channel are compared with recently published analytical solutions showing excellent agreement. Furthermore, interesting results concerning the capacity of a symmetric quantum channel are presented.

High-fidelity quantum computation and quantum state transfer are possible in short spin chains. We exploit a system based on a dispersive qubit-boson interaction to mimic XY coupling. In this model, the usually assumed nearest-neighbor coupling is no longer valid: all the qubits are mutually coupled. We analyze the performances of our model for quantum state transfer showing how preengineered coupling rates allow for nearly optimal state transfer. We address a setup of superconducting qubits coupled to a microstrip cavity in which our analysis may be applied.

We propose an atom optics experiment to measure the stability of the quantum kicked rotor under perturbations of the Hamiltonian. We avail ourselves of the theory of Loschmidt echoes, i.e., we consider the overlap of a quantum state evolved in a perturbed and an unperturbed potential. Atom interferometry allows us to determine the overlap integral in amplitude and phase. A numerical analysis of the kicked rotor in various regimes shows that the quantum signatures of specific classical properties can be detected experimentally.

Optical behavior analogous to electromagnetically induced transparency and absorption is observed in experiments using coupled fused-silica microspheres. This behavior results from interference between coresonant whispering-gallery modes of the two spheres. Coupled-resonator-induced transparency and absorption are observed. Which effect is seen depends on the strength of coupling of incident light from a tapered fiber into the first sphere and on the strength of coupling between the two spheres. The observed effects can enhance microresonator performance in various applications.

Feshbach resonances in ${}^6\mathrm{Li}$ were experimentally studied and theoretically analyzed. In addition to two previously known $s$-wave resonances, three $p$-wave resonances were found. Four of these resonances are narrow and yield a precise value of the singlet scattering length. The position of the broad $s$-wave resonance near $83\mathrm{mT}$ is mostly sensitive to the triplet potential. It was previously determined in a molecule-dissociation experiment for which we, here, discuss systematic shifts.

How does the classical phase-space structure for a composite system relate to the entanglement characteristics of the corresponding quantum system? We demonstrate how the entanglement in nonlinear bipartite systems can be associated with a fixed-point bifurcation in the classical dynamics. Using the example of coupled giant spins we show that when a fixed point undergoes a supercritical pitchfork bifurcation, the corresponding quantum state—the ground state—achieves its maximum amount of entanglement near the critical point. We conjecture that this will be a generic feature of systems whose classical limit exhibits such a bifurcation.

We consider an array of $N$ quantum dot pairs interacting via Coulomb interaction between adjacent dots and hopping inside each pair. We show that at the first order in the ratio of hopping and interaction amplitudes, the array maps in an effective two-level system with energy separation becoming exponentially small in the macroscopic (large-$N$) limit. Decoherence at zero temperature is studied in the limit of weak coupling with phonons. In this case, the macroscopic limit is robust with respect to decoherence. Some possible applications in quantum information processing are discussed.

A photon-number-splitting (PNS) attack against differential-phase-shift (DPS) quantum key distribution (QKD) is described. In the conventional 1984 Bennett-Brassard protocol, using weak laser light, the PNS attacks, which involve installing a lossless transmission line and blocking pulses from which extra photons cannot be picked up, impose a limit on the transmission distance. In contrast, use of a coherent pulse train in DPS QKD prevents the PNS attack and removes the distance limitation imposed by it. We carried out a DPS QKD experiment that simulated the situation where some pulses are blocked. The result showed that extra bit errors are induced in an eavesdropped condition, indicating the robustness of DPS QKD against PNS attacks.