We present a nonperturbative time-dependent theoretical method to study ${\mathrm{H}}_2$ ionization with femtosecond laser pulses when the photon energy is large enough to populate the $Q_1$ $(25–28\mathrm{eV})$ and $Q_2$ $(30–37\mathrm{eV})$ doubly excited autoionizing states. We have investigated the role of these states in dissociative ionization of ${\mathrm{H}}_2$ and analyzed, in the time domain, the onset of the resonant peaks appearing in the proton kinetic energy distribution. Their dependence on photon frequency and pulse duration is also analyzed. The results are compared with available experimental data and with previous theoretical results obtained within a stationary perturbative approach. The method allows us as well to obtain dissociation yields corresponding to the decay of doubly excited states into two H atoms. The calculated $\mathrm{H}(n=2)$ yields are in good agreement with the experimental ones.

The influence of a superposed offset field on the response of highly polarized (quasi-1D) very-high-$n$ Rydberg atoms to a periodic train of unidirectional half-cycle pulses (HCPs) is investigated, both experimentally and theoretically. It is observed that the presence of an offset field strongly influences the dynamics. The electronic states become transiently stabilized and trapped near the ionization threshold when the net average field they experience is near zero irrespective of whether the HCPs, applied parallel to the atomic axis, are directed towards or away from the nucleus. The nature of this stabilization is explored using Poincaré surfaces of section. In large offset fields, direct field ionization becomes possible but it is demonstrated that this can be suppressed by application of an oppositely directed HCP train. The use of such “frustrated field ionization” to measure the polarization of Rydberg atoms is discussed.

The scattering of sound-wave perturbations from vortex excitations in hydrodynamic systems with typical Bose-Einstein-condensate (BEC) parameters is investigated by numerical integration of the associated Klein-Gordon equation. The simulations indicate that at sufficiently high angular speeds, in the perturbative limit where back-reaction effects can be neglected, sound wave packets can extract a sizable fraction of the vortex energy through a mechanism of superradiant scattering. It is conjectured that this superradiant regime may be detectable in BEC experiments.

We describe the design and implementation of a two-dimensional optical lattice of double wells suitable for isolating and manipulating an array of individual pairs of atoms in an optical lattice. Atoms in the square lattice can be placed in a double well with any of their four nearest neighbors. The properties of the double well (the barrier height and relative energy offset of the paired sites) can be dynamically controlled. The topology of the lattice is phase stable against phase noise imparted by vibrational noise on mirrors. We demonstrate the dynamic control of the lattice by showing the coherent splitting of atoms from single wells into double wells and observing the resulting double-slit atom diffraction pattern. This lattice can be used to test controlled neutral atom motion among lattice sites and should allow for testing controlled two-qubit gates.

Anomalous diffusion is discussed in the context of quantum Brownian motion with colored noise. It is shown that earlier results follow simply and directly from the fluctuation-dissipation theorem. The limits on the long-time dependence of anomalous diffusion are shown to be a consequence of the second law of thermodynamics. The special case of an electron interacting with the radiation field is discussed in detail. We apply our results to wave-packet spreading.

This theoretical paper investigates the formation of ground state molecules from ultracold cesium atoms in a two-color scheme. Following previous work on photoassociation with chirped picosecond pulses [Luc-Koenig et al., Phys. Rev. A, 70, 033414 (2004)], we investigate stabilization by a second (dump) pulse. By appropriately choosing the dump pulse parameters and time delay with respect to the photoassociation pulse, we show that a large number of deeply bound molecules are created in the ground triplet state. We discuss (i) broad-bandwidth dump pulses which maximize the probability to form molecules while creating a broad vibrational distribution as well as (ii) narrow-bandwidth pulses populating a single vibrational ground state level, bound by $113{\mathrm{cm}}^{-1}$. The use of chirped pulses makes the two-color scheme robust, simple, and efficient.

The optical scattering force, behind Doppler cooling and magneto-optical trapping, may be amplified without incurring additional spontaneous emission by the state-dependent coherent deflection produced by a pulsed or chirped laser field. At some cost in experimental complexity, amplified forces allow efficient cooling on narrow transitions and permit the compact deceleration of beams with reduced transverse heating, and will be of interest for molecules and atoms with open level schemes where losses following spontaneous emission would otherwise prevail. We present a general analysis of the amplification scheme, and propose an optimized, dynamic cooling scheme that allows the temperature of a sample to be reduced by around a factor of two per excited state lifetime.

By using a one-dimensional nonpolynomial nonlinear mean-field model, we numerically analyze the process of the loading of the Bose-Einstein condensate into a moving one-dimensional optical lattice. We demonstrate that the recently observed dynamical instability of the Bloch states of a repulsive condensate in a moving optical lattice can lead to formation of trains of spatially localized wave packets that can be associated with matter-wave gap solitons. We study the characteristic features of this matter-wave localization under realistic conditions by modeling the dynamics of the condensate beyond the onset of dynamical instability.

We investigate the two-dimensional optical rotor of a weakly modulated vectorial bistable laser submitted to a single or multiple stochastic perturbations. In the Langevin-type equation of the rotor the role of an even or odd input forcing function on the system dynamics is isolated. Through these two inputs of optical and magnetic natures we verify that the stochastic resonance exists only when the periodic modulation acts on the even parity optical input. When two mutually correlated noises are simultaneously submitted to the input functions of opposite parities, we find a critical regime of the noise interplay whereby one stable state becomes noise-free. In this case, the residence time of the light vector in the noise-free state diverges which leads to a collapse of the output signal-to-noise ratio. But, in this critical regime also obtained when one noise drives both the even and odd functions, if the system symmetry is broken through an independent lever control, we can recover the switching cycle due to a new response mechanism, namely, the dual stochastic response, with a specific output signal-to-noise ratio expression. Both the theoretical analysis and the experiment show that the signal-to-noise ratio now displays a robust behavior for a large range of the input noise amplitude, and a plateau with respect to the input signal amplitude. Furthermore, we isolate an original signature of this synchronization mechanism in the residence-time distribution leading to a broadband forcing frequency range. These noise interplay effects in a double well potential are of generic nature and could be found in other nonlinear systems.

Nonlinear optical quantum gates can be created probabilistically using only single-photon sources, linear optical elements, and photon-number-resolving detectors. These gates are heralded but operate with probabilities much less than 1. There is currently a large gap between the performance of the known circuits and the established upper bounds on their success probabilities. One possibility for increasing the probability of success of such gates is feed-forward, where one attempts to correct certain failure events that occurred in the gate’s operation. In this Brief Report we examine the role of feed-forward in improving the success probability. In particular, for the nonlinear sign-shift gate, we find that in a three-mode implementation with a single round of feed-forward the optimal average probability of success is approximately given by $p_{\mathrm{success}}=0.272$. This value is only slightly larger than the general optimal success probability without feed-forward, $p_{\mathrm{success}}=0.25$.

Finding control fields (pulse sequences) that can compensate for the dispersion in the parameters governing the evolution of a quantum system is an important problem in coherent spectroscopy and quantum information processing. The use of composite pulses for compensating dispersion in system dynamics is widely known and applied. In this paper, we make explicit the key aspects of the dynamics that makes such a compensation possible. We highlight the role of Lie algebras and noncommutativity in the design of a compensating pulse sequence. Finally, we investigate three common dispersions in NMR spectroscopy, namely the Larmor dispersion, rf inhomogeneity, and strength of couplings between the spins.

We generalize Hardy’s proof of nonlocality to the case of bipartite mixed statistical operators, and we exhibit a necessary condition which has to be satisfied by any given mixed state $\sigma$ in order that a local and realistic hidden variable model exists which accounts for the quantum mechanical predictions implied by $\sigma$. Failure of this condition will imply both the impossibility of any local explanation of certain joint probability distributions in terms of hidden variables and the nonseparability of the considered mixed statistical operator. Our result can be also used to determine the maximum amount of noise, arising from imperfect experimental implementations of the original Hardy’s proof of nonlocality, in presence of which it is still possible to put into evidence the nonlocal features of certain mixed states.

Both the $K$ absorption edge and the $K$-shell photoelectric cross section have been determined for Hf, Ta, Au, and Pb by using a continuous external bremsstrahlung (EB) radiation from a weak $\beta$ source. In this method the EB photons produced in a nickel foil by the $\beta$ particles from a weak ${}^{90}\mathrm{Sr}\text{−}{}^{90}\mathrm{Y}$ $\beta$ source are incident on a target and the spectrum of the transmitted photons is recorded with a high-purity Ge detector coupled to an 8000 multichannel analyzer. The transmitted spectrum exhibits a sharp fall in intensity at the $K$-shell binding energy; and such a fall is used to determine both the $K$-absorption edge and the photoelectric cross section of the target.

The ionization dynamics of ${{\mathrm{H}}_2}^{+}$ exposed to high-intensity, high-frequency, ultrashort laser pulses is investigated with two theoretical approaches. The time-dependent Schrödinger equation is solved by a direct numerical method, and a simple two-center interference-diffraction model is studied. The energy and angular distributions of the photoelectron for various internuclear distances and relative orientations between the internuclear axis of the molecule and the polarization of the field are calculated. The main features of the photoelectron spectrum pattern are described well by the interference-diffraction model, and excellent quantitative agreement between the two methods is found. The effect of quantal vibration on the photoelectron spectrum is also calculated. We find that vibrational average produces some broadening of the main features, but that the patterns remain clearly distinguishable.

Bragg diffraction divides a Bose-Einstein condensate into two overlapping components, moving with respect to each other with high momentum. Elastic collisions between atoms from distinct wave packets can significantly deplete the condensate. Recently, Ziń et al. [Phys. Rev. Lett. 94, 200401 (2005)] introduced a model of two counterpropagating atomic Gaussian wave packets incorporating the dynamics of the incoherent scattering processes. Here we study the properties of this model in detail, including the nature of the transition from spontaneous to stimulated scattering. Within the first-order approximation, we derive analytical expressions for the density matrix and anomalous density that provide excellent insight into correlation properties of scattered atoms.

Inner-shell photodetachment of ${\mathrm{He}}^{-}$ is studied near the ${\mathrm{He}}^{-}1s2s2p{}^{4}P^o\to \mathrm{He}2s2p{}^{3}P^o$ threshold. The spectrum over this region shows significant suppression of ${\mathrm{He}}^{+}$ signal production due to postcollision interaction (PCI) effects. The observed shape and absolute cross section are in general agreement with theoretical predictions when corrections for semiclassical PCI effects are included.

Entanglement purification protocols play an important role in the distribution of entangled systems, which is necessary for various quantum information processing applications. We consider the effects of photodetector efficiency and bandwidth, channel loss and mode mismatch on the operation of an optical entanglement purification protocol. We derive necessary detector and mode-matching requirements to facilitate practical operation of such a scheme, without having to resort to destructive coincidence-type demonstrations.

We report on the experimental demonstration of high energy-time entanglement in two-photon states created in the process of spontaneous parametric down-conversion. We show that the classical variance product, which we violate by three orders of magnitude, actually represents a lower bound estimate of the number of information eigenmodes $K$. Explicit measurements estimate $K$ to be greater than 100, with theoretical estimates predicting a value of as high as $1\times {10}^6$. These results provide incentive for the practical feasibility of large bandwidth quantum information processing, particularly in cryptography over large distances.

For any specified pure state of quantum open system, we can construct a kind of quantum generalized measurement (QGM) that the state of the system after measurement will be deterministically collapsed into the specified pure state from any initial state. In other words, any pure state of quantum open system is reachable by QGM. Subsequently, whether the qubit is density matrix controllable is discussed in the case of pure dephasing. Our results reveal that combining QGM with coherent control will enhance the ability of controlling the quantum open system. Furthermore, it is found that the ability to perform QGM on the quantum open system, combined with the ability of coherence control and conditions of decoherence-free subspace, allows us to suppress quantum decoherence.