From a microscopic set of equations which takes into account spontaneous emission into lasing mode, we derive a macroscopic quantum model of low-threshold semiconductor lasers that includes the parabolic band structure, Pauli blocking of the injection current, and the carrier distribution dependence on the temperature. This model confirms the predictions of lasing without inversion and inversionless intensity squeezing, that were previously made by Yamamoto and co-authors on the basis of a semiclassical two-level approach. In addition, our analysis demonstrates the existence of an optimum temperature value that minimizes the injection current necessary to obtain lasing and intensity squeezing.

The $3p\to 2s$ line-emission cross section for ${\mathrm{Fe}}^{23+}$ is calculated in the configuration-average distorted-wave approximation. Contributions from $2s\to 3p$ dielectronic recombination, from $2s\to 3p$ direct excitation, and from $2s\to 4l$ resonant excitation-autoionization are included. Agreement between the distorted-wave calculations and the recent electron-beam ion-trap experiments of Gu et al. [Astrophys. J. 518, 1002 (1999)] is very good.

Photodetachment from negative S ions has been studied in external electric fields up to 220 V/cm for laser polarization parallel to the electric field direction. Fast neutrals produced by a pulsed laser beam crossed with a 4 keV ${\mathrm{S}}^{-}$ ion beam were detected to measure the relative cross sections. Increased ion currents, lower background signals, analog detection methods, and more precise wavelength calibration have produced higher quality data than previous direct cross-section ratio measurements on nonhydrogenic ions. The ratio of the electric field-on to electric field-off cross sections is found to be in excellent agreement with theoretical predictions, thus resolving a long-standing discrepancy with previous, less definitive experiments. As predicted, electron wave-function rescattering effects are very small in both the amplitude and the phase of the electric-field-induced oscillations.

The quantum properties of the fluorescence light emitted by diamond nanocrystals containing a single nitrogen-vacancy (NV) colored center are investigated. We have observed photon antibunching with very low background light. This system is therefore a very good candidate for the production of single photon on demand. In addition, we have measured a larger NV center lifetime in nanocrystals than in the bulk, in good agreement with a simple quantum electrodynamical model.

We present a theoretical analysis of inequivalent classes of interference experiments with non-Abelian anyons using an idealized Mach-Zender-type interferometer. Because of the non-Abelian nature of the braid group action one has to distinguish the different possibilities in which the experiment can be repeated, which lead to different interference patterns. We show that each setup will, after repeated measurement, lead to a situation where the two-particle (or multiparticle) state gets locked into an eigenstate of some well-defined operator. Also, the probability to end up in such an eigenstate is calculated. Some representative examples are worked out in detail.

We show how to construct states for which a Greenberger-Horne-Zeilinger-type paradox occurs if each party measures either the position or momentum of his particle. The paradox can be ascribed to the anticommutation of certain translation operators in phase space. We then rephrase the paradox in terms of modular and binary variables. The origin of the paradox is then due to the fact that the associativity of addition of modular variables is true only for c numbers but does not hold for operators.

A procedure for bound-state QED is presented, based upon a covariant form of the time-evolution operator. In contrast to the standard S-matrix formalism, our procedure is applicable also to states that are quasidegenerate. All (quasi)singularities, which appear when an intermediate state is (quasi)degenerate with the initial state, are eliminated. Our procedure is closely related to many-body perturbation theory (MBPT) and may open up possibilities to combine QED and MBPT in a more systematic fashion. The procedure is applied to the fine structure of the heliumlike neon and argon ions, and good agreement is obtained with recent experimental data.

This work presents the x-ray extended range technique for measuring x-ray mass attenuation coefficients. This technique includes the use of multiple foil attenuators at each energy investigated, allowing independent tests of detector linearity and of the harmonic contributions to the monochromated synchrotron beam. Measurements over a wide energy range allow the uncertainty of local foil thickness to be minimized by the calibration of thin sample measurements to those of thick samples. The use of an extended criterion for sample thickness selection allows direct determination of dominant systematics, with an improvement of accuracies compared to previous measurements by up to factors of 20. Resulting accuracies for attenuation coefficients of copper (8.84 to 20 keV) are 0.27–0.5 %, with reproducibility of 0.02%. We also extract the imaginary component of the form factor from the data with the same accuracy. Results are compared to theoretical calculations near and away from the absorption edge. The accuracy challenges available theoretical calculations, and observed discrepancies of 10% between current theory and experiments can now be addressed.

A part of the two-loop self-energy correction, the so-called P term, is evaluated numerically for the $1s$ state to all orders in $Z\alpha .$ Our calculation, combined with the previous investigation [S. Mallampalli and J. Sapirstein, Phys. Rev. A 57, 1548 (1998)], yields the total answer for the two-loop self-energy correction in H-like uranium and bismuth. As a result, the major uncertainty is eliminated from the theoretical prediction for the Lamb shift in these systems. The total value of the ground-state Lamb shift in H-like uranium is found to be 463.95(50) eV.

Collision of the molecular hydrogen ion, ${\mathrm{H}}_2^{+}$ in the energy range 20–100 keV with a H target has been investigated. Production of one or two fast protons or hydrogen atoms formed from the projectile breakup was distinguished by use of a Si-barrier detector. Cross sections were determined by coincidence-counting techniques between the target ions (separated by time-of-flight analysis), electrons, and one or two fast product particles. Capture processes are shown to dominate at the lower energies whereas pure target ionization is demonstrated to be the most likely process at 90 keV and higher energies. Such data are of importance in the understanding of astrophysical and high-temperature laboratory plasmas.

The accuracy of two recently proposed approximate distribution functions for drifting linear ions [R. Baranowski and M. Thachuk, Phys. Rev. A 63, 032503 (2001) is tested at a number of levels of microscopic detail. Through formal mathematical manipulations, low-order expressions are derived for a number of physical observables, including the angular momentum dependence of the drift velocity, and parallel and perpendicular translational temperatures, as well as the dependence of the quadrupolar alignment parameter on the parallel and perpendicular velocity components. Finally, comparisons are made between the approximate forms and basis set expansions of the distribution function utilized in formal solutions of the Boltzmann equation, with the goal of suggesting general features of these basis sets that might help improve convergence.

Spin-exchange cross sections between a ${}^3{\mathrm{He}}^{+}$ ion and a Rb atom were calculated in a ${}^3{\mathrm{He}}^{+}$ impact-energy range from 0.01 to 10 keV/amu by the semiclassical close-coupling method based on the molecular-orbital expansion. The previous calculation in which only two states of the ${}^3{\mathrm{He}}^{+}$ $\left(1s{\hspace{0.5em}}^{2}S\right)+\mathrm{Rb}\left(5s{\hspace{0.5em}}^{2}S\right)$ system were taken into account overestimated the experimental result at 6.33 keV/amu by orders of magnitudes, whereas the present calculation, which allowed the target excitation and electron transfer, showed a rough agreement with the experimental result. In view of the application, the spin-exchange cross sections larger than ${10}^{-14}\hspace{0.5em}{\mathrm{cm}}^2,$ predicted at ${}^3{\mathrm{He}}^{+}$ impact energies lower than 0.3 keV/amu will hopefully provide a powerful tool for producing a highly polarized ${}^3\mathrm{He}$ ion beam.

Calculations of $1s$ electron inner-shell photoionization from the ${1s}^{2}2p{}^2{P}^o$ Li excited state were previously reported using the R-matrix code [Zhou et al., Phys. Rev. A 59, 462 (1999)]. These calculations included important resonances in the partial and total photoionization cross sections for incident photon energies (i) lower than 74 eV and due to the ${1snln}^{\prime }{l}^{\prime }{}^2{L}^e$ autoionizing states for $n=2\mathrm{}$ and 3 and $L=S,$ $P,$ and $D,$ and (ii) larger than 140 eV and due to the ${2ln}^{\prime }{l}^{\prime }{n}^{″}{l}^{″}{}^2{L}^e$ autoionizing states. The latter resonances corresponding to ${2ln}^{\prime }{l}^{\prime }{n}^{″}{l}^{″}$ were recently analyzed by Zhou et al. [Phys. Rev. A 62, 012707 (2000)], using an eigenphase derivative technique along with quantum defect theory. The present paper proposes an analysis of the former resonances in the 58–74 eV incident photon energy range. The determination of the resonant positions for each series converging to a $1s2l{}^{1,3}L$ or $1s3l{}^{1,3}L$ ionization threshold of Li is obtained using the same method as in Zhou et al. [Phys. Rev. A 62, 012707 (2000)]. The identifications are compared with theoretical results of Chung [Phys. Rev. A 57, 3518 (1998)] using the saddle-point method for energies below the ${\mathrm{Li}}^{+}1s2s{}^{1}S$ threshold. Agreement is excellent in most cases and some discrepancies are discussed.

This paper analyzes the result of a temporal coherent control experiment using trains of ultrashort laser pulses in the situation where the laser repetition period is smaller than the relaxation times of the system. We focus on the accumulative effects present in sequential two-photon absorption. It is shown that stimulated emission due to the accumulation of population in the excited state give an important contribution to the coherently controlled process. Excellent agreement is obtained in the comparison between experiments in rubidium vapor and theory, where resonant pulse propagation is also taken into account.

We extend the concept of dark state cooling to enclose the fields in a high-$Q$ resonator. Choosing a sufficiently large detuning between the atomic transition and the driving fields, spontaneous emission during the cooling process is largely suppressed and the energy dissipation occurs dominantly via cavity decay. For a ring cavity with two degenerate counterpropagating waves of opposite circular polarization, the final dark state is identical as in free space. However, we find a strong cavity mediated force with a large capture range as a precooling mechanism. In this scheme spontaneous emission is only important for the final jump to the dark state in the dynamics and the reabsorption problem is strongly reduced. In principle the scheme could also be applied to molecules with a suitable transition.

We demonstrate theoretically the use of genetic-learning algorithms to coherently control the dynamics of a Bose-Einstein condensate. We consider specifically the situation of a condensate in an optical lattice formed by two counterpropagating laser beams. The frequency detuning between the lasers acts as a control parameter that can be used to precisely manipulate the condensate even in the presence of a significant mean-field energy. We illustrate this procedure in the coherent acceleration of a condensate and in the preparation of a superposition of prescribed relative phase.

The response of a trapped Bose-Einstein condensed gas to a density perturbation generated by a two-photon Bragg pulse is investigated by solving the time-dependent Gross-Pitaevskii equation. We calculate the total momentum imparted to the condensate as a function of both the time duration of the pulse and the frequency difference of the two laser beams. The role of the dynamic response function in characterizing the time evolution of the system is pointed out, with special emphasis to the phonon regime. Numerical simulations are compared with the predictions of local-density approximation. The relevance of our results for the interpretation of current experiments is also discussed.

Multiphoton states constructed from photon pairs generated in the process of spontaneous parametric down-conversion possess frequency and space-time correlations that may carry undesired distinguishing information. It is shown that these correlations may be eliminated if certain conditions in the source configuration are satisfied. For the cases in which these conditions cannot be satisfied because of experimental constraints, it is shown that the correlations may be reduced through proper choices of crystal length and pump bandwidth. The advantage of such source engineering is that it yields much higher count rates, since no photon pairs are lost by predetection spectral filtering.

Understanding the saturation behavior of polarization-spectroscopy signals is a vital task for the development of this method as a versatile tool for quantitative detection of trace species. Recent progress in the theoretical treatment of the polarization-spectroscopy process offers the opportunity of studying its saturation behavior thoroughly. This theoretical treatment, referred to as direct numerical integration (DNI) calculations, is based on numerically demanding calculations; that is why we present a simple model that describes the curve shape of polarization spectroscopy power-dependence scans in both the saturated and the unsaturated regime. Polarization-spectroscopy-saturation curves in the copropagating beam geometry from the excitation of OH $A{}^2{\Sigma }^{+}–X{}^2\Pi \mathrm{}(0,0)\mathrm{}$ at the $Q_2\mathrm{}\left(8\right)\mathrm{}$ line in a low-pressure flame were compared to both results from the DNI calculations and to our proposed analytical equation. Our simple model provides excellent fits to polarization-spectroscopy-saturation curves for absorption lines dominated by homogeneous broadening and for narrow-bandwidth excitation sources. The model does not give a good agreement with experiment for lines dominated by inhomogeneous broadening. For this case an empirical equation is proposed and investigated. Our proposed model offers a starting point for a simplification of the underlying polarization-spectroscopy theory, the complexity of which has been a major obstacle to the further development of this theory.

We study the effects of phase fluctuations of a laser on the dynamics of a two-level atom. We assume that the atom is in a cavity and consider two cases in which the atom is either sandwiched between two parallel metal plates or placed inside a rectangular metallic waveguide. We use the well-known master equation and utilize the phase diffusion model for our analysis. We derive expressions for the dipolar and dissipative forces acting on the atom. We then show that the laser fluctuations produce an increase in the magnitude of the dissipative force and a reduction in the dipolar force. We illustrate that the atom-laser interaction gives rise to a potential, which binds the atom to the central part of the cavity. We also show that the atomic velocity produces significant changes on the potential and on the force acting on the atom. The combined effects of the velocity and laser fluctuations on the atomic motion are discussed.