A quantum key distribution scheme whose security depends on the features of pre- and postselected quantum states is described.

We investigate the capabilities of a quantum computer based on cold trapped ions in the presence of nondissipative decoherence. The latter is accounted by using the evolution time as a random variable and then averaging on a properly defined probability distribution. Severe bounds on computational performances are found.

Energies, transition probabilities, and lifetimes are calculated for ${3d}^2$ states in Ca-like ions with nuclear charges Z ranging from 22 to 100. Relativistic many-body perturbation theory (MBPT), including the Breit interaction, is used to evaluate retarded $M1\mathrm{}$ and $E2\mathrm{}$ matrix elements. The calculations start from a ${1s}^2{2s}^2{2p}^6{3s}^2{3p}^6$ Dirac-Fock potential. First-order perturbation theory is used to obtain intermediate coupling coefficients and second-order MBPT is used to determine matrix elements. Contributions from negative-energy states are included in the second-order $M1\mathrm{}$ and $E2\mathrm{}$ matrix elements. The resulting transition energies and lifetimes are compared with experimental values and with results from other recent calculations.

By means of high spectral and spatial resolution x-ray spectroscopic methods we have observed for the first time simultaneously two-electron and higher-order intercombination transitions from autoionizing levels in large scale optically thick laser produced plasmas. Intercombination transitions as well as the two-electron op-satellite transitions in Li-like ions have anomalous large intensities (up to a factor of 14). Sophisticated non-Maxwellian opacity simulations treating extended configuration interaction atomic data employing the MARIA-code reveal, however, good agreement among forbidden transitions.

The relative triple differential cross section for 100-eV electron-impact ionization of the three outer molecular orbitals of ${\mathrm{O}}_2$ has been studied in an asymmetric $\mathrm{}(e,2e)\mathrm{}$ experiment. The angular distribution of low-energy ejected electrons was measured from 30° to 125° with respect to the incident beam direction for an incident electron scattering angle of 4° and ejected-electron energies of 3.5, 6.0, and 11.0 eV. The ionization of the outer $1{\pi }_g$ orbital that leads to the formation of an ${\mathrm{O}}_2^{+}$ ion in the $X{}^2{\Pi }_g$ state was observed to be accompanied by the ejection of a low energy electron 90° to the incident beam direction on the binary side. Binary results for the other two orbitals, $1{\pi }_u$ and $3{\sigma }_g,$ to give the $a{}^4{\Pi }_u$ and $b{}^4{\Sigma }_g^{+}$ states were similar to previous results for the analogous orbitals in ${\mathrm{N}}_2$ with ejection directions of 75° and 45° to the incident beam direction, respectively. In contrast to the ${\mathrm{N}}_2$ results, the recoil peak for all cases was found to be aligned approximately 180° to the momentum transfer direction. Possible mechanisms for the binary low-energy electron ejection at angles so far removed from the momentum transfer direction are discussed.

We investigate inelastic cold collisions in a Rb-Cs mixed-species trap where the rubidium trap loss rate was measured. We present the experimental results and a qualitative discussion for the observation. These studies are relevant for high-resolution atomic and molecular spectroscopy, and for the production of mixed-species Bose-Einstein condensates.

This paper aims at studying the time-dependent effects involved in the photoassociation reaction for a sample of cold alkali-metal atoms, within a two-channel model where the vibrational motion in the excited state is coupled by laser light to the continuum state describing two colliding atoms in the lowest triplet electronic state $\left(a{}^3{\Sigma }_u^{+}\right).\mathrm{}$ Both photodissociation and photoassociation processes are considered at a time scale shorter than the radiative lifetime, so that spontaneous emission does not have to be considered. The characteristic times are then the vibrational period in the excited state, which for alkali-metal dimers can be estimated of the order of a few hundreds of picoseconds, and the Rabi period, depending upon the laser intensity. Numerical calculations using wave-packet propagation are performed for the coupling of the vibrational motion in the ${\mathrm{Cs}}_2{1}_g{(6s+6p}_{3/2})$ and $a^3{\Sigma }_u^{+}\mathrm{}(6s+6s)\mathrm{}$ channels by a cw laser slightly red detuned relative to the ${\mathrm{D}}_2$ resonance line. The results show Rabi oscillations in the populations of the two channels during time intervals when the vibrational motion is stopped at the outer turning point. At intensities of $\approx 250\mathrm{kW}{\mathrm{cm}}^{-2},$ a new characteristic time appears, a factor of 2 larger than the classical vibrational period, which corresponds to vibrational motion in the upper adiabatic potential created by the coupling. Such an effect modifies the scattering length for collisions in the lower state, and it clearly opens a possibility of control by tuning the laser intensity.

We study the use of detection devices in entanglement-based state preparation. In particular we consider optical detection devices such as single-photon sensitivity detectors, single-photon resolution detectors, and detector cascades (with an emphasis on the performance of realistic detectors). We develop an extensive theory for the use of these devices. In entanglement-based state preparation we perform measurements on subsystems, and we therefore need precise bounds on the distinguishability of these measurements (this is fundamentally different from, e.g., tomography, where an ensemble of identical states is used to determine probability distributions, etc.). To this end, we introduce the confidence of preparation, which may also be used to quantify the performance of detection devices in entanglement-based preparation. We give a general expression for detector cascades of arbitrary size for the detection up to two photons. We show that, contrary to the general belief, cascading does not give a practical advantage over detectors with single-photon resolution in entanglement-based state preparation.

We establish the relation between the Wigner function of a field emerging from a cavity, determined through pulsed homodyne tomography, and the phase-space distribution of the original field in the cavity. We show that the reconstructed Wigner function of the outgoing field is equivalent to generalized phase-space distributions of the initial cavity field, the original Wigner distribution being obtained when there is perfect mode matching between the outgoing field and the homodyning pulse.

We consider the influence of refractive index fluctuations (due to randomly distributed inclusions inside a dielectric spherical droplet) on the line shapes of scattering Mie resonances. The significant difference in the spatial distributions of the mode functions participating in the process does not allow one to employ the standard statistical ensembles used in random matrix theory. We propose to model the system by a simplified random ensemble which gives a very good agreement with the available experimental data, and we predict a type of line shape for narrow scattering resonances.

Spontaneous emission (SE) from electrons forming a quasi-two-dimensional Wigner crystal in a dielectric cavity is discussed for a multilayer configuration. All layers in the system are considered as dielectrics with real dielectric constants. Within such a model, we treat SE completely from the quantum-mechanical (QM) point of view, i.e., we use the quantized electromagnetic field in dielectrics and calculate its interaction with electrons in the Wigner crystal. The full QM approach enables us to go beyond the standard dipole approximation and take into account, e.g., the vibration of electrons around their regular positions inside the crystal and the change of electron positions during the deexcitation process. We analyze only the perpendicular transitions, which involve quantum states determined mainly by the image potential. Since this potential is relatively weak for a typical configuration of a Wigner lattice, the corresponding electromagnetic lifetime $\tau$ is estimated to be very large $(\tau \lesssim {10}^{-3}\hspace{0.5em}\mathrm{s}\mathrm{}).\mathrm{}$ We derive a simple expression for the angular distribution of the SE radiation and analyze its possible deviation from the dipole radiation $\left({\sin }^2\theta \right)$ spectrum. For a typical Wigner crystal formed above a liquid He layer we have found no significant corrections to the spectrum obtained in the dipole approximation. We also consider the case of a Wigner crystal in a (micro)cavity between two metallic plates, discuss the effect of the cavity selection rules on the SE rate, and examine the corresponding discrete SE spectrum.

We investigate the quantum decoherence of a mesoscopically superposed states of excitons in a quantum well placed within a leaky cavity. Based on the factorization theory of quantum dissipation, our investigation shows that the coherence of the superposed states of the excitons will decrease in an oscillating form when the cavity field is of the quasimode form. Regarding the effects of the thermal cavity fields on the quantum decoherence of the superposition states of the exciton, we observe that the higher the temperature of the environment, the shorter the decoherence characteristic time.

We discuss the derivation of multimode rate equations for the description of a semiconductor laser with external cavity. We adopt a formulation where the complex field amplitudes are coupled to the nonlinear gains. For N lasing modes, this leads to $2N$ equations that display in-phased and antiphased time-dependent solutions. A simplified reference model is obtained by assuming that the key parameters are frequency independent. A general linear stability analysis leads to the prediction of two types of Hopf bifurcations. A nondegenerate Hopf bifurcation occurs with the relaxation oscillation frequency as the characteristic bifurcation frequency. A $\left(N-1\right)$-degenerate Hopf bifurcation occurs with a lower characteristic frequency. To assess the nature and stability of the solutions emerging from the Hopf bifurcations, we perform a nonlinear stability analysis on a reduced model obtained in the limit of large linewidth enhancement factor. In this asymptotic limit, the steady state is always destabilized in favor of a stable periodic inphased or antiphased state. A numerical analysis yields a bifurcation diagram of the multimode equations, which confirms the analytic results and reveals further complex regimes (quasiperiodic and chaotic, in-phased and antiphased) as the amplitude of the field fed back into the laser is increased.

This paper reports the experimental measurement of Se and Y K-shell ionization cross sections by electron impact. The thin targets with thick substrate are used in the experiment. The influence of the electrons reflected from the substrate on the measurements is discussed and corrected. For Se and Y, in energies from near threshold to 40 keV, the experimental data are reported and compared with the empirical formula.

An experiment to verify the phase in noncyclic SU(2) spatial evolution of split-beam interferometry using spin-polarized neutrons is proposed.

We report the first completely ab initio calculations of doubly differential cross sections for the electron impact ionization of hydrogen at incident energies of 17.6 eV, 25 eV, and 30 eV. These cross sections have been extracted from wave functions obtained by directly solving a finite difference discretized Schrödinger equation without explicit reference to any assumed asymptotic form. Outgoing wave boundary conditions are assured by the use of the exterior complex scaling method. Our calculations suggest the need for additional experiments to augment the one available measurement.

Absorption-free (also known as “interaction-free”) measurement aims to detect the presence of an opaque object using a test particle without that particle being absorbed by the object. Here we consider semitransparent objects that have an amplitude $\alpha$ of transmitting a particle while leaving the state of the object unchanged and an amplitude $\beta$ of absorbing the particle. The task is to devise a protocol that can decide which of two known transmission amplitudes is present while ensuring that no particle is absorbed by the object. We show that the probabilities of being able to achieve this are limited by an inequality. This inequality implies that absorption-free distinction between complete transparency and any partial transparency is always possible with probabilities approaching 1, but that two partial transparencies can only be distinguished with probabilities less than 1.

The wave nature of electrons, epitomized by Young’s two-slit experiment, is also demonstrated in resonant photoemission. We present photoelectron spectra of argon measured in the vicinity of the ${2p}_{3/2}\to 4s$ core excitation. Interference effects are monitored by following the intensity of final states populated through both the resonant and the direct channel. The partial-electron-yield spectra of ${3p}^{-2}\mathrm{nd}$ final states exhibit a rich variety of interference effects. This demonstrates the need for a unified one-step description of the ionization through a direct and a core-excitation mediated channel.

Lifetimes of metastable levels in the ground term of Fe ions within the ${3s}^2{3p}^k,k=1\mathrm{}–5,$ isoelectronic sequences have been measured. These measurements were performed utilizing ions that were selected by mass to charge ratio while transported from an electron cyclotron resonance ion source to a Kingdon ion trap, where they were captured and then confined for periods of up to 2.1 s. During this storage period, selected emission wavelengths of transitions from metastable levels in the visible or near-ultraviolet spectral regions were isolated using interference filters, and the time-dependent fluorescence intensities were measured using a photomultiplier tube. Measurement precisions on the order of 2% were achieved in favorable cases. The measured lifetimes are $\tau (\mathrm{Fe}X,{3s}^2{p}^5{}^2{P}_{1/2})=13.64\mathrm{}\pm 0.25\mathrm{ms},$ $\tau (\mathrm{Fe}\mathrm{XI},{3s}^2{3p}^4{}^1{D}_2)=9.86\mathrm{}\pm 0.22\mathrm{ms},$ $\tau \left(\mathrm{Fe}\mathrm{XII}{,3s}^2{3p}^3{}^2{P}_{3/2}\right)=1.85\mathrm{}\pm 0.24\mathrm{ms},$ $\tau (\mathrm{Fe}\mathrm{XII},{3s}^2{3p}^3{}^2{P}_{1/2})=4.38\mathrm{}\pm 0.42\mathrm{ms},$ $\tau (\mathrm{Fe}\mathrm{XII},{3s}^2{3p}^3{}^2{D}_{3/2})=20.35\mathrm{}\pm 1.24\mathrm{ms},$ $\tau (\mathrm{Fe}\mathrm{XIII},{3s}^2{3p}^2{}^1{D}_2)=6.93\mathrm{}\pm 0.18\mathrm{ms},$ and $\tau (\mathrm{Fe}\mathrm{XIV},{3s}^{2}3p{}^2{P}_{3/2})=17.52\mathrm{}\pm 0.29\mathrm{ms}.$ These results are compared with existing and with new theoretical calculations, which have estimated uncertainties on the order of $10–25\%.\mathrm{}$

Absorption of radiation from sodium dimer molecular states correlating to $\mathrm{Na}\mathrm{}\left(3s\right)-\mathrm{Na}\mathrm{}\left(3s\right)\mathrm{}$ is investigated theoretically. Vibrational bound and continuum transitions from the singlet $X{}^1{\Sigma }_g^{+}$ state to the first excited $A{}^1{\Sigma }_u^{+}$ and $B{}^1{\Pi }_u$ states and from the triplet $a{}^3{\Sigma }_u^{+}$ state to the first excited $b{}^3{\Sigma }_g^{+}$ and $c{}^3{\Pi }_g$ states are studied quantum-mechanically. Theoretical and experimental data are used to characterize the molecular properties, taking advantage of knowledge recently obtained from ab initio calculations, spectroscopy, and ultracold atom collision studies. The quantum-mechanical calculations are carried out for temperatures in the range from 1000 to 3000 K, and are compared with previous calculations and measurements where available.