We consider strong-field index enhancement in three-level systems in which there is a strong spontaneous decay from an auxiliary level into either of two states that the probe field couples. A control field is detuned resonant with the transition between the auxiliary level and one of the dressed states produced by the strong probe field. It is shown that it is possible to achieve maximal atomic coherence, which characterizes an ultralarge index of refraction and vanishing absorption. This scheme is based on selective dressed population trapping, which is established by relying on the control field and the strong decay to transfer population from one dressed state to another through the auxiliary level. An advantage of the present scheme is that the conditions for its realization are accessible experimentally.

The proposal of a far-infrared laser employing intraband transitions in the system of quantum dots is presented. The model for electrically defined Gaussian quantum dots in a narrow Ga(Al)As quantum well is examined and the conditions for inversion of population of the dot matrix are discussed. The laser is planned to be pumped by periodically repeated rapid creation and destruction of the quantum dot matrix allowing for repeated filling of the dot levels with electrons from a quantum well. The kinetics of the electron-photon system is analyzed in order to determine the characteristics of the pumping signal and other laser parameters.

We present a feasible scheme to implement the nondeterministic quantum logic operation of Knill, Laflamme, and Milburn [Nature 409, 46 (2001)] by a teleportation technique. An entangled photon channel is generated by emitting two single photons into an asymmetric beam splitter. In order to generate the classical channel signal, two-photon detectors are needed.

This is a comment on Stahlhofen’s paper [Phys. Rev. A 62, 012112 (2000)]. It is shown by stationary-phase theory that the Goos-Hänchen shift in frustrated total internal reflection (FTIR) is not independent of the group delay (or phase time in the literature). The group delay involves the contribution of Goos-Hänchen shift and is always larger than zero in FTIR. It is also shown that the group delay in the two-dimensional (2D) optical FTIR can be written in the same form as that of the group delay in the 1D quantum tunneling in the sense that the group delay is the derivative of the total phase shift with respect to the angular frequency.

The photonic tunneling time in frustrated total internal reflection is determined by the Goos-Hänchen shift. This fact, derived in the preceding Comment [Phys. Rev. A 65, 066101 (2002)] using a different strategy, suffices to reproduce all experimental data available so far.

We present a phase-space description of the process of quantum teleportation for a system with an N-dimensional space of states. For this purpose we define a discrete Wigner function which is a minor variation of previously existing ones. This function is useful to represent a composite quantum system in phase space and to analyze situations where entanglement between subsystems is relevant (dimensionality of the space of states of each subsystem is arbitrary). We also describe how a direct tomographic measurement of this Wigner function can be performed.

The scattering of orthopositronium off atomic lithium (Li) target has been investigated using the close-coupling approximation (CCA) with different basis sets. The predictions of different projectile-elastic and target-elastic CCA models have been presented along with static-exchange results. We report the scattering length, the effective range, and the zero-energy cross section for singlet and triplet scattering. The spin averaged zero-energy cross section is significantly improved over the static-exchange prediction by the inclusion of the virtual Ps excitation channels in the basis set. All the present models indicate a binding of lithium-positride (LiPs) in the attractive singlet channel. The low-energy (below first Li excitation threshold) and low order (up to $l=2)$ phase shifts are also reported and are found to be variationally consistent. Except the s-wave singlet scattering, other phase shifts are influenced more by the target distortion effect than the projectile inelastic effect. Estimates of the different single-atom excitation cross sections have also been reported along with an estimate of the total cross section. The peak in the total cross-section curve for the present system is not prominent at energies around the Ps inelastic threshold, unlike the Ps-Na case. For incident energy 20 eV or more the Ps ionization is the dominant contributor to the total cross section.

Recently, the method of Lu-Fano plots was extended to potentials with general $R^{-n}$ asymptotic behavior. The aim of this paper is to discuss the efficiency of this method for the interpretation of experimental vibrational spectra of long-range diatomic molecules when two channels are coupled. A Lu-Fano analysis is implemented on experimental data for the ${\mathrm{Cs}}_2{0}_u^{+}{(6S+6P}_{1/2,3/2})$ photoassociation spectra below the ${6S+P}_{1/2}$ and ${6S+P}_{3/2}$ dissociation limits. The parameters are fitted on level energies and predissociation widths, giving information about the asymptotic coefficients $C_3$ for both channels and about a coupling parameter which determines the variation of predissociation widths as a function of detuning. This parameter can be extrapolated above the ${6S+P}_{3/2}$ threshold to yield the fine-structure transition cross section between the two channels. The probability for such a transition in a model where hyperfine structure effects are neglected is found to be 0.54.

Electron-impact differential cross-section measurements for the excitation of the ${2p}^{5}3s$ configuration of Ne are reported. The Ne cross sections are obtained using experimental differential cross sections for the electron-impact excitation of the $n=2\mathrm{}$ levels of atomic hydrogen [Khakoo et al., Phys. Rev. A 61, 012701-1 (1999)], and existing experimental helium differential cross-section measurements, as calibration standards. These calibration measurements were made using the method of gas mixtures (Ne and H followed by Ne and He), in which the gas beam profiles of the mixed gases are found to be the same within our experimental errors. We also present results from calculations of these differential cross sections using the R-matrix and unitarized first-order many-body theory, the distorted-wave Born approximation, and relativistic distorted-wave methods. Comparison with available experimental differential cross sections and differential cross-section ratios is also presented.

The theory of vortex motion in a dilute superfluid of inhomogeneous density demands a boundary layer approach, in which different approximation schemes are employed close to and far from the vortex, and their results matched smoothly together. The most difficult part of this procedure is the hydrodynamic problem of the velocity field, many healing lengths away from the vortex core. This paper derives and exploits an exact solution of this problem in the two-dimensional case of a linear trapping potential, which is an idealization of the surface region of a large condensate. It thereby shows that vortices in inhomogeneous clouds are effectively “dressed” by a nontrivial distortion of their flow fields, that image vortices are not relevant to Thomas-Fermi surfaces, and that for condensates large compared to their surface depths, the energetic barrier to vortex penetration disappears at the Landau critical velocity for surface modes.

We address the use of entanglement to improve the precision of generalized quantum interferometry, i.e., of binary measurements aimed to determine whether or not a perturbation has been applied by a given device. For the most relevant operations in quantum optics, we evaluate the optimal detection strategy and the ultimate bounds to the minimum detectable perturbation. Our results indicate that entanglement-assisted strategies improve the discrimination in comparison with conventional schemes. Possible implementations of entanglement-assisted schemes, in order to approach the performances of the optimal strategies, are also suggested.

We introduce a Werner-like mixture [R. F. Werner, Phys. Rev. A 40, 4277 (1989)] by considering two correlated but different degrees of freedom, one with discrete variables and the other with continuous variables. We evaluate the mixedness of this state and its degree of entanglement, establishing its usefulness for quantum information processing such as quantum teleportation. Then, we provide its tomographic characterization. Finally, we show how such a mixture can be generated and measured in a trapped system like one electron in a Penning trap.

By performing radiation-damped R-matrix scattering calculations for the photorecombination of ${\mathrm{Fe}}^{17+}$ forming ${\mathrm{Fe}}^{16+},$ we demonstrate and discuss the difficulties and fundamental inaccuracies associated with the R-matrix method for treating dielectronic recombination (DR). Our R-matrix results significantly improve upon earlier R-matrix results for this ion. However, we show theoretically that all R-matrix methods are unable to account accurately for the phenomenon of radiative decay followed by autoionization. For ${\mathrm{Fe}}^{17+},$ we demonstrate numerically that this results in an overestimate of the DR cross section at the series limit, which tends to our analytically predicted amount of $40\%.$ We further comment on the need for fine resonance resolution and the inclusion of radiation damping effects. Overall, slightly better agreement with experiment is still found with the results of perturbative calculations, which are computationally more efficient than R-matrix calculations by more than two orders of magnitude.

The multiply differential cross section of the dissociative ionization of ${\mathrm{H}}_2^{+}$ by fast (2 keV) electron impact is determined theoretically using a two-center continuum wave function constructed using the exact solutions of the separable two-center Schrödinger equation in prolate spheroidal coordinates for the slow (50 eV) ejected electron. The comparison of the results to those obtained by approximate approaches, like those using an effective center Coulomb wave or Pluvinage-type two-center wave or the wave packet evolution approach, shows the limits of these approximate methods. Additionally, the variation of the sevenfold differential cross section, which corresponds to the ionization of aligned molecules, reveals interesting mechanisms and gives some insight into the optimal conditions for the dissociative ionization of more complex diatomic targets.

Antihydrogen-formation cross sections in antiproton-positronium collisions are calculated by using a time-dependent coupled-channel (TDCC) method. Nonuniform grid points are taken in the two-dimensional radial space to obtain accurate wave functions. High numerical accuracy of the TDCC method is demonstrated in a calculation of positronium-formation cross sections in positron-hydrogen collisions. The result is somewhat discrepant with previous results; in the present calculation, the cross section has a maximum of 16.19 in units of $\pi a_0^2=0.880\mathrm{}\times {10}^{-16}\hspace{0.5em}{\mathrm{cm}}^2$ at an antiproton energy of 9.2 eV in the center-of-mass frame.

Constructive techniques for preparing a specific unitary generator from a coupled two-spin system, via bounded amplitude radio frequency selective single spin pulses, are presented. The frequency of any pulse in the sequence is one of the two Larmor frequencies, while the phase takes one of two values. The fact that the amplitude is bounded implies that the Larmor frequency separation between the two spins need not be large to maintain selectivity. In addition, the contribution of the J coupling term to single spin evolution is not neglected, but instead plays a crucial role. The procedure is based on a certain decomposition of SU(4) available in the literature. A method for determining the parameters entering this factorization, in terms of the entries of the target unitary generator, is also provided.

We present a magnetic trapping scheme for neutral atoms based on a hybrid of Ioffe-Pritchard and time-averaged orbiting potential traps. The resulting double-well magnetic potential has readily controllable barrier height and well separation. This offers a new tool for studying the behavior of Bose condensates in double-well potentials, and in particular for atom optics and interferometry. We formulate a description for the potential of this magnetic trap and discuss practical issues such as loading with atoms, evaporative cooling and manipulating the potential.