The coherent-information concept is used to analyze a variety of simple quantum systems. Coherent information was calculated for the information decay in a two-level atom in the presence of an external resonant field, for the information exchange between two coupled two-level atoms, and for the information transfer from a two-level atom to another atom and to a photon field. The coherent information is shown to be equal to zero for all full-measurement procedures, but it completely retains its original value for quantum duplication. Transmission of information from one open subsystem to another one in the entire closed system is analyzed to learn quantum information about the forbidden atomic transition via a dipole active transition of the same atom. It is argued that coherent information can be used effectively to quantify the information channels in physical systems where quantum coherence plays an important role.

This paper deals with the production of orthodimuonium (OM) $({\mu }^{+}{\mu }^{-}$ atom in triplet state) in collisions of high-energy electrons with nuclei or atoms. This reaction was previously studied by Holvik and Olsen [Phys. Rev. D 35, 2124 (1987)] on the basis of a bremsstrahlung mechanism where OM is produced by only one virtual photon. In the present paper, we consider a competing three-photon mechanism where the production of OM results from the collision of a photon generated by the electron with two photons emitted by the nucleus. We derive the corresponding energy spectrum and production rate of OM and show that the three-photon mechanism is the dominant one for a heavy atom target.

Excitation processes in ${\mathrm{Li}}^{+}-\mathrm{N}\mathrm{e}$ collisions have been studied over a wide range of laboratory angles of $2°<~\theta <~92°,$ and at laboratory collision energies of $200<~E_{\mathrm{lab}}<~500\mathrm{eV},$ by means of differential energy-transfer measurements. One- and two-electron excitations for both charge-exchange reactions and direct excitations of the Ne atoms were observed at reduced laboratory angles of $E_{\mathrm{lab}}\theta >6\mathrm{}\mathrm{keV}\mathrm{}\mathrm{deg}.$ The electronic transitions take place through diabatic potential crossings at internuclear distances of $R<~0.57\AA .$ The excitation mechanisms near threshold have been investigated by referring to ab initio potentials, and the mechanisms of dominant one- and two-electron excitations could be well understood. Analysis of the theoretical potentials suggests that one-electron excitations are due to avoided-crossing interactions, while two-electron excitation occurring through two successive one-electron transitions can be interpreted by a combination of avoided-crossing and noncrossing interactions.

Fragmentation channels in collisions between fullerenes were studied as a function of collision energy (115–1400 eV center of mass) and scattering angle using a rotatable reflectron time-of-flight mass spectrometer. The products from fusion reactions and those from inelastically scattered (but not fused) projectile ions represent the two fragmentation channels in the collisions studied. The fragment size dependence is discussed as a function of collision energy and scattering angle. The results are in good qualitative agreement with theoretical models. A change in fragmentation behavior due to the onset of a finite-system analog of a phase transition in the reaction products can be seen in the mass and angular distributions of the fragment ions.

A trajectory approach is taken to the hydrodynamical treatment of collective excitations of a Bose-Einstein condensate in a harmonic trap. The excitations induced by linear deformations of the trap are shown to constitute a broad class of solutions that can be fully described by a simple nonlinear matrix equation. An exact closed-form expression is obtained for the solution describing the mode $\{n=0,$ $m=+2\mathrm{}\}$ in a cylindrically symmetric trap, and the calculated amplitude-dependent frequency shift shows good agreement with the experimental results of the JILA group.

We reinvestigate the dynamics of the grow and collapse of Bose-Einstein condensates in a system of trapped ultracold atoms with negative scattering lengths, and found a new behavior in the long time scale evolution: the number of atoms can go far beyond the static stability limit. The condensed state is described by the solution of the time-dependent nonlinear Schrödinger equation, in a model that includes atomic feeding and three-body dissipation. Our results for the model show that, by changing the feeding parameter and when a substantial depletion of the ground-state exists, a chaotic behavior is found. We consider a criterion proposed by Deissler and Kaneko [Phys. Lett. A 119, 397 (1987)] to diagnose spatiotemporal chaos.

The concept of time refraction is introduced to describe the effects of a sudden change of the optical properties of a dielectric medium. This can be seen as the most elementary process associated with photon acceleration and frequency upshifting. The quantum theory of such a process shows that the initial wave splits into time-transmitted and time-reflected waves propagating in opposite directions after the occurrence of a time discontinuity of the refractive index. The time refraction laws, analogous to the well known Fresnel formulas and Snell’s law, are also derived. It is shown that, in quantum terms, time refraction is equivalent to a squeezing transformation.

The theory of continuous measurement has been applied to photon counting by means of an atomic beam crossing a cavity and impinging on a field-ionization detector behind. An interesting feature of the proposed measurement consists in an incoherent frequency down-conversion coupling of the measured mode to another mode, which, on the assumption that the latter is initially in the vacuum state, results in a second registration of the counts.

We propose a scheme to generate a superposition of several coherent states with arbitrary coefficients on a line in phase space and directly measure the Wigner characteristic function for the motion of a trapped ion. Unlike previous schemes for motional quantum-state engineering and endoscopy, the present scheme works in the strong-excitation regime, which may be of experimental importance in view of decoherence.

We study the evaporative cooling of magnetically trapped atomic hydrogen on the basis of the kinetic theory of a Bose gas. The dynamics of trapped atoms is described by the coupled differential equations, considering both the evaporation and dipolar spin relaxation processes. The numerical time-evolution calculations quantitatively agree with the recent experiment of Bose-Einstein condensation with atomic hydrogen. It is demonstrated that the balance between evaporative cooling and heating due to dipolar relaxation limits the number of condensates to $9\times {10}^8$ and the corresponding condensate fraction to a small value of 4% as observed experimentally.

We analyze the multitime correlations of a laser-induced Bose-Einstein condensate. We use quantum stochastic methods to obtain, under certain circumstances, a Fokker-Planck equation that describes the phase-diffusion process and an analytical expression of the two-time correlations. We also perform quantum Monte Carlo numerical simulations of the correlations, which are in good agreement with the predicted analytical results.

We investigate the quantum Zeno effect in the case of electron tunneling out of a quantum dot in the presence of continuous monitoring by a detector. It is shown that the Schrödinger equation for the whole system can be reduced to Bloch-type rate equations describing the combined time development of the detector and the measured system. Using these equations we find that continuous measurement of the unstable system does not affect its exponential decay to a reservoir with a constant density of states. The width of the energy distribution of the tunneling electron, however, is not equal to the inverse lifetime—it increases due to the decoherence generated by the detector. We extend the analysis to the case of a reservoir described by an energy-dependent density of states, and we show that continuous measurement of such quantum systems affects both the exponential decay rate and the energy distribution. The decay does not always slow down, but might be accelerated. The energy distribution of the tunneling electron may reveal the lines invisible before the measurement.

We show that optimal universal cloning of the polarization state of photons can be achieved via stimulated emission in three-level systems, both of the Λ and the $V$ type. We establish the equivalence of our systems with coupled harmonic oscillators, which permits us to analyze the structure of the cloning transformations realized. These transformations are shown to be equivalent to the optimal cloning transformations for qubits discovered by Bužek and Hillery and Gisin and Massar. The down-conversion cloner discovered previously by some of the authors is obtained as a limiting case. We demonstrate an interesting equivalence between systems of Λ atoms and systems of pairwise entangled V atoms. Finally we discuss the physical differences between our photon cloners and the qubit cloners considered previously and prove that the bounds on the fidelity of the clones derived for qubits also apply in our situation.

Absolute cross sections are reported for the electron impact dissociative excitation of the deuterated ions ${\mathrm{ND}}_n^{+}$ $(n=2\mathrm{}-4)$ and ${\mathrm{OD}}_n^{+}$ $\mathrm{}(n=2,3\mathrm{})\mathrm{}$ from threshold up to 70 eV using a crossed beams technique. The method focuses on measuring light dissociation-product ions from a heavy target. For all studied targets, ${\mathrm{D}}^{+}$ formation is the dominant channel, with absolute cross section values in the range of $\left(1.3\mathrm{}-2\right)\times {10}^{-16}{\mathrm{cm}}^2,$ once the initial rise from the threshold is past, continuing to 70 eV. The present measurements for dissociation of ${\mathrm{OD}}_2^{+}$ and ${\mathrm{OD}}_3^{+}$ yielding ${\mathrm{D}}^{+}$ are compared with recent results obtained in the storage ring experiments, these cross sections are found to be in a reasonable agreement within the quoted uncertainties over the part of the energy range where they are expected to be the same. The cross sections for obtaining ${\mathrm{D}}_2^{+}$ are about an order of magnitude smaller than the cross sections for ${\mathrm{D}}^{+}$ formation from the same targets.

We present an extension of the method of discrete momentum representation to the calculation of elastic electron scattering from polar molecules. The essence of this method is the subtraction of the first Born term from the Lippmann-Schwinger equation, which is solved for a modified interaction potential from which the long-range part is removed. Forward scattering is described in the first Born approximation. The use of this method is demonstrated for electron scattering from the ${\mathrm{H}}_2\mathrm{O}$ molecule in the static-exchange approximation. The results are in good agreement with former calculations as well as with experiments.

We study the stability and dynamics of vortices in two-species condensates using a two-mode model. The recent experimental results obtained at JILA [M. R. Matthews et al., Phys. Rev. Lett. 83, 2498 (1999)] and recent numerical simulations based on the Gross-Pitaevskii equations are explained using this simple model. We also make an exhaustive analysis of the stability properties of the system when the relative populations of the two species and/or their relative scattering lengths are changed and prove that stabilization of otherwise unstable configurations can be attained by controlling the relative population of both species.

We consider the interaction of a two-level atom inside an optical parametric oscillator. In the weak-driving-field limit, we essentially have an atom-cavity system driven by the occasional pair of correlated photons, or weakly squeezed light. We find that we may have holes, or dips, in the spectrum of the fluorescent and transmitted light. This occurs even in the strong-coupling limit when we find holes in the vacuum-Rabi doublet. Also, spectra with a subnatural linewidth may occur. These effects disappear for larger driving fields, unlike the spectral narrowing obtained in resonance fluorescence in a squeezed vacuum; here it is important that the squeezing parameter N tends to zero so that the system interacts with only one correlated pair of photons at a time. We show that a previous explanation for spectral narrowing and spectral holes for incoherent scattering is not applicable in the present case, and propose an alternative explanation. We attribute these anomalous effects to quantum interference in the two-photon scattering of the system.

Factorial moments for the light from a degenerate parametric oscillator mixed with a coherent local oscillator are calculated and shown to reveal novel nonclassical features of light. We have found novel regimes where the nonclassical character of the field is reflected not in the violations of inequalities based on the second-order moments but in those based on higher order moments. These violations can be observed in photoelectric measurements of light.

The theory of the optical parametric oscillator is examined and compared to the experiment in the regime of far below threshold. It is found that the output state has no difference from spontaneous parametric down-conversion except that the bandwidth of down-conversion is reduced to that of the resonator and the conversion rate is enhanced by cavity resonance. The reduction of the bandwidth of the down-converted fields makes it possible for a direct measurement of the time interval distribution between two down-converted photons. The observed distributions are well explained by the theory. Such a narrow-band two-photon source will find wide applications in quantum information processing.

We investigate the mechanism of enhanced ionization in two-electron molecules by solving exactly the time-dependent Schrödinger equation for a one-dimensional ${\mathrm{H}}_2$ in an ultrashort, intense $(I>~{10}^{14}{\mathrm{W}/\mathrm{c}\mathrm{m}}^2)$ laser pulse $(\lambda =1064\mathrm{}\mathrm{nm}).\mathrm{}$ Enhanced ionization in two-electron systems differs from that in one-electron systems in that the excited ionic state ${\mathrm{H}}^{\mathrm{-}}{\mathrm{H}}^{+}$ regarded as the dominant doorway state to ionization crosses the covalent ground state HH in field-following time-dependent adiabatic energy. An analytic expression for the crossing condition obtained in terms of the lowest three states agrees with the numerical results. The gap at the avoided crossing decreases the initial covalent component and promotes electron transfer to ${\mathrm{H}}^{\mathrm{-}}{\mathrm{H}}^{+}.$ As the internuclear distance R decreases, the population of the ${\mathrm{H}}^{\mathrm{-}}{\mathrm{H}}^{+}$ created increases, whereas the ionization rate from a ${\mathrm{H}}^{\mathrm{-}}{\mathrm{H}}^{+}$ decreases owing to the stronger attraction by the distant nucleus. As a result, the rate has a peak at $R\approx 6\mathrm{a}.\mathrm{u}.\mathrm{},$ where most adiabatic states avoid each other with considerable gaps.