We discuss a point model for the collective emission of light from $N$ two-level atoms in a photonic band-gap material, each with an atomic resonant frequency near the edge of the gap. In the limit of a low initial occupation of the excited atomic state, our system is shown to possess atomic spectra and population statistics that are radically different from free space. For a high initial excited-state population, mean-field theory suggests a fractionalized inversion and a macroscopic polarization for the atoms in the steady state, both of which can be controlled by an external dc field. This atomic steady state is accompanied by a nonzero expectation value of the electric field operators for field modes located in the vicinity of the atoms. The nature of homogeneous broadening near the band edge is shown to differ markedly from that in free space due to non-Markovian memory effects in the radiation dynamics. Non-Markovian vacuum fluctuations are shown to yield a partially coherent steady-state polarization with a random phase. In contrast with the steady state of a conventional laser, near a photonic band edge this coherence occurs as a consequence of photon localization in the absence of a conventional cavity mode. We also introduce a classical stochastic function with the same temporal correlations as the electromagnetic reservoir, in order to stochastically simulate the effects of vacuum fluctuations near a photonic band edge.

Fidelity has always been an important concept in quantum optics. Recently, it was found that fidelity can also play a key role in quantum information and communication theory. Fidelity can be interpreted as the probability that a decoded message possesses the same information content as the message prior to coding and transmission. In this paper, we give a formula of Bures fidelity for displaced squeezed thermal states directly by the displacement and squeezing parameters and briefly discuss how the results can apply to quantum information theory.

We report, theoretically and experimentally, how polarization fluctuations in vertical-cavity semiconductor lasers are affected by optical anisotropies. We develop a spin-eliminated (class A) description of laser polarization and show how the various model parameters can be extracted from the experimental data. In practice, the linear anisotropies are often much stronger than the nonlinear anisotropies, so that the polarization modes defined by the linear anisotropies form a useful basis. For this case we derive a one-dimensional model for polarization noise, with simple expressions for the relative strength of the polarization fluctuations and the rate of polarization switches. For the other, more extreme, case where the nonlinear anisotropies are as strong (or even stronger) than the linear anisotropies, the spin-eliminated description remains valid. However, in this case the concept of polarization modes is shown to lose its meaning, as a strong four-wave-mixing peak appears in the optical spectrum and polarization fluctuations become highly nonuniform.

Interaction-free measurement is discussed on a three-state basis, including the unitary, nondetected phase of absorption. The interaction-free output can be regarded as part of the forward-scattered wave accompanying absorption. For a perfect absorber the forward-scattered wave is locked both in amplitude and in phase. For an imperfect one it has a nontrivial phase of dynamical origin, measurable by interferometry.

A complex spectral decomposition is developed for the survival probability and spectral density of a simple metastable state in an open quantum system. The time interval before exponential decay occurs is shown to be dominated by the short-lived quasibound states of the system.

The internal wave functions of the eight lowest intrashell triply excited states of Li atoms are analyzed. By visualizing the equidensity surfaces we show that in four of the eight states the most probable shape of the three electrons is a coplanar equilateral triangle with the nucleus at the center. The relative energies of these four states resemble the rotational spectrum of a symmetric top. The remaining four states have nodal surfaces in their internal wave functions and thus have higher excitation energies.

Reciprocity relations are derived between time-independent transition probabilities for absorption and emission of photons within a system governed by a time-dependent Hamiltonian. The proof combines the use of absorbing boundary conditions to express the probabilities and the ${(t,t}^{\prime })$ formalism. The case of a particle incident on an asymmetric double-barrier potential and interacting with an oscillatory field is given for illustration.

A quantum-defect theory of atomic collisions and molecular vibration spectra is presented. Based on the exact solutions of the Schrödinger equation for an attractive ${1/r}^6$ potential, the theory provides a systematic interpretation of molecular bound states and atom-atom scattering properties and establishes the relationship between the two. Applications to systems including ${}^7{\mathrm{Li}}_2$ and ${}^{23}{\mathrm{Na}}_2$ are discussed.

Faddeev equations for Lorentz invariant T operators for a relativistic system of two and three particles are considered. The Faddeev equations involve the particle creation-annihilation interaction in the invariant mass of the system, and effective two- and three-body interactions derived from this interaction. Correction terms to conventional two- and three-body T operators are given.

Correlated photoionization processes in ${\mathrm{H}}^{-}$ are calculated by a numerical solution of the time-dependent Schrödinger equation on a partitioned lattice. Projection of the time evolved wave function onto lattice eigenstates for H yields cross sections for photoionization with excitation and double photoionization. The lattice results confirm the surprisingly large double to single photoionization ratios for ${\mathrm{H}}^{-}$ found in the recent R-matrix calculations of Meyer, Greene, and Esry, Phys. Rev. Lett. 78, 4902 (1997).

A theoretical study of the static dipole polarizability of carbon clusters ${\mathrm{C}}_n$ with $n<~8$ has been presented. The dipole polarizabilities have been calculated using density functionals of the hybrid type in combination with the finite field method. Large basis sets have been investigated in order to obtain reliable results. It has been shown that the dipole polarizabilities could be an important quantity for the identification of clusters with different numbers of atoms and even for the separation of isomers. In particular, it is predicted that the jet formed by the two isomers of ${\mathrm{C}}_6,$ cyclic and linear, will split up in the presence of an electric field.

A universal mechanism destroying coherence in a $\Lambda$ system driven by two resonant laser fields due to four-photon interactions is analyzed theoretically. It is shown that this mechanism gives rise to novel spectral structures in resonance fluorescence. The “dark resonance” in absorption (dispersion) spectra is affected as well.

We present analytical results for the steady-state resonance fluorescence spectrum produced by a driven two-level atom excited under two strong coherent fields in the presence of a broadband squeezed vacuum. Here the central frequency of the squeezed vacuum is considered to be resonant as well as off-resonant with the average driving field frequency while maintaining resonance between the atomic transition frequency and the average driving field frequency. We observe a ringing phenomenon in the fluorescence spectrum due to the small frequency difference in the bichromatic field and this phenomenon is found to be sensitive to the detuning of the carrier frequency of squeezed vacuum with respect to the central frequency of the bichromatic field.

It is argued here that in a large class of lasers there exist mechanisms by which large coherent fields could be generated, and hence there seems to be no compelling reason to regard optical coherence as a “fiction.” Some brief comments on the use of a “pointer basis” to resolve statistical mixtures are also presented.

The ambiguity of representing the density matrix of laser light by coherent states or by the number states is “inherited” from a similar ambiguity in the description of lasing mechanisms. Atomic excitation by electron impact is analyzed, and it is concluded that in normal discharge lasers, the lasing atoms become energetically entangled with the electrons and no mean dipoles are excited.

The study of the $5p$ photoemission from laser polarized (aligned or oriented) Eu atoms was used to deduce the ratio of the magnitude of the electric dipole amplitudes and the phase difference between the two outgoing $\epsilon s$ and $\epsilon d$ electron waves. The combination of magnetic dichroism experiments, the measurement of the difference spectra for two atomic polarizations, and phase tilt experiments, the observation of the modulation of the photoelectron signal on the angle η of linear laser polarization, is a very promising method for performing a “complete” photoionization experiment.

Using the random matrix description of open quantum chaotic systems we calculate in closed form the universal autocorrelation function and the probability distribution of the total photodissociation cross section in the regime of quantum chaos.

Target ionization in collisions of singly charged ${\mathrm{Ne}}^{+}$ ions with Ne has been investigated at projectile velocities from 0.25 to 0.55 a.u. using electron and recoil momentum imaging techniques. The momentum distributions of the ejected electrons were found to carry a distinct signature strongly suggesting that ionization is taking place by successive promotions through molecular orbitals. The observed recoil transverse momentum distributions are donut-shaped, indicating that single ionization is confined to a well-defined impact-parameter window.