A theoretical description of near-threshold ionization of inner atomic shells by short electromagnetic pulses is developed. It is based on a nonstationary approach to the Fano-Feshbach problem of photoexcitation with simultaneous Auger decay of the produced vacancy. We suggest a method of solving the system of time-dependent Schrödinger equations describing a resonant state embedded into continuum. The method gives a nonstationary description of the post-collision interaction in near-threshold photoionization including the recapture of the slow photoelectron in high Rydberg states. As an example, $1s$ photoionization in Ne with the following $\mathit{KLL}$ Auger decay is considered. The influence of the duration of the pulse on the recapture probability and shape of the emitted electron peak is studied.

The results of ab initio global optimizations of the structures of ${\mathrm{Si}}_n\mathrm{H}$, $n=4–10$, atomic clusters using a parallel genetic algorithm are presented. Driving the global search with the parallel implementation of the genetic algorithm are presented and using the density functional theory as implemented in the Carr-Parinello molecular dynamics code to calculate atomic cluster energies and perform the local optimization of their structures, we have been able to demonstrate that it is possible to perform global optimizations of the structure of atomic clusters using ab initio methods. The results show that this approach is able to find many structures that were not previously reported in the literature. Moreover, in most cases the new structures have considerable lower energies than those previously known. The results clearly demonstrate that these calculations are now possible and in spite of their larger computational demands provide more reliable results.

This work explores the effect of spontaneous emission on the population transfer efficiency in stimulated Raman adiabatic passage (STIRAP). The approach uses adiabatic elimination of weakly coupled density matrix elements in the Liouville equation, from which a very accurate analytic approximation is derived. The loss of population transfer efficiency is found to decrease exponentially with the factor ${\Omega }_0^2∕\Gamma$, where $\Gamma$ is the spontaneous emission rate and ${\Omega }_0$ is the peak Rabi frequency. The transfer efficiency increases with the pulse delay and reaches a steady value. For large pulse delay and large spontaneous emission rate STIRAP degenerates into optical pumping.

We study the effect of lower dimensional geometry on the frequency splitting of the quadrupole oscillations of a harmonically trapped Bose-Einstein condensate due to the presence of a quantized vortex. To study the effect of two-dimensional geometry we consider a pancake-shaped condensate and employ various models for the coupling parameter depending on the thickness of the condensate relative to the value of the scattering length. Using these models and the sum-rule approach we obtain analytical expressions for the frequency splitting. These expressions are valid for positive scattering length and large $N$. We show that the frequency splitting of the quadrupole oscillations are significantly altered by the reduced dimensionality and also study the evolution of the splitting as the system makes transition from one scattering regime to the other.

We study the Landau damping of Bogoliubov excitations in two- and three-dimensional optical lattices at finite temperatures, extending our recent work on one-dimensional (1D) optical lattices. We use a Bose-Hubbard tight-binding model and the Popov approximation to calculate the temperature dependence of the number of condensate atoms $n^{\mathrm{c}0}\left(T\right)$ in each lattice well. As with 1D optical lattices, damping only occurs if the Bogoliubov excitations exhibit anomalous dispersion (i.e., the excitation energy bends upward at low momentum), analogous to the case of phonons in superfluid ${}^4\mathrm{He}$. This leads to the disappearance of all damping processes in a $D$-dimensional simple cubic optical lattice when $U{n}^{\mathrm{c}0}⩾6DJ$, where $U$ is the on-site interaction, and $J$ is the hopping matrix element.

Quantum mechanics provides us with probability densities-wave functions modulus squared. Such a probability density is experimentally recovered as an average over many repeated measurements performed on a system in a given wave function. Sometimes it is important to be able to theoretically predict not just the average but also a possible outcome of a single measurement. It is very difficult to make exact predictions of this kind in the case of many-body systems due to correlations in the corresponding many-body wave functions. Here I propose an approximate way of simulating the outcomes of a single-experiment density measurement that is performed on variety of states of $N$ bosons. The approximation is accurate if occupation of single-particle modes is macroscopic.

We study properties of excited states of an array of weakly coupled quasi-two-dimensional Bose condensates by using the hydrodynamic theory. The spectrum of the axial excited states strongly depends on the coupling among the various discrete radial modes in a given symmetry. By including mode coupling within a given symmetry, the complete excitation spectrum of axial quasiparticles with various discrete radial nodes are presented. A single parameter which determines the strength of the mode coupling is identified. The excitation spectrum in the zero angular momentum sector can be observed by using the Bragg scattering experiments.

Within the framework of exact quantization of the electromagnetic field in dispersing and absorbing media the input-output problem of a high-$Q$ cavity is studied, with special emphasis on the absorption losses in the coupling mirror. As expected, the cavity modes are found to obey quantum Langevin equations, which could be also obtained from quantum noise theories, by appropriately coupling the cavity modes to dissipative systems, including the effect of the mirror-assisted absorption losses. On the contrary, the operator input-output relations obtained in this way would be incomplete in general, as the exact calculation shows. On the basis of the operator input-output relations the problem of extracting the quantum state of an initially excited cavity mode is studied and input-output relations for the $s$-parametrized phase-space function are derived, with special emphasis on the relation between the Wigner functions of the quantum states of the outgoing field and the cavity field.

We show that the criticism of a recent Comment by Gao et al. on the insecurity of the quantum secret sharing protocol [proposed in S. Bagherinezhad and V. Karimipour, Phys. Rev. A 67, 044302 (2003)] is based on a misconception about the meaning of security and hence is invalid.

We introduce an extended version of a previous all-versus-nothing proof of impossibility of Einstein-Podolsky-Rosen’s local elements of reality for two photons entangled both in polarization and path degrees of freedom (A. Cabello, quant-ph/0507259), which leads to a Bell’s inequality where the classical bound is 8 and the quantum prediction is 16. A simple estimation of the detection efficiency required to close the detection loophole using this extended version gives $\eta >0.69$. This efficiency is lower than that required for previous proposals.

We investigate many-body phase diagrams of atomic boson-fermion mixtures loaded in the two-dimensional optical lattice. Bosons mediate an attractive, finite-range interaction between fermions, leading to fermion pairing phases of different orbital symmetries. Specifically, we show that by properly tuning atomic and lattice parameters it is possible to create superfluids with $s$-, $p$-, and $d$-wave pairing symmetry as well as spin and charge density wave phases. These phases and their stability are analyzed within the mean-field approximation for systems of ${}^{40}\mathrm{K}\text{−}{}^{87}\mathrm{Rb}$ and ${}^{40}\mathrm{K}\text{−}{}^{23}\mathrm{Na}$ mixtures. For the experimentally accessible regime of parameters, superfluids with unconventional fermion pairing have transition temperature around a percent of the Fermi energy.

We present an experimental method to engineer polarization-momentum hyperentangled two-photon states, using linear optics and a single type-I nonlinear crystal. These states have been completely characterized and their nonlocal behavior has been verified by an “all versus nothing” test of local realism, which represents a generalization of the Greenberger-Horne-Zeilinger (GHZ) to the case of two entangled particles and two observers. The manipulation of these states may represent a useful control in quantum state engineering and Bell state measurements and, more in general, in quantum information applications.

A quantum algorithm is proposed to solve the satisfiability (SAT) problems by the ground-state quantum computer. The scale of the energy gap of the ground-state quantum computer is analyzed for the $3\text{−}\text{bit}$ exact cover problem. The time cost of this algorithm on the general SAT problems is discussed.

We find that the asymptotic entanglement of assistance of a general bipartite mixed state is equal to the smaller of its two local entropies. Our protocol gives rise to the asymptotically optimal Einstein-Podolsky-Rosen (EPR) pair distillation procedure for a given tripartite pure state, and we show that it actually yields EPR and Greenberger-Horne-Zeilinger (GHZ) states; in fact, under a restricted class of protocols, which we call “one-way broadcasting,” the GHZ rate is shown to be optimal. This result implies a capacity theorem for quantum channels where the environment helps transmission by broadcasting the outcome of an optimally chosen measurement. We discuss generalizations to $m$ parties and show (for $m=4$) that the maximal amount of entanglement that can be localized between two parties is given by the smallest entropy of a group of parties of which the one party is a member, but not the other. This gives an explicit expression for the asymptotic localizable entanglement and shows that any nontrivial ground state of a spin system can be used as a perfect quantum repeater if many copies are available in parallel. Finally, we provide evidence that any unital channel is asymptotically equivalent to a mixture of unitaries and any general channel to a mixture of partial isometries.

The creation and detection of specific angular momentum states in Rydberg atoms is a significant advance in quantum-information processing. Detection of high angular momentum states in alkali-metal atoms is difficult because they are nearly-degenerate in energy. We present theoretical results on the detection of such states using terahertz-frequency half-cycle pulses (HCPs). Calculations are done for $n=15,\ell =0,\dots ,14$ states in hydrogen and cesium using a grid-based pseudopotential method. Our results indicate that HCP redistribution is a reliable method of distinguishing angular momentum Rydberg states.

In this work we present an investigation on the quantum dynamics of ultracold electronically excited atoms exposed to an external magnetic quadrupole field. We present a general Hamiltonian which describes the quantum dynamics of an atom in an arbitrary linear magnetic field. This makes our approach applicable to a wide range of atoms and magnetic field configurations. The system is solved by incorporating an adiabatic separation of the electronic and center-of-mass dynamics. We provide the adiabatic energy surfaces and discuss under which conditions trapped center-of-mass states can be achieved. We present energies and wave functions of the corresponding quantum states. By analyzing the properties of the combined center-of-mass and electronic quantum states we demonstrate that the extension of the electronic wave function can exceed that of the center-of-mass motion. Therefore such atoms cannot be considered as being pointlike. A discussion of electromagnetic transitions is also provided.

Hyperpolarized ${}^{129}\mathrm{Xe}$ finds numerous applications in NMR spectroscopy and magnetic resonance imaging. The production of hyperpolarized ${}^{129}\mathrm{Xe}$ by spin-exchange optical pumping is therefore an important experimental issue. We model the three-dimensional transport processes within a so-called batch mode pump cell via numerical finite element method simulations and compare the results with experimental data. In particular, the influence of different experimental parameters, such as temperature, xenon and nitrogen partial pressure, laser power, and radius-to-length ratio of a cylindrical pump cell, is evaluated. The developed numerical method is capable of describing the spin-exchange optical pumping process in a realistic manner.

We examine the static and dynamic stability of the solutions of the Gross-Pitaevskii equation and demonstrate the intimate connection between them. All salient features related to dynamic stability are reflected systematically in static properties. We find, for example, the obvious result that static stability always implies dynamic stability and present a simple explanation of the fact that dynamic stability can exist even in the presence of static instability.

Conversion of an expanding Bose-Einstein condensate of Cs atoms to a molecular one with an efficiency of more than 30% was observed recently in experiments by M. Mark et al. [Europhys. Lett. 69, 706 (2005)]. The theory presented here describes the experimental results. Values of resonance strength of 8 mG and rate coefficients for atom-molecule deactivation of $1\times {10}^{-11}{\mathrm{cm}}^3∕\mathrm{s}$ and molecule-molecule one of $1.5\times {10}^{-9}{\mathrm{cm}}^3∕\mathrm{s}$ are estimated by a fit of the theoretical results to the experimental data. Near the resonance, where the highest conversion efficiency was observed, the results demonstrate strong sensitivity to the magnetic field ripple and inhomogeneity. A conversion efficiency of about 60% is predicted by non-mean-field calculations for densities and sweep rates lower than the ones used in the experiments.

Based on the standard many-fermion field theory, we construct models describing ultracold fermions in a one-dimensional optical lattice by implementing a mode expansion of the fermionic field operator where modes, in addition to space localization, take into account the quantum numbers inherent in local fermion interactions. The resulting models are generalized Hubbard Hamiltonians whose interaction parameters are derived by a fully analytical calculation. The special interest for this derivation resides in its model-generating capability and in the flexibility of the trapping techniques that allow the tuning of the Hamiltonian interaction parameters over a wide range of values. While the Hubbard Hamiltonian is recovered in the very low-density regime, in general, far more complicated Hamiltonians characterize high-density regimes, revealing a rich scenario for both the phenomenology of interacting trapped fermions and the experimental realization of devices for quantum-information processing. As a first example of the different situations that may arise beyond the models well known in the literature (the unpolarized-spin fermion model and the noninteracting spin-polarized fermion model), we derive a rotational Hubbard Hamiltonian describing the local rotational activity of spin-polarized fermions. Based on standard techniques we obtain the mean-field version of our model Hamiltonian and show how different dynamical algebras characterize the cases of attractive and repulsive two-body potentials.