We discuss quantum enhancement of the quasiclassical vibrational predissociation (VP) rate of an atom (A)-diatom (BC) van der Waals (vdW) complex $A-\mathrm{BC}$ from a state lying close to the dissociation threshold. The enhancement is due to the accumulation of a noticeable fraction of the state population near the outer turning point of the $A-\mathrm{BC}$ bond. For potentials behaving as power-laws at large separations, the enhancement manifests itself in the variation of the energy dependence of the VP frequency factor from the classical frequency to its quantum counterpart. We show that it is related to the complex scattering length associated with the vibrational relaxation of BC in collisions with A. We also discuss the corrected quasiclassical quantization condition for energy levels of a diatom lying very close to the dissociation threshold. Our results generalize those of J. Trost, C. Eltshka, and H. Friedrich [J. Phys. B 31, 361 (1998)] and C. Boisseau et al. [Eur. Phys. J. D 12, 199 (2000)] for the calculation of complex energy levels of an anharmonic oscillator that dissociates from highly excited states under the action of a weak high-frequency perturbation.

We present ab initio calculations of cross sections for single and autoionizing double electron capture in collisions of ${\mathrm{N}}^{5+}$ with ${\mathrm{H}}_2,$ for impact energies between 0.2 and 10 keV/amu. Calculations have been carried out by means of a close-coupling molecular treatment using the sudden approximation for rotation and vibration of the diatomic molecules. Since the molecular states involved are infinitely excited, a configuration interaction method, with a block-diagonalization procedure, has been employed to evaluate potential energy surfaces and dynamical couplings.

In this paper we investigate the effective scattering length a of spin-polarized ${\mathrm{Ne}}^{*}.$ Due to its anisotropic electrostatic interaction, its scattering length is determined by five interaction potentials instead of one, even in the spin-polarized case, a unique property among the Bose condensed species and candidates. Because the interaction potentials of ${\mathrm{Ne}}^{*}$ are not known accurately enough to predict the value of the scattering length, we investigate the behavior of a as a function of the five phase integrals ${\Phi }_{\Omega }$ corresponding to the five interaction potentials. We find that the scattering length has five resonances instead of only one and cannot be described by a simple gas-kinetic approach or the degenerate internal state (DIS) approximation. However, the probability for finding a positive or large value of the scattering length is not enhanced compared to the single-potential case. We find that the induced dipole-dipole interaction enables strong coupling between the different $|J\Omega {\mathrm{PM}}_P〉$ states, resulting in an inhomogeneous shift of the resonance positions and widths in the quantum-mechanical calculation as compared to the DIS approach. The dependence of the resonance positions and widths on the input potentials turns out to be rather straightforward. The existence of two bosonic isotopes of ${\mathrm{Ne}}^{*}$ enables us to choose the isotope with the most favorable scattering length for efficient evaporative cooling towards the Bose-Einstein condensation transition, greatly enhancing the feasibility to reach this transition.

We study molecular alignment by trains of strong, ultrashort laser pulses. Single-pulse alignment is analyzed both in classical and quantum-mechanical regimes. Moreover, we suggest multipulse excitation schemes leading to dramatically enhanced molecular alignment, and define conditions for femtosecond experiments of this kind.

We present a simple model of ultracold cesium dimer formation in which the photoassociation into rovibrational levels of the double-minimum ${\mathrm{Cs}}_2$ $3{}^1{\Sigma }_u^{+}$ state above its internal barrier enables spontaneous or stimulated emission into the lowest rovibrational levels of the ${\mathrm{Cs}}_2$ $1\left(X\right)\mathrm{}{}^1{\Sigma }_g^{+}$ ground electronic state. The transition dipole moment should be sufficiently large for observation of such spectral transitions. The use of the ${\mathrm{Cs}}_2$ $3{}^1{\Sigma }_u^{+}$ double-minimum potential could be extended to molecular Bose-Einstein condensates in which an optical trap can be employed for trapping of both atomic and molecular species.

We have demonstrated a high-flux source of polarization-entangled photons using a type-II phase-matched periodically poled ${\mathrm{KTiOPO}}_4$ parametric down-converter in a collinearly propagating configuration. We have observed quantum interference between the single-beam down-converted photons with a visibility of 99% together with a measured coincidence flux of $300{\mathrm{s}}^{-1}/\mathrm{mW}$ of pump. The Clauser-Horne-Shimony-Holt version of Bell’s inequality was violated with a value of $2.711\pm \mathrm{0.017.}$

We investigate theoretically the quantum fluctuations of the fundamental field in the output of a nondegenerate second-harmonic generation process occurring inside a laser cavity. Due to the nondegenerate character of the nonlinear medium, a field orthogonal to the laser field is for some operating conditions independent of the fluctuations produced by the laser medium. We show that this fact may lead to perfect squeezing for a certain polarization mode of the fundamental field. The experimental feasibility of the system is also discussed.

This paper addresses the question of the distillation of entanglement from a finite number of multipartite mixed states. It is shown that, if one can distill a pure entangled state from n copies of a mixed state ${\sigma }_{\mathrm{ABC}\cdot \cdot \cdot },$ there must be at least a subspace in the whole Hilbert space of all the copies such that the projection of ${\sigma }_{\mathrm{ABC}\cdot \cdot \cdot }^{\otimes n}$ onto the subspace is a pure entangled state. We also show that the purification of entanglement or distillation of entanglement can be carried out by local joint projective measurements with the help of classical communication and local general positive operator valued measurements on a single particle, in principle. Finally, we discuss experimental realizability of the entanglement purification.

Quantum spectroscopy was performed using the frequency-entangled broadband photon pairs generated by spontaneous parametric down-conversion. An absorptive sample was placed in front of the idler photon detector, and the frequency of signal photons was resolved by a diffraction grating. The absorption spectrum of the sample was measured by counting the coincidences, and the result is in agreement with the one measured by a conventional spectrophotometer with a classical light source.

We discuss some classes of atomic lenses capable of generating atomic vortex beams. For certain lens classes one may associate a topological charge with the beam in the focal region, and it is shown that an energy of the order $ħ/\tau$ is needed to alter the topological charge by $\Delta n=\pm 1,$ where $\tau$ is the atom interaction time with the lens. We also find that the vortex can be manipulated by the properties of the atomic lens. In particular, it is suggested that the circulation varies locally in space.

To observe or control a quantum system, one must interact with it via an interface. This article exhibits simple universal quantum interfaces—quantum input/output ports consisting of a single two-state system or quantum bit that interacts with the system to be observed or controlled. It is shown that under very general conditions the ability to observe and control the quantum bit on its own implies the ability to observe and control the system itself. The interface can also be used as a quantum communication channel, and multiple quantum systems can be connected by interfaces to become an efficient universal quantum computer. Experimental realizations are proposed, and implications for controllability, observability, and quantum information processing are explored.

In this paper we study quantum communication channels with correlated noise effects, i.e., quantum channels with memory. We derive a model for correlated noise channels that includes a channel memory state. We examine the case where the memory is finite, and derive bounds on the classical and quantum capacities. For the entanglement-assisted and unassisted classical capacities it is shown that these bounds are attainable for certain classes of channel. Also, we show that the structure of any finite-memory state is unimportant in the asymptotic limit, and specifically, for a perfect finite-memory channel where no information is lost to the environment, achieving the upper bound implies that the channel is asymptotically noiseless.

We derive two lower bounds on entanglement of formation for arbitrary mixed Gaussian states by two distinct methods. To achieve the first one we use a local measurement procedure that symmetrizes a general Gaussian state and the fact that entanglement cannot increase under local operations and classical communications. The second one is obtained via a generalization to mixed states of an interesting result already known for pure states, which says that squeezed states are those that, for a fixed amount of entanglement, maximize Einstein-Podolsky-Rosen-like correlations.

The quantum electrodynamical theory of the cascades and two-photon transitions in few-electron highly charged ions is presented. The numerical calculations are performed for the $E1\mathrm{}-M1\mathrm{}$ transition $2{}^3{P}_0–1^1{S}_0$ in the He-like ions with the nuclear charge $30<~Z<~100.$

A complete system of the B-spline solutions for the Dirac equation with the parity-nonconserving (PNC) weak-interaction effective potential is obtained. This system can be used for the accurate evaluation of the radiative corrections to the PNC amplitudes in the multicharged ions and neutral atoms. The use of the scaling procedure allows for the evaluation of the PNC matrix elements with relative accuracy ${10}^{-7}.$

Anions of the second and third periods—${\mathrm{B}}^{-},$ ${\mathrm{O}}^{-},$ ${\mathrm{F}}^{-},$ ${\mathrm{Al}}^{-},$ ${\mathrm{S}}^{-},$ and ${\mathrm{Cl}}^{-}$—had their total electron detachment cross sections measured, in the 0.3–1.5 a.u. velocity range, for He, Ne, and Ar targets. It is observed that, for any given target, the cross sections present similar velocity dependence and a conspicuous maximum at the same velocity, which differs for distinct targets. It was also obtained that these cross sections scale with target-independent factors, and the ratio of factors within the same group of the periodic table is also nearly group independent. Explanations for these behaviors are presented considering the presence of low- and high-velocity regions, and describing the latter by a free-electron model. The roles of the double ionization and the strong electron correlation in the detachment process are also discussed.

Electronic processes leading to dissociative ionization of polyatomic molecules in strong laser fields are investigated experimentally, theoretically, and numerically. Using time-of-flight ion mass spectroscopy, we study the dependence of fragmentation on laser intensity for a series of related molecules and report regular trends in this dependence on the size, symmetry, and electronic structure of a molecule. Based on these data, we develop a model of dissociative ionization of polyatomic molecules in intense laser fields. The model is built on three elements: (i) nonadiabatic population transfer from the ground electronic state to the excited-state manifold via a doorway (charge-transfer) transition; (ii) exponential enhancement of this transition by collective dynamic polarization of all electrons, and (iii) sequential energy deposition in both neutral molecules and resulting molecular ions. The sequential nonadiabatic excitation is accelerated by a counterintuitive increase of a large molecule’s polarizability following its ionization. The generic theory of sequential nonadiabatic excitation forms a basis for quantitative description of various nonlinear processes in polyatomic molecules and ions in strong laser fields.

Two remote parties that have never interacted each other can be entangled through entanglement swapping operation done by a third party. Currently existing entanglement swapping experiments are done probabilistically by postselection, i.e., once a successful swapping is verified, the resultant entanglement is destructed. We propose a simple nonpostselection scheme to demonstrate the high-quality quantum entanglement swapping with the spontaneous parametric down-conversion process. Our scheme only requires the normal photon detectors which only distinguish the vacuum and nonvacuum Fock states.

This paper reports the elastic scattering cross sections for a few low-lying partial waves and also the converged elastic cross sections with added partial waves in the energy range ${10}^{-10}–{10}^{-2}$ a.u. for the $\mathrm{H}¯-\mathrm{H}$ system using atomic orbital techniques. The present s-wave predictions are in good agreement with the other existing theoretical estimates. Nonzero low-order partial-wave elastic cross sections show dips like for s-wave scattering. The converged elastic cross section shows structurelike behavior in the energy range $4.2\times {10}^{-4}–{10}^{-2}\mathrm{a}.\mathrm{u}.$