Angular distributions of the two ejected electrons resulting from the double ionization of helium by electron impact have been measured by means of a multicoincidence multiangle $\mathrm{}(e,3e)\mathrm{}$ spectrometer at an incident energy of about 0.6 keV and equal outgoing energies $E_b{=E}_c=11\mathrm{}\mathrm{eV}.$ We identify various regimes of kinematical parameters where substantial differences are found with respect to the first-Born convergent close-coupling calculations: an angular shift of the position of the main lobe and the presence of additional lobes. These differences are attributed to high-order contributions in the projectile-target interaction. This conclusion is supported by recent $\mathrm{}(e,3e)\mathrm{}$ calculations performed within the second-Born approximation.

We have measured the strongly suppressed electric tensor polarizability of the Cs ground state using an optical pump-probe technique in a thermal atomic beam. The result ${\alpha }_2\mathrm{}(F=4\mathrm{})/h=-{3.34\left(2\right)\mathrm{}}_{\mathrm{stat}}{\mathrm{}\left(25\right)\mathrm{}}_{\mathrm{syst}}$ ${10}^{-8}\mathrm{Hz}/(\mathrm{V}/\mathrm{c}\mathrm{m}{)}^2$ agrees with a previous measurement and confirms the long-standing discrepancy with a theoretical value. The anticipated future reduction of the total uncertainty to the 1% level makes this quantity a valuable test for atomic structure calculations involving short-range inner-atomic interactions.

Considering an effectively attractive quasi-one-dimensional Bose-Einstein condensate of atoms confined in a toroidal trap, we find that the system undergoes a phase transition from a uniform to a localized state, as the magnitude of the coupling constant increases. Both the mean-field approximation, as well as a diagonalization scheme, are used to attack the problem.

We show that ground-state Zeeman coherence prepared by two-photon Raman transitions in alkali atoms results in steep controllable and sign-reversible dispersion. Pulse propagation with small negative as well as positive group velocity of light $(-c/5100\mathrm{}$ and $c/41\mathrm{}000)$ in a Cs vapor cell is reported. Energy exchange between copropagating light components through long-lived Zeeman coherence with enhanced absorption or transmission has been observed.

We propose a whole family of physical states that yield a maximal violation of Bell inequalities, when using quadrature-phase homodyne detection. This result is based on a binning process called root binning, that is used to transform the continuous-variable measurements into binary results needed for the tests of quantum mechanics versus local realistic theories. A physical process in order to produce such states is also suggested. The use of high-efficiency spacelike separated homodyne detections with these states and this binning process would result in a conclusive loophole-free test of quantum mechanics.

We introduce the nonadiabatic, or Aharonov-Anandan, geometric phase as a tool for quantum computation and show how this phase on one qubit can be monitored by a second qubit without any dynamical contribution. We also discuss how this geometric phase could be implemented with superconducting charge qubits. While the nonadiabatic geometric phase may circumvent many of the drawbacks related to the adiabatic (Berry) version of geometric gates, we show that the effect of fluctuations of the control parameters on nonadiabatic phase gates is more severe than for the standard dynamic gates. Similarly, fluctuations also affect to a greater extent quantum gates that use the Berry phase instead of the dynamic phase.

We show that universal ion trap computation can be performed on decoherence-free subspaces (DFS’s) using only two-qubit operations. One logical qubit is minimally encoded into three physical qubits. The encoded qubit is in a DFS under collective decoherence. Encoded single- and two-qubit logical operations are implemented via the Sørensen-Mølmer interaction. We show that alternation of the effective Hamiltonians for two particular phase configurations of control fields approximates an anisotropic exchange interaction that has recently been shown to be universal on these encodings. Physically realistic rapid alternation of the control fields also approximates evolution in a DFS. Our scheme is scalable in a recently proposed array-based architecture.

Single-differential and total cross sections of positron-impact ionizations of the helium atom are calculated near the ionization threshold. We investigate the threshold law of positron-impact ionization for models with various asymptotic charges in the two-potential distorted-wave approximation. The threshold power behaviors of impact-ionization cross sections depend only on the asymptotic charges experienced by the scattered positron and the ejected electron. Our numerical results agree well with experimental data and other theoretical results.

We study the energy loss of fast, hydrogenlike ions in thin solid foils, in the regime prior to the establishment of the ion-charge equilibrium. The projectile-charge evolution is described by a nonstationary, continuous-time Markov process, while the target response is described by a time-dependent dielectric-response formalism. We first derive the projectile self-energy in the presence of charge exchange, which is used to determine the bound-electron density in a self-consistent manner, by minimizing the total projectile energy in an adiabatic approximation. An expression for the ion energy-loss distribution is then used to derive the average value of the stopping power as a function of the traversal time in the foil, taking into account the projectile screening by the bound electron. The results of calculations for He ions in Al foils show significant coherence effects on the energy losses in the pre-equilibrium regime, which are interpreted by the overlap between the time delay in the target response and the characteristic time scale for the charge-changing collisions of the projectile with the target atoms.

We demonstrate that strongly polarized quasi-one-dimensional very-high-n (potassium) Rydberg atoms can be produced by photoexcitation of selected Stark states in the presence of a weak dc field. Calculations show that, for $m=0\mathrm{}$ states, significant photoexcitation occurs only in the vicinity of the Stark-shifted s, p, and d levels, and that those states located near the Stark-shifted d level have sizable polarizations. These predictions are confirmed by experiment. The degree of polarization of the product states is analyzed by studying differences in their ionization characteristics when subject to short pulsed electric fields applied parallel and antiparallel to the dc field.

A detailed theoretical investigation of rare-gas atom clusters under intense short laser pulses reveals that the mechanism of energy absorption is akin to enhanced ionization first discovered for diatomic molecules. The phenomenon is robust under changes of the atomic element (neon, argon, krypton, xenon), the number of atoms in the cluster (16–30 atoms have been studied), and the fluence of the laser pulse. In contrast to molecules it does not disappear for circular polarization. We develop an analytical model relating the pulse length for maximum ionization to characteristic parameters of the cluster.

It is impossible to discriminate four Bell states through local operations and classical communication, if only one copy is provided. To complete this task, two copies will suffice and be necessary. When $2n$ copies are provided, we show that the distillable entanglement is exactly $2n-2.$ Surprisingly, when $2n-1$ copies are provided, the distillable entanglement is also $2n-2.$

We introduce a measure of compatibility between quantum states—the likelihood that two density matrices describe the same object. Our measure is motivated by two elementary requirements, which lead to a natural definition. We list some properties of this measure, and discuss its relation to the problem of combining two observers’ states of knowledge.

The paper by Woodward [Phys. Rev. A 62, 052105 (2000)] claimed to have proved that Lagrangian theories with a nonlocality of finite extent are necessarily unstable. In this Comment we propose that this conclusion is false.

The objections recently raised by Llosa [Phys. Rev. A 67, 016101 (2003)] to the constructive proof that Lagrangians with nonlocality of finite extent inherit the full Ostrogradskian instability have been replied in this paper.

We discuss properties of entanglement measures called I-concurrence and tangle. For a bipartite pure state, I-concurrence and tangle are simply related to the purity of the marginal density operators. The I-concurrence (tangle) of a bipartite mixed state is the minimum average I-concurrence (tangle) of ensemble decompositions of pure states of the joint density operator. Terhal and Vollbrecht [Phys. Rev. Lett. 85, 2625 (2000)] have given an explicit formula for the entanglement of formation of isotropic states in arbitrary dimensions. We use their formalism to derive comparable expressions for the I-concurrence and tangle of isotropic states.

The collision of an antiproton and a hydrogen atom at ultralow energies is described in hyperspherical coordinates. An adiabatic approximation leads to potential-energy curves that can be used to describe the dynamics of capture in the $\overline{p}-\mathrm{H}\mathrm{}\left(1s\right)\mathrm{}$ collision. A unified formulation of the collision and capture processes leading to formation of $p\overline{p}$ bound states emerges. Both $\mathrm{Pn}{\mathrm{}\left(\mathrm{nl}\right)-e}^{-}$ “atomic-type” and $\overline{p}-\mathrm{H}{(n}^{\prime }{l}^{\prime })$ “molecular-type” states are represented. These potential curves are calculated to more than 10 000 protonium Bohr radii, where low-energy antiprotons are stopped in collisions with hydrogen atoms. Elastic and inelastic partial and total protonium formation cross sections in the mass-scaled $\overline{p}$-H system are calculated. The existence of protonium negative (or positive) ion in its ground and excited states is discussed.

This work treats four-wave mixing (4WM) in Bose-Einstein condensates (BEC), focusing on the nonlinear phase mismatch, maximum output, and optimal input combination. We show that the nonlinear phase mismatch decreases the 4WM efficiency. It was found that the 4WM efficiency depends on both the coupling coefficient (i.e., the product of the total number of atoms, the scattering length, and the overlap integral) and the ratios among the three initial input beams. The 4WM efficiency increases with the increase of the coupling coefficient when it is small, then saturates, and finally decreases at high coupling coefficient due to both pump depletion and phase-modulation effects. A maximum output efficiency of about 50% in our case is predicted. In order to get the maximum output, the two pump beams should have equal amplitude and the probe beam should be as small as possible. In addition, a large coupling coefficient (>π/2), which is determined by the ratio of the probe beam to the total input, is required. On the other hand, when the coupling coefficient is fixed, a maximum output for this case can be obtained by optimizing the input ratios among the three input beams. Other ratio combinations will decrease the 4WM efficiency.

We report the magnetic trapping of metastable ${}^3{P}_2$ atomic strontium. Atoms are cooled in a magneto-optical trap (MOT) operating on the dipole-allowed ${}^1{S}_0{-}^1{P}_1$ transition at 461 nm. Decay via ${}^1{\overrightarrow{P_1}}^1{\overrightarrow{D_2}}^3{P}_2$ continuously loads a magnetic trap formed by the quadrupole magnetic field of the MOT. Over ${10}^8$ atoms at a density of $8\times {10}^9{\mathrm{cm}}^{-3}$ and temperature of 1 mK are trapped. The atom temperature is significantly lower than what would be expected from the kinetic and potential energies of atoms as they are transferred from the MOT. This suggests the occurrence of thermalization and evaporative cooling in the magnetic trap.