We have measured the cross section for the detachment of three electrons from the ${\mathrm{Cl}}^{-}$ ion following the absorption of a single photon. The triple-photodetachment cross section is compared to our earlier measurement of the cross section for double photodetachment from the same ion. Possible direct and indirect mechanisms responsible for multiple-electron detachment are discussed. The direct process of ejection of three valence electrons seems to be the most likely process. This conjecture needs theoretical verification.

Classical orbit derivations of the triply differential cross section for electron bremsstrahlung (BrS) in a Coulomb field are presented; the classical origin of the circular dichroism effect in the electron scattering angular distributions is established for both spontaneous and laser-stimulated BrS.

It is shown that the response of a three-level atom interacting with an optical field in a ”dark state geometry” (single field, degenerate ground states) depends critically on whether or not spontaneously generated coherence (SGC) is present. In the absence of SGC, the system evolves to a dark state. With SGC present, the dark state population is a constant of the motion and the steady-state values for density-matrix elements depend on their initial values at the time the field is applied.

We consider the inverse problem of in-line holography, applied to minimally destructive imaging of cold atom clouds. Absorption imaging near resonance provides a simple, but destructive measurement of atom column density. Imaging off resonance greatly reduces heating, and sequential images may be taken. Under the conditions required for off-resonant imaging, the generally intractable inverse problem may be linearized. A minimally destructive, quantitative and high-resolution image of the atom cloud column density is then retrieved from a single diffraction pattern.

Ultracold bosons in optical superlattices are expected to exhibit fractional-filling insulating phases for sufficiently large repulsive interactions. On strictly one-dimensional systems, the exact mapping between hard-core bosons and free spinless fermions shows that any periodic modulation in the lattice parameters causes the presence of fractional-filling insulator domains. Here, we focus on two recently proposed realistic two-dimensional structures where such mapping does not hold, i.e., the two-leg ladder and the trimerized kagomé lattice. Based on a cell strong-coupling perturbation technique, we provide quantitatively satisfactory phase diagrams for these structures, and give estimates for the occurrence of the fractional-filling insulator domains in terms of the intercell to intracell hopping amplitude ratio.

We theoretically study the properties of highly prolate-shaped dielectric microresonators. Such resonators sustain whispering-gallery modes that exhibit two spatially well-separated regions with enhanced field strength. The field per photon on the resonator surface is significantly higher than, e.g., for equatorial whispering-gallery modes in microsphere resonators with a comparable mode volume. At the same time, the frequency spacing of these modes is much more favorable, so that a tuning range of several free spectral ranges should be attainable. We discuss the possible application of such resonators for cavity quantum electrodynamics experiments with neutral atoms and reveal distinct advantages with respect to existing concepts.

We propose a quantum circuit for the quantum search problem. The quantum circuit is superior to Grover’s algorithm in some realistic cases. The reasons for the superiority are in short as follows: In the quantum circuit proposed in this paper, all the operators except for the oracle can be written as direct products of single-qubit gates. Such separable operators can be executed much faster than multiparticle operators, such as controlled-NOT gates and Toffoli gates, in many realistic systems. The idea of this quantum circuit is inspired by the Hamiltonian used in the adiabatic quantum computer. In addition, the scaling of the number of oracle calls for this circuit is the same as that for Grover’s algorithm, i.e., $O\left(2^{n∕2}\right)$.

We consider problems of short-time dynamics of a polyelectronic atomic nonstationary state, $V$, assumed to be formed as a wave packet at $t=0$. We focus on two cases, for which the role of the quasicontinuum of the upper part of the Rydberg states, with which the $V$ state has nonzero coupling matrix elements, is investigated. In the first case, the position of the $V$ state is just above the ionization threshold, $E=0$, and so $V$ dissipates into the free electron continuum as an autoionizing state. The question is how the presence of the Rydberg series converging to $E=0$ affects the time evolution of the autoionizing $V$. In the second case, the position of $V$ is embedded in the quasicontinuum of the Rydberg series below threshold. The question is whether there are distinct features in the time evolution of this $V$, although its position is in the discrete part of the energy spectrum. In this case, by focusing on short times and by evaluating analytically certain infinite sums, analogous to Fourier integrations, the following result is obtained: For small times, the $V$ state evolves as an exponentially decaying state. However, in addition to the term describing exponential decay, there is a term, entering with a small coefficient, which describes exponential growth and eventually dominates. It is shown that exponential decay holds for times shorter than the time $t_p$ needed by the wave packet to reach the outer classical turning point. For the decay to be physically meaningful, this time must be smaller than the time $t_d$ which equals the inverse of the half-width in atomic units. We examined a model system of $V$-Rydberg state interaction based on the Boron ${}^{2}S$ spectrum. The results indicate that the effect is observable on the scale of femtoseconds.

We present first steps toward understanding the ultracold scattering properties of polar molecules in strong electric field-seeking states. We have found that the elastic cross section displays a quasiregular set of potential resonances as a function of the electric field, which potentially offers intimate details about the intermolecular interaction. We illustrate these resonances in a “toy” model composed of pure dipoles, and in more physically realistic systems. To analyze these resonances, we use a simple WKB approximation to the eigenphase, which proves both reasonably accurate and meaningful. A general treatment of the Stark effect and dipolar interactions is also presented.

We propose a scheme to generate four Bell states and a four-atom entangled cluster state in a thermal cavity. The photon-number-dependent parts in the effective Hamiltonian are canceled with the assistance of a strong classical field. The cavity field is only virtually excited; no quantum information will be transferred from the atoms to the cavity and thus the scheme is insensitive to the cavity decay and the thermal field. The scheme can also be used to generate the cluster state for the trapped ions.

Recently, there have been several suggestions that weak Kerr nonlinearity can be used for generation of macroscopic superpositions and entanglement and for linear optics quantum computation. However, it is not immediately clear that this approach can overcome decoherence effects. Our numerical study shows that nonlinearity of weak strength could be useful for macroscopic entanglement generation and quantum gate operations in the presence of decoherence. We suggest specific values for real experiments based on our analysis. Our discussion shows that the generation of macroscopic entanglement using this approach is within the reach of current technology.

Experimental total cross sections for direct electron-impact ionization of the valence electron of several Li-like ions are seen to follow a new ab initio scaling law which is inspired by a Coulomb-Born model and the frozen-core Hartree-Fock approximation. The predictive character of this scaling law should be very useful to experimentalists and can be used to complete data tables needed for plasma or astrophysical studies. A single-parameter fit of the best available experimental data, once scaled, provides us with a single formula, for moderately charged Li-like ions, which is more accurate than Lotz semiempirical formula.

In this Comment we deal with a recent study of Ying Wu et al. [Phys. Rev. A 67, 013811 (2003)] on a four-wave mixing process. We show that the authors’ claim, that a large enhancement of the generation efficiency can be obtained by suppression of photon absorption from electromagnetically induced transparency, is not correct.

We study Bragg scattering at one-dimensional (1D) optical lattices. Cold atoms are confined by the optical dipole force at the antinodes of a standing wave generated inside a laser-driven high-finesse cavity. The atoms arrange themselves into a chain of pancake-shaped layers located at the antinodes of the standing wave. Laser light incident on this chain is partially Bragg reflected. We observe an angular dependence of this Bragg reflection which is different from what is known from crystalline solids. In solids, the scattering layers can be taken to be infinitely spread (three-dimensional limit). This is not generally true for an optical lattice consistent of a 1D linear chain of pointlike scattering sites. By an explicit structure factor calculation, we derive a generalized Bragg condition, which is valid in the intermediate regime. This enables us to determine the aspect ratio of the atomic lattice from the angular dependance of the Bragg scattered light.

We study the dynamic response of ultracold bosons trapped in one-dimensional optical lattices using Quantum Monte Carlo simulations of the boson Hubbard model with a confining potential. The dynamic structure factor reveals the inhomogeneous nature of the low temperature state, which contains coexisting Mott insulator and superfluid regions. We present new evidence for local quantum criticality and discuss implications for the experimental excitation spectrum of ${}^{87}\mathrm{Rb}$ atoms confined in one dimension.

We present the probability-preserving description of the decaying particle within the framework of quantum mechanics of open systems, taking into account the superselection rule prohibiting the superposition of the particle and vacuum. In our approach the evolution of the system is given by a family of completely positive trace-preserving maps forming a one-parameter dynamical semigroup. We give the Kraus representation for the general evolution of such systems, which allows one to write the evolution for systems with two or more particles. Moreover, we show that the decay of the particle can be regarded as a Markov process by finding explicitly the master equation in the Lindblad form. We also show that there are remarkable restrictions on the possible strength of decoherence.

Quantum secret sharing schemes encrypting a quantum state into a multipartite entangled state are treated. The lower bound on the dimension of each share given by Gottesman [Phys. Rev. A 61, 042311 (2000)] is revisited based on a relation between the reversibility of quantum operations and the Holevo information. We also propose a threshold ramp quantum secret sharing scheme and evaluate its coding efficiency.

Theoretical calculations and experimental crossed-beam measurements are compared for electron-impact single ionization of ${\mathrm{Pb}}^{q+}$ ions for $q=1–10$. We compare with two main theoretical methods. First, we check against configuration-average distorted-wave calculations, which include both direct-ionization and indirect excitation-autoionization contributions. Second, for ion stages ${\mathrm{Pb}}^{+}$ through to ${\mathrm{Pb}}^{5+}$, we calculate the dominant excitation-autoionization channels using level-resolved distorted-wave theory to evaluate the excitation cross sections. We find that for ion stages ${\mathrm{Pb}}^{+}$, ${\mathrm{Pb}}^{2+}$, and ${\mathrm{Pb}}^{3+}$, distorted-wave theory significantly overestimates the total-ionization cross section, due to an overestimation of the direct-ionization cross section from the $5d$ subshell. For ion stages ${\mathrm{Pb}}^{4+}$ through to ${\mathrm{Pb}}^{10+}$ there is good agreement between theory and experiment. We find evidence for significant metastable fraction in the ion beam of the experiment for ion stages ${\mathrm{Pb}}^{2+}$, ${\mathrm{Pb}}^{3+}$, ${\mathrm{Pb}}^{4+}$, ${\mathrm{Pb}}^{5+}$, and ${\mathrm{Pb}}^{6+}$. For ion stage ${\mathrm{Pb}}^{3+}$ we find that the level-resolved distorted-wave calculation of the excitation autoionization results in a slight reduction of the configuration-average theoretical results, due to splitting of levels within the autoionizing configurations. We also investigate two semiempirical methods of calculating the direct-ionization cross sections: namely, the Lotz method and the binary encounter Bethe method. We find that both methods provide results which are significantly lower than the distorted-wave method for the $5d$-subshell direct ionization of ${\mathrm{Pb}}^{+}$, ${\mathrm{Pb}}^{2+}$, and ${\mathrm{Pb}}^{3+}$. For the higher ion stages, both methods are lower than the distorted-wave direct-ionization cross-section results, trending towards the distorted-wave results as the ion stage increases.

The scattering amplitude for positron-hydrogen collisions has been conveniently derived within the framework of distorted-wave theory by approximating the distortion potential in a particular channel as the average of perturbation of that particular channel over the bound states. A real advantage of this method is that a few straightforward calculations yield the expression of scattering amplitude as a function of partial wave. The method has been successfully applied to study $1s\text{−}2s$ excitation of hydrogen atom by positron impact and Ps$\left(ns\right)$ ($n=1$,2,3) formation in positron-hydrogen collisions in the intermediate and high energy range. Further to bring the scattering amplitudes in tracktable forms we have formulated an effective and efficient method for evaluating general three-denominator Lewis integral. The results compare nicely with other theoretical and experimental results available in the literature.