Optical-field-ionization x-ray lasers in an optically preformed plasma waveguide for pure xenon, krypton, and argon gases, respectively, are achieved. In addition to the 46.9 nm main lasing line for Ne-like argon, the 45.1 and 46.5 nm lasing lines are also observed, indicative of the strong enhancement effect and the large gas density in the plasma waveguide. With this technique multispecies parallel x-ray lasing is also demonstrated in a Kr-Ar mixed-gas waveguide. Extensive experimental results including the pump-energy dependence, the density dependence, and the effects of parameters that control the waveguide fabrication are reported and discussed.

We analyze the dynamics of ultracold atoms in optical lattices induced by a sudden shift of the underlying harmonic trapping potential. In order to study the effect of strong interactions, dimensionality, and lattice topology on transport properties, we consider bosonic atoms with arbitrarily strong repulsive interactions on a two-dimensional square lattice and a hexagonal lattice. On the square lattice we find insulating behavior for weakly interacting atoms and slow relaxation for strong interactions, even when a Mott plateau is present, which in one dimension blocks the dynamics. On the hexagonal lattice the center of mass relaxes to the new equilibrium for any interaction strength.

We propose how to generate genuine multipartite entanglement of electron spin qubits in a chain of quantum dots using the naturally available single-qubit rotations and two-qubit Heisenberg exchange interaction in the system. We show that the minimum number of required operations to generate entangled states of the GHZ, cluster, and W type scales linearly with the number of qubits and estimate the fidelities of the generated entangled cluster states. As the required single and two-qubit operations have recently been realized, our proposed scheme opens the way for experimental investigation of multipartite entanglement with electron spin qubits.

A numerical approach in scattering theory based on the path-integration technique is considered. An evaluation of a discretized Feynman path integral with an accurate Gaussian expansion of the integrand was used for the calculation of the differential cross sections for the scattering of a wave packet by a three-dimensional spherically symmetric Gaussian potential. The results are compared with the precise calculations using the phase functions method. This reveals separately discretization errors of the path integral, which depends on the single parameter—the number of discrete path points. High accuracy of the approach is demonstrated.

We calculate ab initio the fraction of outgoing negative hydrogen ions that are normally incident on an unreconstructed $\mathrm{Si}\left(100\right)$ surface with kinetic energies between $50$ and $150\mathrm{eV}$. The ground-state electronic structure of the surface is derived from a self-consistent screened Thomas–Fermi–von Weizsäcker pseudopotential including Wang-Teter shell structure corrections. Orbitals and energies of the electronic states in this potential are obtained by solving Kohn-Sham equations. The dynamics of the transfer of a single electron during the ion-surface collision is represented within the Newns-Anderson model, including image-charge interactions and electron translation factor. We show that the outgoing ${\mathrm{H}}^{-}$ fraction evolves at large distances from the surface due to nonresonant transitions from the valence band levels of the substrate into the affinity level of ${\mathrm{H}}^{-}$. In particular, we show that electron capture from dangling-bond surface-state resonances determines the final negative-ion fraction. We find good qualitative agreement with the experimental results of Maazouz et al. [Surf. Sci. 398, 49 (1998)] for the scattering of hydrogen atoms and ions on silicon surfaces, even though our calculations do not include the effects of reconstruction and projectile motion parallel to the surface.

The ionization of nonsymmetric heteronuclear diatomic molecules by intense few-cycle laser pulses linearly polarized along the internuclear axis has been investigated. It is found that enhanced ionization (EI) occurs in nonsymmetric molecules and is accompanied by enhanced excitation (EE). We show that the nonsymmetric distribution of the electron cloud between the two nuclei leads to a strong dependence of EI and EE on the carrier envelope phase of few-cycle pulses, and on the orientation of the molecule parallel or antiparallel with the peak electric field of the pulse. This effect is as strong as the pulse duration is short and disappears with increasing pulse duration. The field ionization model, and mainly the “energy level crossing” mechanism, are used to explain these phase effects. The newly proposed energy level crossing mechanism, which is relevant to nonsymmetric molecules, occurs as the driving field moves the dressed ground and excited states closer to each other until their energy levels cross, leading to an enhancement of excitation and ionization. A semiclassical nonadiabatic model derived to interpret the level crossing mechanism also predicts the critical internuclear distance $R_c$ at which EI, EE, and energy crossings occur as a function of charge asymmetry and laser intensity, in good agreement with quantum-mechanical simulations.

We compute the shear viscosity of a superfluid atomic Fermi gas in the unitarity limit. The unitarity limit is characterized by a divergent scattering length between the atoms, and it has been argued that this will result in a very small viscosity. We show that in the low temperature $T$ limit the shear viscosity scales as ${\xi }^5∕T^5$, where the universal parameter $\xi$ relates the chemical potential and the Fermi energy, $\mu =\xi {\epsilon }_F$. Combined with the high temperature expansions of the viscosity our results suggest that the viscosity has a minimum near the critical temperature $T_c$. A naïve extrapolation indicates that the minimum value of the ratio of viscosity over entropy density is within a factor of $\sim 5$ of the proposed bound $\eta ∕s\ge \hslash ∕\left(4\pi k_B\right)$.

We study the conversion of ${}^{40}\mathrm{K}$ and ${}^{87}\mathrm{Rb}$ atoms into stable molecules through the stimulated Raman adiabatic passage (STIRAP) in photoassociation assisted with Feshbach resonance. Starting with the mean-field Lagrange density, we show that the atom-to-molecule conversion efficiency by STIRAP aided by Feshbach resonance is much larger than that by bare Feshbach resonance. We also study the influence of the population imbalance on the atom-to-molecule conversion.

We study the phase diagram for a dilute Bardeen-Cooper-Schrieffer superfluid Fermi-Fermi mixture (of distinct mass) at zero temperature using energy densities for the superfluid fermions in one (1D), two (2D), and three (3D) dimensions. We also derive the dynamical time-dependent nonlinear Euler-Lagrange equation satisfied by the mixture in one dimension using this energy density. We obtain the linear stability conditions for the mixture in terms of fermion densities of the components and the interspecies Fermi-Fermi interaction. In equilibrium there are two possibilities. The first is that of a uniform mixture of the two components, the second is that of two pure phases of two components without any overlap between them. In addition, a mixed and a pure phase, impossible in 1D and 2D, can be created in 3D. We also obtain the conditions under which the uniform mixture is stable from an energetic consideration. The same conditions are obtained from a modulational instability analysis of the dynamical equations in 1D. Finally, the 1D dynamical equations for the system are solved numerically and by variational approximation (VA) to study the bright solitons of the system for attractive interspecies Fermi-Fermi interaction in 1D. The VA is found to yield good agreement to the numerical result for the density profile and chemical potential of the bright solitons. The bright solitons are demonstrated to be dynamically stable. The experimental realization of these Fermi-Fermi bright solitons seems possible with present setups.

This work reports studies of a mixture of two interacting Bose condensates in a quasi-one-dimensional geometry using a finite-temperature path-integral Monte Carlo method. For weakly interacting dilute gases, the interactions between atoms are modeled by a hard-sphere potential whose core radius equals its corresponding scattering length. The effect of both the temperature and the interspecies interaction on the static properties, such as the density profile and the superfluid fraction, is discussed. Our results show that for repulsive intra- and interspecies interactions, the presence of the interspecies interactions dramatically changes the structure of both the density profile and the superfluid fraction, and two species are seen to partially separate in space, and there is a finite-size effect on the phase separation between two partially separating Bose-Einstein condensates. In general, our results agree with the theoretical predictions and experimental observations.

A wave-packet propagation method is developed and applied to investigate the quantum dynamics of scattering processes of identical and distinguishable atoms in harmonic waveguides. The quantum dynamics of the confinement-induced resonances (CIRs) for ultracold collisions of identical particles, $s$-wave CIRs for bosons and $p$-wave CIRs for fermions, is explored in detail. Our multigrid approach allows us to fully take into account the coupling between the center-of-mass (c.m.) and relative motions in the case of distinguishable atoms. The latter includes, in particular, $s$- and $p$-partial-wave mixing, caused by the confining trap, which acts differently on the different atomic species. Specifically, we explore in detail the recently discovered [J. I. Kim, V. S. Melezhik, and P. Schmelcher, Phys. Rev. Lett. 97, 193203 (2006)] dual CIR, which is based on a destructive interference mechanism leading to complete transmission in the waveguide, although the corresponding scattering in free space exhibits strong $s$- and $p$-wave scattering.

We investigate the polarization-to-phase conversion of space-variant polarized light waves. Our analysis can be considered as an extension of Pancharatnam’s phase to the case of space-variant polarization. In two particular cases—azimuthally and radially variant polarization—we found striking consequences. Using an azimuthally variant polarizer combined with a circular analyzer, it is possible to induce spin-to-orbital angular momentum exchange and to generate optical vortices of arbitrary charge. Radially variant polarizers with linear or quadratic radial dependence of the transmission direction generate nondiffracting beams or focus light, respectively. Our results are potentially relevant for the design of achromatic wavefront forming (and correcting) elements based strictly on space-variant polarization optics.

We predict the existence of lattice-cavity solitons for a quadratic nonlinear cavity, where the linear losses are compensated for by the optical pump at second harmonic (degenerate optical parametric oscillator), and which is endowed with a one-dimensional photonic lattice. In the limit of strong discreteness (weak coupling) this kind of soliton solution contains as the subclass the quadratic discrete cavity solitons. The nonlinear coupling between the Bloch waves of different photonics bands allows for the formation of a reach variety of localized solutions. In particular, different types of multiband lattice-cavity solitons can be identified. Most types of lattice-cavity solitons do not have counterparts, neither in conventional planar microresonators nor in genuine discrete systems as an array of weakly coupled cavities. We show that these solitons may destabilize as a consequence of the competition between Bloch waves of different photonic bands.

We study the quantum walk subjected to measurements with a Lévy waiting-time distribution. We find that the system has a sub-ballistic behavior instead of a diffusive one. We obtain an analytical expression for the exponent of the power law of the variance as a function of the characteristic parameter of the Lévy distribution.

Fine-structure intervals in $n=17$ and 20 barium Rydberg levels with $6\le L\le 11$ have been measured precisely with the resonant excitation Stark ionization spectroscopy microwave technique. The measured intervals were analyzed using the long-range polarization model, including allowance for the large nonadiabatic effects of the ${\text{Ba}}^{+}$ $5d$ level and the contributions of higher-order terms. The analysis determined improved values in atomic units of both dipole and quadrupole polarizabilities of ${\text{Ba}}^{+}$, ${\alpha }_1=123.88\left(5\right)$ and ${\alpha }_2=4420\left(250\right)$.

Ultracold photoassociation is a process in which two cold atoms combine to form a molecule. This process is crucially dependent on the atom pair density at close distance termed the photoassociation window. We explore the possibilities for increasing the pair density at the photoassociation window by using a prepulse to accelerate the pairs of atoms at large interatomic separation toward each other. We show that the signal of a subsequent photoassociation pulse could be enhanced by a factor of one to two orders of magnitude with respect to the conventional continuous wave experiment. For neutral encounters on the ground potential surface which scales as $R^{-6}$ the acceleration becomes negligible. The electronic excited state potentials scale for homonuclear $S\to P$ transitions more favorably as $R^{-3}$. A possible pump-dump mechanism for acceleration, excites a pair of atoms at large distance employing the natural acceleration on the excited state. Then a dump pulse moves the population back to the ground surface. By controlling the phase and the amplitude of the light field this scenario can be optimized. In addition the momentum partitioned between the ground and excited surfaces can also be controlled. The possibility for pure quantum light-induced acceleration due to a gradient of the transition dipole is analyzed. Significant acceleration can be obtained only for pulses with intensities above ${10}^{14}\mathrm{W}∕{\mathrm{cm}}^2$ and pulse duration of $10\mathrm{ps}$.

We report experimental measurements of the transmission spectrum of an optical cavity coupled with cold Rb atoms. We observe the multiatom vacuum Rabi splitting of a composite cavity and atom system. When a coupling field is applied to the atoms and induces the resonant two-photon Raman transition with the cavity field in a $\Lambda$-type system, we observe a cavity transmission spectrum with two vacuum Rabi sidebands and a central peak representing the intracavity dark state. The central peak linewidth is significantly narrowed by the dark-state resonance and its position is insensitive to the frequency change of the empty cavity.

We present an efficient architecture for quantum repeaters based on single-photon sources in combination with quantum memories for photons. Errors inherent to previous repeater protocols using photon-pair sources are eliminated, leading to a significant gain in efficiency. The requirements on the single-photon sources and on the photon detectors are within reach of current technology.

We present an analysis of noisy atomic channels involving qutrits. We choose a three-level atom with $\mathsf{V}$ configuration to be the qutrit state. Gell-Mann matrices and a generalized Bloch vector (eight-dimensional) are used to describe the qutrit density operator. We introduce quantum quasidistributions for qutrits that provide a simple description of entanglement. Studying the time evolution for the atomic variables we find the Kraus representation of spontaneous emission quantum channel (SE channel). Furthermore, we consider a generalized Werner state of two qutrits and investigate the separability condition in the presence of spontaneous emission noise. The influence of spontaneous emission on the separability of Werner states for qutrit and qubit states is compared.