Classical cutoffs for the momenta of electrons ejected in laser-induced nonsequential double ionization are derived for the recollision-impact-ionization scenario. Such simple cutoff laws can aid in the interpretation of the observed electron spectra.

We report on measurements of the polarization for ${}^3\mathrm{He}$ using optically pumped potassium vapor. We achieve a ${}^3\mathrm{He}$ polarization greater than 40% in a mid-sized double-chamber glass target cell filled with ${}^3\mathrm{He}$ gas using a single Ti:sapphire laser. Applied to the production of polarized ${}^3\mathrm{He}$ as an atomic beam source for colliders, these developments could have significant impact on future studies of neutron spin structure. The results also motivate the development of high power diode lasers tuned to the appropriate potassium wavelengths.

We present calculated elastic differential cross sections for positron-acetylene scattering, obtained by using the Schwinger multichannel method. Our results are in very good agreement with quasielastic experimental data of Kauppila et al. [Nucl. Instrum. Meth. Phys. Res. B $192,$ 162 (2002)]. We also discuss the existence of a virtual state (zero-energy resonance) in $e^{+}-{\mathrm{C}}_2{\mathrm{H}}_2$ collisions, based on the behavior of the integral cross section and of the s-wave phase shift. As expected the fixed-nuclei cross section and annihilation parameter ${(Z}_{\mathrm{eff}})$ present the same energy dependence at very low impact energies. As the virtual state energy approaches zero, the magnitude of both cross section and $Z_{\mathrm{eff}}$ are extremely enhanced (at zero impact energy). The possibility of shifting from a low-lying virtual state to a shallow bound state is not expected to significantly affect room-temperature annihilation rates.

We have studied the spatial and temporal dynamics of a cold atom cloud in the conservative force field of a ferromagnetic guide, after laser cooling has been switched off suddenly. We observe outgoing “waves” that correspond to caustics of individual trajectories of trapped atoms. This provides detailed information on the magnetic field, the energy distribution, and the spin states.

In this paper we have investigated the propagation of the ultrashort pulsed beam with ultrabroad spectral width in the dispersive medium. A general $\mathrm{}(3+1)\mathrm{}$-dimensional $\left[\mathrm{}\right(3+1\left)\mathrm{}\mathrm{D}\right]$ propagation equation first order in the propagation coordinate is derived by using the extended paraxial approximation, which does not resort to the envelope and carrier frequency. This equation can provide an accurate description of the evolution of the ultrashort pulsed beam through the nonlinear dispersive medium, with the numerical value of the spectral bandwidth being bigger than the carrier frequency and considering the absolute frequency. The dispersive and loss coefficients are introduced, and their impacts on the pulse propagation are discussed. In the case of linear propagation, a family of exact solutions of the $\mathrm{}(3+1)\mathrm{}\mathrm{D}$ equation in dispersive media has been obtained, which represents the ultrashort pulse evolving due to gain (loss), dispersion, and diffraction.

A light beam normally incident upon an uniformly moving dielectric medium is, in general, subject to bendings due to a transverse Fresnel-Fizeau light drag effect. In most familiar dielectrics, the magnitude of this bending effect is very small and hard to detect. Yet, the effect can be dramatically enhanced in strongly dispersive media where slow group velocities in the m/s range have been recently observed taking advantage of the electromagnetically induced transparency effect. In addition to the usual downstream drag that takes place for positive group velocities, we discuss a significant anomalous upstream drag which is expected to occur for negative group velocities. Furthermore, for sufficiently fast speeds of the medium, higher-order dispersion terms are found to play an important role and to be responsible for light propagation along curved paths or the restoration of the time and space coherence of an incident noisy beam. The physics underlying this class of slow-light effects is thoroughly discussed.

The first Born differential cross section for Mott scattering of a Dirac-Volkov electron is reviewed. The expression (26) derived by Szymanowski et al. [Phys. Rev. A $56,$ 3846 (1997)] is corrected. In particular, we disagree with the expression of $(d\sigma /d\Omega )$ they obtained and we give the exact coefficients multiplying the various Bessel functions appearing in the scattering differential cross section. Comparison of our numerical calculations with those of Szymanowski et al. shows qualitative and quantitative differences when the incoming total electron energy and the electric-field strength are increased particularly in the direction of the laser propagation.

We analyze the quantum entanglement properties of bosonic particles hopping over graph structures. Mode entanglement of a graph vertex with respect to the rest of the graph is generated, starting from a product state, by turning on for a finite time a tunneling along the graph edges. The maximum achieved during the dynamical evolution by this bipartite entanglement characterizes the entangling power of a given hopping Hamiltonian. We studied this entangling power as a function of the self-interaction parameters, i.e., nonlinearities, for all the graphs up to four vertices and for two different natural choices of the initial state. The role of graph topology and self-interaction strengths in optimizing entanglement generation is extensively studied by means of exact numerical simulations and by perturbative calculations

We propose a directly measurable criterion for the entanglement of two qubits. We compare the criterion with other criteria, and we find that for pure states, and some mixed states, it coincides with the state’s concurrence. The measure can be obtained with a Bell-state analyzer and the ability to make general local unitary transformations. However, the procedure fails to measure the entanglement of a general mixed two-qubit state.

The effect of unitary noise on the discrete one-dimensional quantum walk is studied using computer simulations. For the noiseless quantum walk, starting at the origin $\mathrm{}(n=0\mathrm{})\mathrm{}$ at time $t=0,$ the position distribution $P_t\mathrm{}\left(n\right)\mathrm{}$ at time t is very different from the Gaussian distribution obtained for the classical random walk. Furthermore, its standard deviation, $\sigma \mathrm{}\left(t\right)\mathrm{}$ scales as $\sigma \mathrm{}\left(t\right)\mathrm{}\sim t,$ unlike the classical random walk for which $\sigma \mathrm{}\left(t\right)\mathrm{}\sim \sqrt{t}.$ It is shown that when the quantum walk is exposed to unitary noise, it exhibits a crossover from quantum behavior for short times to classical-like behavior for long times. The crossover time is found to be $T\sim {\alpha }^{-2},$ where $\alpha$ is the standard deviation of the noise.

Individual members of an ensemble of identical systems coupled to a common probe can become entangled with one another, even when they do not interact directly. We investigate how this type of multipartite entanglement is generated in the context of a system consisting of two two-level atoms resonantly coupled to a single mode of the electromagnetic field. The dynamical evolution is studied in terms of the entanglements in the different bipartite partitions of the system, as quantified by the I tangle. We also propose a generalization of the so-called residual tangle that quantifies the inherent three-body correlations in our tripartite system. This enables us to completely characterize the phenomenon of entanglement sharing in the case of the two-atom Tavis-Cummings model, a system of both theoretical and experimental interest.

Inelastic electron scattering from light atomic species is of fundamental importance and has significant applications in fusion-plasma modeling. Therefore, it is of interest to apply advanced nonperturbative, close-coupling methods to the determination of electron-impact excitation for these atoms. Here we present the results of R matrix with pseudostate (RMPS) calculations of electron-impact excitation cross sections through the $n=4\mathrm{}$ terms in Be, ${\mathrm{Be}}^{+},$ ${\mathrm{Be}}^{2+},$ and ${\mathrm{Be}}^{3+}.$ In order to determine the effects of coupling of the bound states to the target continuum in these species, we compare the RMPS results with those from standard R-matrix calculations. In addition, we have performed time-dependent close-coupling calculations for excitation from the ground and the metastable terms of ${\mathrm{Be}}^{+}$ and the metastable term of ${\mathrm{Be}}^{3+}.$ In general, these results are found to agree with those from our RMPS calculations. The full set of data resulting from this work is now available on the Oak Ridge National Laboratory Controlled Fusion Atomic Data Center web site, and will be employed for collisional-radiative modeling of Be in magnetically confined plasmas.

In this paper we analyze the dynamic evolution of molecular and atomic fragments of small hydrogen clusters interacting with thin solid foils. We compare the vicinage forces, calculated within the dielectric formalism, for ${\mathrm{H}}^{+},$ ${\mathrm{H}}^0,$ and ${\mathrm{H}}_2^{+}$ fragments. Using a molecular dynamics numerical code we determine the energy distribution of the fragments after interacting with the target. This distribution is compared to experimental results for protons coming from the fragmentation of $v=2.02\mathrm{}\mathrm{a}.\mathrm{u}.\mathrm{}$ ${\mathrm{H}}_2^{+}$ ions impinging on an aluminum foil; a fraction of neutral ${\mathrm{H}}^0$ is needed to be included in the simulation to get a good agreement with the experimental results. The ${\mathrm{H}}_2^{+}$ energy spectra for $v=5.42\mathrm{}\mathrm{a}.\mathrm{u}.\mathrm{}$ ${\mathrm{H}}_3^{+}$ interacting with amorphous carbon is also determined. The asymmetry in the Coulomb peaks appearing in the energy spectra both experimentally and in our calculation is opposite for ${\mathrm{H}}_2^{+}$ than in ${\mathrm{H}}^{+};$ kinematic effects and differences in the electronic stopping are enough to reproduce the difference in the alignment of ${\mathrm{H}}_2^{+}$ and ${\mathrm{H}}^{+}$ fragments.

We have observed multifrequency resonances in a system with a spin 1/2 located in dc magnetic field and irradiated simultaneously by a multiple-pulse radio frequency sequence and a low-frequency field swept in the range 0–80 kHz. The used excitation scheme allowed us to measure the effective field of the radio frequency sequence. A peculiarity of this scheme is that the intensity of the resonance lines decreases slowly with the mode number. The theoretical description of the effect is presented using both the rotating frame approximation and the Floquet theory. Both approaches give identical results at the calculation of the resonance frequencies, transition probabilities, and shifts of resonance frequency. The calculated magnetization vs the frequency of the low-frequency field agrees well with the obtained experimental data. The multifrequency spectra give a way for studying slow atomic motion in solids.

Subjecting polar linear molecules to appropriately designed half-cycle electromagnetic pulses induces strong orientation of the molecules. This is deduced by inspecting the quantum dynamics within a simplified model which yields analytical conditions for the parameters of the pulses that lead to strong molecular orientation sustainable for hundreds of picoseconds. These analytical predictions are largely confirmed by a full numerical time-dependent study of the orientation process for NaI molecules. Further strategies for increasing and maintaining the molecular orientation are proposed and numerically illustrated. Our finite-temperature calculations demonstrate that the molecular orientation persists at considerable temperatures.

We find stable families of vortex dipoles in nonrotating Bose-Einstein condensates. The vortex dipoles correspond to topological excited collective states of the condensed atoms. They exist and are dynamically and structurally stable for a broad range of parameters. We show that they can be generated by phase-imprinting techniques on the ground state of condensates.

We provide a self-contained quantum description of the interference produced by macromolecules diffracted by a grating, with particular reference to fullerene interferometry experiments. We analyze the processes inducing loss of coherence consisting in beam preparation (collimation setup and thermal spread of the wavelengths of the macromolecules) and in environmental disturbances. The results show a good agreement with experimental data and highlight the analogy with optics. Our analysis gives some hints for planning future experiments.

The fundamental gates of linear optics quantum computation are realized by using single photons sources, linear optics, and photon counters. Success of these gates is conditioned on the pattern of photons detected without using feedback. Here it is shown that the maximum probability of success of these gates is typically strictly less than 1. For the one-mode nonlinear sign shift, the probability of success is bounded by 1/2. For the conditional sign shift of two modes, this probability is bounded by 3/4. These bounds are still substantially larger than the highest probabilities shown to be achievable so far, which are 1/4 and 2/27, respectively.

Using the molecular dynamics method, we have calculated the interaction-induced polarizability correlation functions and spectra of the depolarized light scattering in a ${\mathrm{C}}_{60}$ fullerene molecule surrounded by an argon atmosphere. The liquid phase of $\left({\mathrm{C}}_{60}\right){\mathrm{Ar}}_n(n=36,40,44)$ system has been studied.

A necessary separability criterion that relates the structures of the total density matrix and its reductions is given. The method used is based on the realignment method [K. Chen and L. A. Wu, Quant. Inf. Comput. 3, 193 (2003)]. The separability criterion naturally generalizes the reduction separability criterion introduced independently in the previous work [M. Horodecki and P. Horodecki, Phys. Rev. A 59, 4206 (1999) and N. J. Cerf, C. Adami, and R. M. Gingrich, Phys. Rev. A 60, 898 (1999)]. In special cases, it recovers the previous reduction criterion and the recent generalized partial transposition criterion [K. Chen and L. A. Wu, Phys. Lett. A 306, 14 (2002)]. The criterion involves only simple matrix manipulations and can therefore be easily applied.