Measurements of doubly differential single and multiple ionization of krypton atoms have been performed for $750\text{eV}$ positron and electron impact. Data were measured as a function of projectile energy-loss and scattering angle. For electrons, the energy-loss range was $0–85\%$ of the initial projectile energy and scattering angles were between $\pm 22°$. Following the procedure adopted previously for argon, the electron impact data were placed on an absolute scale by normalizing to total ionization cross sections available in the literature. The present results for krypton show differences between positron and electron impact that are less pronounced than was found for argon. The difference between the two targets can be understood due to the role of inner-shell ionization.

By solving the time-dependent Schrödinger equation of a two-electron system, we demonstrate that efficient high-order harmonics can be emitted by inner electrons of Rydberg atoms exposed to an intense laser pulse, while the Rydberg electron turns out to be stabilized. Due to stabilization of the Rydberg electron, more intense laser fields may essentially interact with inner electrons, which have high ionization potentials, thereby efficiently generating high-order harmonics. The process has the advantage of producing the very high harmonics, seen in intense laser-ion interactions, while maintaining the higher target densities of a neutral Rydberg $\left(n\sim 10\right)$ gas.

We present theoretical and experimental studies of the dynamics of Bose-Einstein condensates in a one-dimensional optical lattice and a three-dimensional harmonic trap. For low atom densities and inertial forces, the condensate performs regular Bloch oscillations, and its center-of-mass motion closely follows semiclassical single-particle trajectories, shaped by the lowest-energy band. But in other regimes, the center-of-mass motion disrupts the internal structure of the condensate by generating solitons and vortex rings, which can trigger explosive expansion of the atom cloud. We use images of the atom cloud to provide experimental evidence for this internal disruption, and find that the process occurs most readily in high-density condensates undergoing slow Bloch oscillations.

We study the characteristics of the quantum state of light produced by a conditional preparation protocol totally performed in the continuous-variable regime. It relies on conditional measurements on quantum intensity correlated bright twin beams emitted by a nondegenerate optical parametric oscillator above threshold. Analytical expressions as well as computer simulations of the selected state properties and preparation efficiency are developed and show that a sub-Poissonian state can be produced by this technique. Projection onto a given trigger value is studied and then extended to a finite band. The continuous-variable regime offers the unique possibility to improve dramatically the preparation efficiency by choosing multiple selection bands and thus to generate a great number of sub-Poissonian states in parallel.

While it has been possible to build fields in high-$Q$ cavities with a high degree of squeezing for some years, the engineering of arbitrary squeezed states in these cavities has only recently been addressed [Phys. Rev. A 68, 061801(R) (2003)]. The present work examines the question of how to squeeze any given cavity-field state and, particularly, how to generate the squeezed displaced number state and the squeezed macroscopic quantum superposition in a a high-$Q$ cavity.

We measure for the first time, to our knowledge, $N$-photon intrashell resonances, widths, and Bloch-Siegert shifts up to $N=23$. Rydberg atoms of Li $\left(n=25\right)$ in a rotating electric field and a static, in-plane electric or magnetic field are used. The measurements, in conjunction with theoretical estimates, precisely determine the characteristics of the resonances (shifts and widths) and their dependence on the strength of the rotating field.

We report squeezed light generation using four-wave mixing in semiconductor microcavities in the strong coupling regime. The geometry of the experiment corresponds to degenerate four-wave mixing, which gives rise to a bistability threshold. Spatial effects in the nonlinear regime are evidenced, and spatial filtering is required in order to optimize the measured squeezing which is observed close to the bistability turning point. By measuring the noise of the outgoing light, we infer a $6\%$ squeezing on the internal mode field which is a part-light, part-matter field.

By defining and solving from first principles, using the state-specific expansion approach, a time-dependent pump-probe problem with real atomic states, we show computationally that, if time resolution reaches the attosecond regime, strongly correlated electronic “motion” can be probed and can manifest itself in terms of time-dependent mixing of symmetry-adapted configurations. For the system that was chosen in this study, these configurations, the $\text{He}2s2p,2p3d,$ and $3s3p{}^{1}P^o$, whose radials are computed by solving multiconfigurational Hartree-Fock equations, label doubly excited states (DES) of He inside the $1s\epsilon p{}^{1}P^o$ scattering continuum and act as nonstationary states that mix, and simultaneously decay exponentially to $1s\epsilon p{}^{1}P^o$ via the atomic Hamiltonian, $H_A$. The herein presented theory and analysis permitted the computation of attosecond snapshots of pairs of electrons in terms of time-dependent probability distributions of the angle between the position vectors of the two electrons. The physical processes were determined by solving ab initio the time-dependent Schrödinger equation, using as initial states either the $\text{He}1s^2$ or the $1s2s{}^{1}S$ discrete states and two femtosecond Gaussian pulses of $86\text{fs}$ full width at half-maximum, having frequencies in resonance with the energies of the correlated states represented by the $2s2p$ and $2p3d$ configurations. We calculated the probability of photoabsorption and of two-photon resonance ionization and of the simultaneous oscillatory mixing of the configurations $2s2p,2p3d,3s3p,$ and $1s\epsilon p{}^{1}P^o$, within the attosecond scale, via the interactions present in $H_A$. Among the possible channels for observing the attosecond oscillations of the occupation probabilities of the DES, is the de-excitation path of the transition to the $\text{He}1s3d{}^{1}D$ discrete state, which emits at $6680\mathrm{\AA }$.

Measurement of energies of ions that result from disassembly of atomic and molecular clusters [${\text{Ar}}_n$, $n=2000–\mathrm{40\hspace{0.17em}000}$; ${\left({\mathrm{N}}_2\right)}_n$, $n=50–3000$] in strong optical fields provides evidence for “charge-flipping” acceleration that gives rise to ions with energies in excess of the Coulombic limit. Measurements of ion spectra as a function of cluster size, and as a function of laser polarization, demonstrate different facets of charge-flipping acceleration.

The effect of the relativistic spin rotation on two-particle spin states, conditioned by the setting of the spins of the particles in their rest frames and by the noncommutativity of the Lorentz transformations along noncollinear directions, is discussed. Particularly, the transition from the center-of-mass system (c.m.s.) of two spin-1∕2 particles to the laboratory is considered. When the vectors of the c.m.s. particle velocities are not collinear with the velocity vector of the c.m.s., the angles of the relativistic spin rotation for the two particles are different. As a result, the relative fractions of the singlet and triplet states in the relativistic system of two spin-1∕2 particles with a nonzero vector of relative momentum depend on the concrete frame in which the two-particle system is analyzed.

A generalization of exponential and Gaussian trial functions is proposed for variational calculations of few-body Coulomb systems. It is shown that the new functions allow us to express the matrix elements of the Hamiltonian in a closed form. In comparison with other methods they allow us, in the case of two-center Coulomb systems, to reduce the length of variational expansion by an order without loss of accuracy. The efficiency of the method is demonstrated in calculations of three-particle systems $ppe$, $dde$, $tte$, $\mu \mu e$ and four-particle systems ${\mathrm{H}}_2$ and $\text{He}{\mathrm{H}}^{+}$ of different isotopic composition.

In an earlier publication we addressed the problem of splitting an electron beam in the Stern-Gerlach experiment. In contrast to arguments put forward in the early days of quantum theory, we concluded that there are no issues of principle preventing the observation of electron spin during free flight. In that paper, however, we considered only a sudden switch off of the separating magnetic field. In this work we consider the possible effects of finite switching times at the beginning and the end of the interaction period. We consider a model where the coupling between the electron and the field is time dependent. As a result of the time dependence, the field also acquires an electric component, but this seems to cause no significant change of our conclusions. On the other hand, the smooth change of the interaction enforces the same longitudinal velocity on the electron both at the beginning and end of the interaction period because of conservation laws; this effect was missing in our earlier calculations. As the electrons are supposed to travel as a beam, this feature helps by restoring the beam quality after the interaction.

In this paper we investigate the quantum Zeno and anti-Zeno effects without using any particular model of the measurement. Making a few assumptions about the measurement process we derive an expression for the jump probability during the measurement. From this expression the equation, obtained by Kofman and Kurizki [Nature (London) 405, 546 (2000)] can be derived as a special case.

We study the collective motion of atoms confined in an optical lattice operating inside a high-finesse ring cavity. A simplified theoretical model for the dynamics of the system is developed upon the assumption of adiabaticity of the atomic motion. We show that in a regime where the light shift per photon times the number of atoms exceeds the linewidth of the cavity resonance, the otherwise tiny retroaction of the atoms upon the light field becomes a significant feature of the system. As a result dispersive optical bistability can arise and the lattice positions is determined by the strength of the atom-cavity coupling rather than by the phases of the incoupled light beams. Solving the complete set of classical equations of motion confirms these findings, however, additional nonadiabatic phenomena are predicted, such as, for example, self-induced radial breathing oscillations. We compare these results with experiments involving laser-cooled ${}^{85}\mathrm{Rb}$ atoms trapped in an optical lattice inside a ring cavity with a finesse of $1.8\times {10}^5$. Our observations are in excellent agreement with our theoretical predictions.

The effect of interference stabilization is shown to exist in a system of two atomic levels coupled by a strong two-color laser field, the two frequencies of which are close to a two-photon Raman-type resonance between the chosen levels, with open channels of one-photon ionization from both of them. We suggest an experiment, in which a rather significant (up to $90\%$) suppression of ionization can take place and which demonstrates explicitly the interference origin of stabilization. Specific calculations are made for H and He atoms and optimal parameters of a two-color field are found. The physics of the effect and its relation with such well-known phenomena as light induced continuum structure and population trapping in a three-level system are discussed.

We present a detailed theory of spectacular semiclassical catastrophes happening during the time evolution of a kicked quantum rotor [Phys. Rev. Lett. 87, 163601 (2001)]. Both two- and three-dimensional rotational systems are analyzed. It is shown that the wave function of the rotor develops a cusp at certain delay after a kick, which results in a sharply focused rotational wave packet. The cusp is followed by a fold-type catastrophe manifested in the rainbow-type moving singularities. In the three-dimensional case, the rainbows are accompanied by additional singular features similar to the glory structures known in the wave optics. These catastrophes in the time-dependent angular wave function are well described by the appropriate tools of the quasiclassical wave mechanics, i.e., by Airy and Bessel approximations and Pearcey’s functions. A scenario of “cumulative squeezing” is also presented in which a specially designed train of short kicks produces an unlimited narrowing of the rotor angular distribution. This scenario is relevant to the molecular alignment by short laser pulses, and also to the atom lithography schemes in which cold atoms are focused by an optical standing wave.

Anyons obtained from a finite gauge theory have a computational power that depends on the symmetry group. The relationship between group structure and computational power is discussed in this paper. In particular, it is shown that anyons based on finite groups that are solvable but not nilpotent are capable of universal quantum computation. This extends previously published results to groups that are smaller and therefore more practical. Additionally, a new universal gate set is built out of an operation called a probabilistic projection, and a quasiuniversal leakage correction scheme is discussed.

We report on the implementation of quantum state tomography for an ensemble of ${\text{Eu}}^{3+}$ dopant ions in a ${\mathrm{Y}}_2{\text{SiO}}_5$ crystal. The tomography was applied to a qubit based on one of the ions’ optical transitions. The qubit was manipulated using optical pulses and measurements were made by observing the optical free induction in a phase sensitive manner. Fidelities of $>90\%$ for the combined preparation and measurement process were achieved. Interactions between the ions due to the change in the ions’ permanent electric dipole moment when excited optically were also measured. In light of these results, the ability to do multiqubit quantum computation using this system is discussed.

The $L_1\text{−}{L}_3$ Coster-Kronig (CK) yield $f_{13}$ for the ${}_{70}\mathrm{Yb}$, ${}_{71}\mathrm{Lu}$, ${}_{74}\mathrm{W}$, ${}_{75}\mathrm{Re}$, ${}_{79}\mathrm{Au}$, ${}_{80}\mathrm{Hg}$, and ${}_{81}\mathrm{Tl}$ elements was deduced using measured intensities of the $L\alpha$ x rays emitted following decay of the $L_i$-subshell $\left(i=1,2,3\right)$ vacancies produced in widely different proportions by the $59.54\text{keV}$ $\gamma$ rays $\left(B_{L_1}<E_{inc}<B_K\right)$ and the $\text{Ge}∕\text{Se}∕\text{Rb}$ $K$ x rays $\left(B_{L_3}<E_{K\alpha }<B_{L_2},B_{L_1∕L_2}<E_{K\beta }<B_K\right)$; where ${\mathrm{B}}_{K∕Li}$ is the $K$-shell∕$L_i$-subshell binding energy of the target element. An energy-dispersive x-ray fluorescence set up, involving photon sources consisting of a ${}^{241}\mathrm{Am}$ annular source in the direct and secondary excitation modes along with the $\text{Ge}∕\text{Se}∕\text{RbCl}$ secondary exciter and a $\text{Si}\left(\text{Li}\right)$ detector, was used for the measurements. The measured $f_{13}$ yields for ${}_{70}\mathrm{Yb}$, ${}_{71}\mathrm{Lu}$, and ${}_{74}\mathrm{W}$ are found to agree with the values based on the relativistic Dirac-Hartree-Slater (RDHS) calculations, while those for ${}_{79}\mathrm{Au}$, ${}_{80}\mathrm{Hg}$, and ${}_{81}\mathrm{Tl}$ are lower by $\sim 20\%$. The measured $f_{13}$ yields exhibit a jump from 0.354(24) for ${}_{74}\mathrm{W}$ to 0.450(27) for ${}_{75}\mathrm{Re}$ compared to that from 0.352 to 0.640 as predicted by the RDHS calculations due to onset of the intense $L_1\text{−}{L}_3{M}_5$ CK transition.

We have investigated the ground-state structures of neutral and charged $X{\mathrm{H}}_4$$(X=\mathrm{C},$ Si, Ge, Sn, and Pb) molecules using the first-principles electronic structure methods. The structure of positively charged molecules for $X=\mathrm{Si},$ Ge, Sn, and Pb is characterized by a severe distortion from tetrahedral structure and an unusual H-H bond while the negatively charged molecules get distorted by pushing two hydrogen atoms away from each other. However, ${\mathrm{CH}}_4^{+}$ and ${\mathrm{CH}}_4^{-}$ are exceptions to this behavior. We provide an insight into the symmetry breaking mechanism and unusual H-H bonding using simple electrostatic arguments based on the unequal charge distribution on H atoms. Those charged molecules having unequal charge distribution on H atoms get distorted due to different electrostatic forces between the atoms. We show that the directionality and occupation of the highest occupied molecular orbital play an important role in creating charge asymmetry in these molecules.