We investigate whether it is possible to store and retrieve an intense probe pulse using a medium that can be modeled as a set of atoms with the relevant energy levels in $\Lambda$ configuration. We demonstrate that it is indeed possible to store and retrieve the probe pulses that are not necessarily weak. We find that the retrieved pulse remains a replica of the original pulse, although there is overall broadening and loss of the intensity. The loss of intensity can be understood in terms of the dependence of absorption on the intensity of the probe. Our calculations include the dynamics of the control field, which becomes especially important as the intensity of the probe pulse increases. We use the adiabatic theory of Grobe et al. [Phys. Rev. Lett. $73,$ 3183 (1994)] to understand our numerical results on the storage and retrieval of light pulses at moderate powers.

The principle of teleportation can be used to perform a quantum computation even before its quantum input is defined. The basic idea is to perform the quantum computation at some earlier time with qubits that are part of an entangled state. At a later time a generalized Bell-state measurement is performed jointly on the then defined actual input qubits and the rest of the entangled state. This projects the output state onto the correct one with a certain exponentially small probability. The sufficient conditions are found under which the scheme is of benefit.

$K$-shell photodetachment from ${\mathrm{C}}^{-}$ has been investigated in the photon energy range between 280 and 285 eV using the merged ion-beam–photon-beam technique. ${\mathrm{C}}^{-}$ ions were produced using a Cs sputtering negative-ion source, while the photons were produced by the undulator beam line 10.0.1 of the Advanced Light Source. ${\mathrm{C}}^{+}$ ions formed by double detachment were detected as a function of incident photon energy. Using this collinear arrangement, the relative cross sections were measured and compared with theoretical predictions. The measured spectrum shows the first experimental evidence of the ${1s2s}^2{2p}^4{(}^{4}P)$ shape resonance near 281.7 eV, which is in excellent agreement with two independent R-matrix calculations for the $1s$ photodetachment cross section of ${\mathrm{C}}^{-}$ producing ${\mathrm{C}}^{+}.$

We present a study of the superfluid properties of atomic Bose gases in optical lattice potentials using the Bose-Hubbard model. To do this, we use a microscopic definition of the superfluid fraction based on the response of the system to a phase variation imposed by means of twisted boundary conditions. We compare the superfluid fraction to other physical quantities, i.e., the interference pattern after ballistic expansion, the quasimomentum distribution, and number fluctuations. We have performed exact numerical calculations of all these quantities for small one-dimensional systems. We show that the superfluid fraction alone exhibits a clear signature of the Mott-insulator transition. Observables like the fringe visibility, which probe only ground-state properties, do not provide direct information on superfluidity and the Mott-insulator transition.

We show that extremely short uv and vuv pulses with a duration in the range of 1 fs can be generated by pulse compression using hollow waveguides filled with a gaseous Raman medium. The specifics of phase-amplitude modulation, the mechanism of pulse compression and physical limitations due to phase mismatch, velocity walk-off, and pump pulse changing as well as improvement by the use of pump pulse sequences and dispersion control are studied by analytical and numerical solutions.

We propose a protocol for conditional quantum logic between two four-state atoms inside a high-$Q$ optical cavity. The process detailed in this paper utilizes a direct four-photon two-atom resonant process and has the added advantage of commonly addressing the two atoms when they are inside the high-$Q$ optical cavity.

We study band-structure effects on the energy loss of protons scattered off the Cu (111) surface. The distance dependent stopping power for a projectile traveling parallel to the surface is calculated within the linear response theory. The self-consistent electronic response of the system is evaluated within the random-phase approximation. In order to characterize the surface band structure, the electronic single-particle wave functions and energies are obtained by solving the Schrödinger equation with a realistic one-dimensional model potential. This potential reproduces the main features of the Cu (111) surface: the energy band gap for electron motion along the surface normal, as well as the binding energy of the occupied surface state and the first image state. Comparison of our results with those obtained within the jellium model allows us to characterize the band-structure effects in the energy loss of protons interacting with the Cu (111) surface.

An anisotropic pseudopotential arising in the context of collisions of two particles polarized by an external field is rigorously derived and its properties are investigated. Such a low-energy pseudopotential may be useful in describing collective properties of dilute quantum gases, such as molecules polarized by an electric field or metastable ${}^3{P}_2$ atoms polarized by a magnetic field. The pseudopotential is expressed in terms of the reactance (K) matrix and derivatives of the Dirac $\delta$ function. In most applications, it may be represented as a sum of a traditional spherically symmetric contact term and an anisotropic part. The former contribution may be parametrized by a generalized scattering length. The anisotropic part of the pseudopotential may be characterized by the off-diagonal scattering length for dipolar interactions and off-diagonal scattering volume for quadrupolar interactions. The two-body matrix element of the pseudopotential in a basis of plane waves is also derived.

We examine a typical two-source optical interference apparatus consisting of two cavities, a beam splitter, and two detectors. We show that field-field interference occurs even when the cavities are not initially in coherent states but rather in other nonclassical states. However, we find that the visibility of the second-order interference, that is, the expectation values of the detectors’ readings, changes from 100%, when the cavities are prepared in coherent states, to zero visibility when they are initially in single Fock states. We calculate the fourth-order interference, and for the latter case find that it corresponds to a case where the currents oscillate with 100% visibility, but with a random phase for every experiment. Finally, we suggest an experimental realization of the apparatus with nonclassical sources.

The size-dependent decoherence of the exciton states, resulting from the spontaneous emission, is investigated in a semiconductor spherical microcrystallite under the condition $a_B\ll R_0<~\lambda .$ In general, the larger size of the microcrystallite corresponds to the shorter coherence time. If the initial state is a superposition of two different excitonic coherent states, the coherence time depends on both the overlap of two excitonic coherent states and the size of the microcrystallite. When the system with fixed size is initially in the even or odd coherent state, the larger average number of the excitons corresponds to the faster decoherence. When the average number of the excitons is given, the bigger size of the microcrystallite corresponds to the faster decoherence. The decoherence of the exciton states for the materials GaAs and CdS is numerically studied by our theoretical analysis.

A theoretical model of coherent control of nuclear-spin isomers by microwave radiation has been developed. The model accounts the M degeneracy of molecular states and molecular center-of-mass motion. The model has been applied to the ${}^{13}{\mathrm{CH}}_3\mathrm{F}$ molecules. Microwave radiation excites the para state $(J=11,$ $K=1)$ which is mixed by the nuclear-spin-spin interaction with the ortho state (9,3). Dependences of the isomer enrichment and conversion rates on the radiation frequency have been calculated. Both spectra consist of two resonances situated at the centers of nuclear-spin-allowed and -forbidden transitions of the molecule. Larger enrichment, up to 7%, can be produced by strong radiation resonant to the forbidden transition. The spin conversion rate can be increased by two orders of magnitude at this resonance.

A unitary transformation of the Dirac operator in a magnetic field is presented, which leads to a reformulation of the interaction of a Dirac particle with a magnetic field, in which, as in nonrelativistic theory, diamagnetic and paramagnetic contributions appear naturally, but at a four-component-spinor level. The diamagnetic contribution to the magnetic susceptibility consists of two terms, each of which is evaluated as a simple expectation value with the unperturbed relativistic wave function. One of the two terms closely resembles its nonrelativistic counterpart. The proposed formalism is analyzed in the context of the direct perturbation theory of relativistic effects. It is compared with the more traditional sum-over-states approach including negative-energy states, as well as with a Fock-space formulation. In the latter, the vacuum energy depends on the external magnetic field. The creation of a particle (electron or positron) is accompanied by a change of the vacuum energy via a kind of exclusion effect. This change can be identified with the diamagnetism of the particle. The access to diamagnetism and paramagnetism based on the Gordon decomposition of the induced current density is to some extent, but not entirely, equivalent to that which results from the unitary transformation. For a physically meaningful decomposition of the current density a combination of the Gordon approach with the unitary transformation is recommended. Neither the interpretation nor the computation of diamagnetic contributions in terms of negative-energy states is encouraged.

High-resolution measurements of the photoinduced $L_3-M_1$ and $L_3-M_{4,5}$ x-ray emission lines were performed with a reflection-type bent-crystal spectrometer for a variety of elements ranging from xenon $\mathrm{}(Z=54\mathrm{})\mathrm{}$ to iridium $\mathrm{}(Z=77\mathrm{}).$ The measurements were performed at the European Synchrotron Radiation Facility in Grenoble, France. From the observed linewidths of the $L_3-M_1$ x-ray transitions, the widths of the $M_1$ levels were determined using the $L_3$ level widths extracted from the measured $L_3-M_5$ transitions, and assuming for the $M_5$ levels the widths reported recently by Campbell and Papp [At. Data Nucl. Data Tables 77, 1 (2001)]. In the rare-earth region a Z-dependent broadening of the $M_1$ level was observed.

The spectrum of the $({\mathrm{Li}}^{3+}+\overline{p}+2e)$ four-body system was calculated in an adiabatic approach. The two-electron energies were approximated by a sum of two single-electron effective charge two-center energies as suggested by J. S. Briggs, P. T. Greenland, and E. A. Solov’ev [J. Phys. B. 32, 197 (1999)]. While the structure of the spectrum does not exclude the existence of long-lived states, their experimental observability is still to be clarified.

Krypton $3p$ excitations and their subsequent resonant Auger decay have been studied. The absolute photoabsorption cross section was determined in the region of the $3p$ resonances, and some structures were detected in the absorption spectrum. Hartree-Fock calculations including correlation between ${3p}^{-1}\mathrm{nl}$ and ${3d}^{-2}{n}^{\prime }{l}^{\prime }\mathrm{nl}$ configurations were used to interpret the structures of the $3p$ resonant excitation series and of the corresponding decay spectra. Furthermore, the decay spectra confirmed the assignment of the main structures in the absorption spectrum.

We report integral, differential, and momentum-transfer cross sections for elastic scattering of low-energy electrons by the ${\mathrm{C}}_3{\mathrm{H}}_4$ isomers allene, propyne, and cyclopropene, which belong to the $D_{2d},$ $C_{3v},$ and $C_{2v}$ groups, respectively. We use the Schwinger multichannel method with pseudopotentials [Bettega et al., Phys. Rev. A 47, 1111 (1993)] at the static-exchange approximation to compute the cross sections for energies up to 40 eV. We compare our results with available experimental results and find very good agreement. Our results confirm the existence of the shape resonances in the cross sections of allene and propyne, and the isomer effect, both reported by the experimental studies.

When a projectile travels inside a metal, it interacts with the electron gas, producing both binary and collective excitations (plasmons). Within the nearly-free-electron-gas scheme, Rösler and co-workers showed that plasmons decay in first order and a conduction electron is emitted (interband transition). Working within the frame of atomic collisions, we develop a simple model to describe this decay. The first-order Born expansion is used to approximate the electron wave functions. The influence of the lattice potential on the excited electron is considered in the calculations in order to balance the momentum-conservation equation. It gives contributions associated with sites of the reciprocal lattice. The potential expansion coefficients are obtained following Animalu and co-workers [Philos. Mag. 9, 451 (1964)]. First- and second-differential spectra (in energy and angle) are analyzed discriminating contributions due to different lattice momenta. In all cases, contributions due to binary excitations of the valence electrons and inner-shell ionization are presented to establish a comparison.

The present paper reports on the study of deterministic instabilities in the atomic cloud of a magneto-optical trap. Giant periodic and erratic self-oscillations are experimentally observed and analyzed through a simple original model taking into account the shadow effect and the spatial distribution of the atoms in the cloud. We show that giant oscillations are induced by a homoclinic orbit merging in the neighborhood of a Hopf bifurcation.

We consider the time evolution of the occupation probabilities for the $2s-2p$ transition in a hydrogen atom interacting with an external field $V\left(t\right).$ A two-state model and a dipole approximation are used. In the case of degenerate energy levels an analytical solution of the time-dependent Schrödinger equation for the probability amplitudes exists. The form of the solution allows one to choose the ratio of the field amplitude to its frequency that leads to temporal trapping of electrons in specific states. The analytic solution is valid when the separation of the energy levels is small compared to the energy of the interacting radiation.

We discuss the superfluid to Mott insulator transition of an atomic Bose gas in an optical lattice with high filling factors. We show that in this multiband situation, the long-wavelength physics is described by a single-band Bose-Hubbard model. We determine the many-body renormalization of the tunneling and interaction parameters in the effective Bose-Hubbard Hamiltonian, and consider the resulting model at nonzero temperatures. We show that, in particular, for a one- or two-dimensional optical lattice, the Mott-insulator phase is more difficult to realize than anticipated previously.