We show that a $\mathsf{V}$-type three-level system with additional coherence due to a static magnetic field coupling the two excited levels displays peculiar interference effects which can be controlled through the magnetic-field strength. In particular, emission suppression and trapping of the population in a nondecaying state can be obtained.

We give a complete analysis of covariant measurements on two spins. We consider the cases of two parallel and two antiparallel spins, and we consider both collective measurements on the two spins and measurements that require only local quantum operations and classical communication (LOCC). In all cases we obtain the optimal measurements for arbitrary fidelities. In particular, we show that if the aim is to determine as accurately as possible the direction in which the spins are pointing, it is best to carry out measurements on antiparallel spins (as already shown by Gisin and Popescu), second best to carry out measurements on parallel spins, and worst to be restricted to LOCC measurements. If the aim is to determine as accurately as possible a direction orthogonal to that in which the spins are pointing, it is best to carry out measurements on parallel spins, whereas measurements on antiparallel spins and LOCC measurements are both less suitable but equivalent.

Calculation of cross sections for low-energy electron scattering by atoms and molecules requires accurate treatment of multipole polarization response, an aspect of bound-free electronic correlation. A formally exact theory is developed that extends the static-exchange model of electron scattering to include electronic correlation. This formalism is valid both for short-range Coulomb-cusp correlation and for long-range multipole polarization response.

We propose a scheme for the generation and reconstruction of entangled states between the internal and external (motional) degrees of freedom of a trapped electron. Such states also exhibit quantum coherence at a mesoscopic level.

We study low-energy collective excitations in a trapped superfluid Fermi gas. These excitations originate from slow variations of the phase of the superfluid order parameter and, in this sense, are similar to the Bogolyubov sound in a homogeneous superfluid Fermi gas. Well below the critical temperature the eigenfrequencies of the lowest collective excitations are of the order of the trap frequency, and these modes manifest themselves in the density oscillations under periodic modulations of the trap frequencies. This gives a clear signature of the presence of a superfluid phase.

Semiclassical periodic orbit theory is used in many branches of physics. However, most applications of the theory have been to systems that involve only single-particle dynamics. In this paper, we develop a semiclassical formalism to describe the density-of-states for two noninteracting particles. This includes accounting properly for particle exchange symmetry. As specific examples, we study two identical particles in a disk and in a cardioid. In each case, we demonstrate that the semiclassical formalism correctly reproduces the quantum densities.

We use the Barnett-Pegg formalism of angle operators to study a rotating particle with and without a flux line. Requiring a finite-dimensional version of the Wigner function to be well defined, we find a natural time quantization that leads to classical maps from which the arithmetical basis of quantum revivals is seen. The flux line, which fundamentally alters the quantum statistics, forces this time quantum to be increased by a factor of a winding number and determines the homotopy class of the path. The value of the flux is restricted to the rational numbers, a feature that persists in the infinite-dimensional limit.

In this paper, we study theoretically the process of electron capture between one-optical-electron atoms (e.g., hydrogenlike or alkali atoms) and ions at low-to-medium impact velocities ${(v/v}_e\approx 1)$ working on a modification of an already developed classical overbarrier model [V. Ostrovsky, J. Phys. B 28, 3901 (1995)], which allows us to give a semianalytical formula for the cross sections. The model is discussed and then applied to a number of test cases, including experimental data as well as data coming from other sophisticated numerical simulations. It is found that the accuracy of the model, with the suggested corrections and applied to quite different situations, is rather high. Furthermore, even ionization can be computed within the same framework.

Interelectronic-interaction correction of first order in $1/Z$ is studied for radiative recombination of an electron with a heavy He-like ion in the ground state. A rigorous relativistic treatment is compared with two frequently applied approximations. A calculation is carried out for radiative recombination of an electron into $2s,$ ${2p}_{1/2},$ and ${2p}_{3/2}$ states of He-like uranium for impact energies between 10 and 700 MeV/u.

We present an efficient algorithm able to predict the trajectories of individual cluster constituents as they penetrate relatively thick amorphous targets. Our algorithm properly treats both the intracluster Coulomb repulsion and the collisions between cluster constituents and target atoms. We have compared our simulation predictions to experimental measurements of the distribution of lateral exit velocities, and demonstrated that the in-target Coulomb explosion of 2MeV/atom carbon clusters in carbon foils must be shielded with a screening length of less than 2.5 Å. We also present a simple phenomenological model for the suppression of the exit-side charge of ions in clusters which depends on the enhanced ionization potential that an electron near an ion feels due to the ion’s charged comoving neighbors. By using our simulation algorithm we have predicted the exit correlations of the cluster constituents and verified that the charge suppression model fits the observed charge suppression of ions in clusters to within the experimental uncertainties.

We present data on the photoabsorption spectrum of lithium atoms in an electric field at energies between the saddle point of the Stark potential and below the field-free ionization threshold. The spectrum displays a sequence of sharp resonances and a sequence of broad ones. We find that the broad resonances arise from the classically forbidden reflection of waves above a dynamical potential barrier. The recurrence spectrum is also observed and it is dramatically affected by above-barrier reflections. We have developed a semiclassical theory that interprets the spectra using quasiclassical trajectories that undergo above-barrier reflection.

We report five cases of integrability for a hydrogenic atom under three static external fields: a magnetic field, an electric field, and a van der Waals interaction. Exact integrals of motion and corresponding quantum operators are obtained explicitly for each case. Integrals of motion (quantum operators) can be expressed as components of a suitably generalized Runge-Lenz vector (operator). Quadratic quantum operators are found to have the amazing property of requiring a nonclassical extra term proportional to ${ħ}^2.$ The structuring of the classical phase space is investigated numerically via Poincaré surfaces of section and corroborates the analytical results.

A theoretical model of soft x-ray lasers in hot laser-produced plasmas using a Maxwell-Bloch formalism has been developed and implemented in a numerical code (COLAX). First simulation results in the case of Ne-like Zn from solid targets are presented, including spontaneous emission generation and amplification, refraction, saturation, and coherence properties. A detailed simulation is presented of the effect on amplification and coherence of double-passing the radiation in the amplifying medium by using a half-cavity mirror.

Entangled-photon pairs (biphotons) generated by spontaneous optical parametric down-conversion exhibit a number of properties that are analogous to those of ordinary photons generated by incoherent sources. The spatial pump-field distribution and the two-particle wave function in the biphoton case play the respective roles of the source intensity distribution and the second-order coherence function in the incoherent case. The van Cittert–Zernike theorem, which is applicable for incoherent optical sources emitting independent photons, has a counterpart for biphotons. Likewise, the partial-coherence theory of image formation has an analogous counterpart for biphoton beams transporting spatial information. However, an underlying duality, rather than analogy, accompanies the mathematical similarity between incoherent and biphoton emissions if the comparison is made between the photon count rate in the incoherent case and the biphoton count rate in the entangled-photon case. The smaller the size of an incoherent source, the more separable is the coherence function and the more coherent is the field, and therefore the higher the visibility of ordinary interference fringes. In contrast, the narrower the size of a biphoton pump source, the more separable is the wave function and the less entangled is the field, and therefore the lower the visibility of biphoton interference fringes. This duality is similar to the complementarity between single and two-photon interference exhibited for biphotons.

At very high radiation field intensities in vacuum, Maxwell equations need to be modified to account for the QED photon-photon interaction. We show that such modified equations support nondiffracting spatial radiation solitons that propagate for very long distances without changing their shapes. These solitons, and the underlying self-focusing and instability effects, should be observable in the near future.

The properties of dipole-allowed and -forbidden (quadrupole) transitions in an atom placed near an ideally conducting cylinder are considered. Explicit analytical expressions for transition rates for different orientations of dipole and quadrupole are found. It is shown that the decay rates of dipole and quadrupole transitions with radially oriented moments tend to infinity when the cylinder radius tends to zero. On the other hand, for tangential orientation (φ orientation), the dipole transition rate decreases while the quadrupole transition rate increases substantially. As a result, the quadrupole decay rates may approach the dipole decay rates. Such behavior has analogy in neither spherical nor plane geometry of the metal interface.

The mediated photon-photon interaction due to the resonant Kerr nonlinearity in an inhomogeneously broadened atomic vapor is considered. The time scale for photon-photon scattering is found to be determined by the inhomogeneous broadening and the magnitude of the momentum transfer. This time can be shorter than the atomic relaxation time. Effects of atom statistics are included and the special case of small-angle scattering is considered. In the latter case, the time scale of the nonlinear response remains fast, even though the linear response slows as the inverse of the momentum transfer. Measurements of temporal and angular correlations show good agreement with theory.

A ${\chi }^{\mathrm{}\left(2\right)\mathrm{}}$ nonlinear optical crystal is pumped by two temporally well-distinguishable femtosecond laser pulses to generate entangled photon pairs in the process of spontaneous parametric down-conversion. We have observed first- and second-order interference between amplitudes generated from the first and the second pump pulse as a function of the time delay between the two pump pulses. The criteria for first- and second-order interference are found to be very different, which reflect the quantum entanglement nature of the state of spontaneous parametric down-conversion.

We have measured quasi-instantaneous transverse patterns in a broad aperture laser. Nonordered patterns yielding boundary-determined regular structures in progressive time-integrated recording are observed. The linear analysis and numerical integration of the full Maxwell-Bloch equations allow us to interpret the features of the experiment. We show the limitations imposed by the boundary conditions to the theoretical predictions of the linear stability analysis.

The cloning of conjugate continuous quantum variables is analyzed based on the concept of Gaussian cloning machines, i.e., transformations that yield copies that are Gaussian mixtures centered on the state to be copied. The optimality of Gaussian cloning machines that transform N identical input states into M output states is investigated, and bounds on the fidelity of the process are derived via a connection with quantum estimation theory. In particular, the optimal N-to-$M$ cloning fidelity for coherent states is found to be equal to $MN/(MN+M-N).\mathrm{}$