Atomic level lifetimes have been measured on magnetic dipole $\mathrm{}\left(M1\mathrm{}\right)\mathrm{}$ transitions in Si-like, P-like, Ar-like, K-like, and Ca-like ions of Kr, using an electron-beam ion trap in magnetic trapping mode. The lifetimes are compared to various predictions, including the results of different calculations. In Ar-like ions, magnetic quadrupole $\mathrm{}\left(M2\mathrm{}\right)\mathrm{}$ transitions have been identified as significant competing decay channels.

We consider low-dimensional model systems with a fixed two-body interaction and a variable (nonlocal) one-body potential. It is shown explicitly that an extended domain of allowed (N-representable) one-body density matrices cannot be generated in this way, the excluded domain depending on the two-body interaction under consideration. This stands in contrast to the behavior of the diagonal part of the density matrix.

We scrutinize individual interchannel coupling effects on atomic dipole and nondipole $(E1\mathrm{}-E2)$ photoelectron angular distribution parameters of valence electrons far above thresholds, choosing $2p$ photoionization of N and Ne in the photon energy range from 1 to 10 keV as case studies. It is found that individual correlation effects are strong far above thresholds. However, a cancellation effect is also discovered that largely obviates the net correlation effect on photoelectron angular distributions. It is shown that the cancellation can be removed, i.e., strong correlation effects can be observed, by considering core-ionized (core-excited) initial states; this is expected to be quite general. The importance of this work is that it shows that the tacit belief in the insignificance of correlation in nondipole parameters far above thresholds is quite misleading. In addition, it suggests studies of core-ionized or core-excited atoms as a means of exploring these large correlation effects in nondipole photoelectron angular distributions far above thresholds.

The use of the Bethe cross section in the binary-encounter-dipole (BED) model for electron-impact ionization is studied. While the dipole contribution in the Born approximation accounts for the longest-range interaction in electron-neutral atom/molecule inelastic collisions at any incident energy, the Bethe formula is applicable only at high energies. To derive a suitable representation of the Born cross section for dipole-allowed transitions, a convergent series representation for the generalized oscillator strength (GOS) of electron- impact ionization is studied. It is shown that by transforming to a new variable determined by the location of the singularities of the GOS on the complex plane of momentum transfer K, a series representation for the GOS is obtained that is convergent at all physically attainable values of K. An approximate representation of the GOS that truncates the series representation to the first three terms is also given. The approximate GOS describes the interaction of the electron with a shielded dipole potential and satisfies both Lassettre’s limit theorem at $K=0\mathrm{}$ and the asymptotic behavior at large K derived by Rau and Fano [A. R. P. Rau and U. Fano, Phys. Rev. 162, 68 (1967)]. The dipole-Born cross section so obtained is applicable at all incident energies and goes to the Bethe cross section at the high-energy limit. It provides a more suitable representation of the dipole contribution in the BED model than the Bethe cross section and is valid over the entire energy range. A similar analysis of the optical-oscillator strength (OOS) as a function of the complex momentum for the ejected electron $k_p,$ plus the requirement that the OOS satisfies both the low- and $\mathrm{high}{-k}_p$ limits produces an analogous series representation for the OOS. An approximate one-term representation of the OOS is also developed that can be used in modeling calculations. Numerical examples of total ionization cross sections of ${\mathrm{N}}_2,$ ${\mathrm{H}}_2\mathrm{O},$ ${\mathrm{CO}}_2,$ ${\mathrm{CH}}_4,$ and ${\mathrm{CF}}_4$ using the new analytical representation are presented to illustrate the applicability of the improved BED model.

We present a simple half-collision model that allows the approximate calculation of absolute cross sections for multiple ionization by breaking this process down into a primary ionization event followed by a half-scattering event in which additional electrons are ionized. As a critical test for the feasability of this approach, we consider the double ionization of two-electron systems, which we describe in terms of the single ionization of the “primary” electron followed by impact ionization of the slow “secondary” electron. For triple ionization of lithium, the model decomposes the three-electron breakup process into a double ionization of the two inner electrons followed by electron-electron half-scattering of the receding electrons at the residual “spectator” $2s$ electron. We find surprisingly good agreement with recent experimental data.

We investigate the mechanism of enhanced ionization that occurs at a critical internuclear distance $R_c$ in the two-electron symmetric linear triatomic molecule ${\mathrm{H}}_3^{+}$ subjected to an ultrashort, intense laser pulse by solving exactly the time-dependent Schrödinger equation for a one-dimensional model of ${\mathrm{H}}_3^{+}.$ Results of the simulations are analyzed by using three essential adiabatic field states |1〉, |2〉, and |3〉 that are adiabatically connected with the lowest three electronic states $X^1{\Sigma }_g^{+},$ $B^1{\Sigma }_u^{+},$ and $E^1{\Sigma }_g^{+}$ of the field free ion. We give also a simple MO (molecular orbital) picture in terms of these three states to illustrate the important electronic configurations in an intense field. The states |1〉, |2〉, and |3〉 are shown to be composed mainly of the configurations ${\mathrm{HHH}}^{+},$ ${\mathrm{HH}}^{+}\mathrm{H},$ and ${\mathrm{H}}^{+}\mathrm{HH},$ respectively in the presence of the field. We conclude that the overall level dynamics is governed mainly by transitions at the zero-field energy quasicrossings of these three states. The response of ${\mathrm{H}}_3^{+}$ to a laser field can be classified into two regimes. In the adiabatic regime $(R<\mathrm{}{R}_c),$ the electron transfers from one end of the molecule to the other end every half optical cycle thus creating the ionic component ${\mathrm{H}}^{+}{\mathrm{H}}^{+}{\mathrm{H}}^{\mathrm{-}}.$ In the diabatic regime $(R>\mathrm{}{R}_c),$ internuclear electron transfer is suppressed due to electron repulsion and laser induced localization. In the intermediate $\left(R\simeq R_c\right)$ region, where enhanced ionization occurs, the state |3〉 is most efficiently created by the field-induced nonadiabatic transitions between the states at quasicrossing points. The “quasistatic” laser-induced potential barriers are low enough for the electron to tunnel from the ascending (upper) well, thus confirming the quasistatic model at high intensities. Analytic expressions for the critical distance $R_c$ are obtained from this model and collective electron motion is inferred from the detailed time-dependent two-electron distributions.

Ionization of the hydrogen atom in an intense laser field is calculated using the classical-trajectory Monte Carlo method with allowance of tunneling at the classical turning points. With an initial condition of zero angular momentum, excellent agreement with an accurate quantum-mechanical calculation is obtained. The usually mentioned classical threshold for ionization at field strength ${0.0625e/a}_0^2$ is shown to be irrelevant to realistic pulses; the effective classical thresholds for an adiabatically ramped dc pulse are found to be ${0.080e/a}_0^2$ in one dimension and ${0.130e/a}_0^2$ in three dimensions.

Using 120-fs, 800-nm Ti:sapphire laser pulses, ionic yields of singly and doubly charged magnesium ions were measured as a function of intensity and laser field ellipticity. A clear “nonsequential” enhancement of the doubly charged ion is observed for circularly polarized light in addition to linearly polarized light. Over the entire intensity range the double-ionization yield is considerably higher for linear polarization than it is for circular polarization.

The vortex phase diagram in the external rotation frequency versus temperature is calculated for dilute Bose-Einstein condensed gases. It is determined within the Bogoliubov-Popov theory for a finite temperature where the condensate and noncondensate fractions are treated on an equal footing. The temperature dependences of various thermodynamic instability lines for the vortex nucleation are computed to construct the phase diagram. Experiments are proposed to resolve a recent controversy on the vortex creation problem associated with the quantized vortex observation in ${}^{87}\mathrm{Rb}$ atom gases.

For a Bose-Einstein condensate placed in a rotating trap, we give a simplified expression of the Gross-Pitaevskii energy in the Thomas Fermi regime, which only depends on the number and shape of the vortex lines. Then we check numerically, that when there is one vortex line, our simplified expression leads to solutions with a bent vortex for a range of rotational velocities and trap parameters that are consistent with the experiments.

The non-Markovian correlation spectra of spontaneous emission of an excited two-level atom are derived including the effect of finite size of the atom and all the possible contribution of allowed multipole radiations. The emission process is then analyzed by the quantum stochastic trajectory approach. The non-Markovian effect is counted in by expanding the original system to an enlarged system with Markovian reservoirs. In the case of a hydrogenlike atom with large atomic number Z, the deviation from the Weisskopf-Wigner result is quite evident.

The target used in x-ray lasers (XRL) is designed as an one-dimensional photonic crystals (1D PC’s) with 12 bilayers of ${\mathrm{SiO}}_2$ and ${\mathrm{TiO}}_2,$ at the center of which a defect layer made of target material is embedded. The optical thicknesses of the target layer and each layer of the others are one-half and one-quarter of the pumping laser wavelength, respectively, so that the pumping laser is the localized mode of the 1D PC’s. Compared with the common slab target, the pumping laser intensity in the target layer will be enhanced by about two orders. This is very important to the progress of x-ray lasers towards compactness, shorter wavelength, and higher-conversion efficiency. The gain coefficient will increase by a factor of five, hence, the target length in saturated XRL is reduced to one-fifth. X-ray laser intensity will increase by about four orders, and so does the conversion efficiency. The average ionization state increases with pumping laser intensity, therefore, x-ray lasers will shift towards shorter wavelength.

We study the amplification of a resonant standing-wave light field due to the interaction with a beam of monovelocity two-level atoms moving in the Raman-Nath regime and in the Bragg regime. The atomic density is low so that, at most, one atom is inside the cavity at a time. This system is very similar to the well-known micromaser but it is operating in the optical region of the field frequencies. Therefore, the situation corresponds to a microlaser. Unlike the micromaser system, the momentum transfer between the atoms and photons in the microlaser essentially effects the center-of-mass motion of the atoms and the evolution of the field.

A relation connecting the integrated total x-ray scattering intensity and the electron-pair relative-motion density at the origin or electron-electron coalescence density, I(0), is put forward. This link shows that the electron-electron coalescence density can be determined by the experiment. In addition, the proposed relation opens the possibility for assessing the quality of the theoretical calculations of the electron-electron coalescence density against experimental data. A series of atomic and molecular integrated x-ray scattering intensities are reported from the theoretically determined electron-electron coalescence data. Additionally, the integrated elastic and inelastic components are determined and their importance in describing the charge concentration and the exchange and correlation effects in molecular systems is discussed.

Two beams of collinear type-I biphotons generated via spontaneous parametric down-conversion (SPDC) from coherent pump beams are transformed without a loss into a state of correlated photons with orthogonal (in the general case, elliptical) polarizations. This alternative state manifests remarkable properties: while having the spectrum of type-I SPDC, it has polarization properties similar to type-II SPDC. To test the state, we use the anticorrelation effect (“anticorrelation dip”).

Motivated by decoherence theory, a heuristic formulation of quantum measurement theory is produced in which the macroscopic measuring instrument is explicitly included on the same footing as the quantum system being measured. The theory takes the form of a master equation for the joint density matrix of the measuring instrument and quantum system. The master equation refers to both systems in a symmetric way. Solution of the master equation using quantum trajectory theory furnishes a quasi-Hamiltonian evolution of the measuring process, punctuated by quantum jumps involving both the atomic system and the states of the measuring instrument. The projection postulate of quantum measurement theory emerges as a result from the formalism. A pervasive quantum Zeno effect and delayed detection are two phenomena, usually masked by the standard treatment of quantum measurement theory, which appear in this formalism.

We propose a protocol that enables a dealer to share a quantum secret with n players using less than n quantum shares for several access structures. For threshold schemes we derived an expression that shows how many quantum shares can be saved in this scheme. Also, several features that are available for classical secret-sharing schemes (and previously not known to be possible for quantum secret-sharing) become available with this protocol.

In a quantum computer the hardware and software are intrinsically connected because the quantum Hamiltonian (or more precisely its time development) is the code that runs the computer. We demonstrate this subtle and crucial relationship by considering the example of electron-spin-based solid-state quantum computer in semiconductor quantum dots. We show that multielectron quantum dots with one valence electron in the outermost shell do not behave simply as an effective single-spin system unless special conditions are satisfied. Our work compellingly demonstrates that a delicate synergy between theory and experiment (between software and hardware) is essential for constructing a quantum computer.

A large-scale, relativistic configuration-interaction (RCI) method has been developed for precision calculations of transition oscillator strengths. It is based on the no-pair Hamiltonian and employs finite B-spline basis functions. For the ${2s}^2{}^1{S}_0–2s2p{}^3{P}_1$ intercombination transition in berylliumlike carbon, the present RCI expansions reach close to 200 000 configurations, and include all single and double excitations from valence-valence, core-valence, and core-core interactions, along with dominant triple and quadruple excitations. Resulting length- and velocity-gauge transition rates are very well converged, but still differ by a factor of 2. This strong gauge dependence is found to arise from the neglect of negative-energy states which has negligible effects on length-gauge results but can affect velocity-gauge results significantly. The present intercombination transition rate for C III of $101.6\pm 0.7\hspace{0.5em}{\sec }^{-1}$ differs from the measured value of $102.94\pm 0.14\hspace{0.5em}{\sec }^{-1}$ [Doerfert et al., Phys. Rev. Lett. 78, 4355 (1997)] by about 1.3%.

A recent study suggests the existence of highly polar and nonpolar long-range Rydberg molecules under temperature and density conditions representative of those found in atomic Bose-Einstein condensates. The electronic wave functions of these Rydberg molecules are characterized by an elliptically shaped nodal structure. We use a combination of quantum and semiclassical methods to explore this unique nodal structure and its correlations with the Born-Oppenheimer potential curves of the molecules. We demonstrate a special “quasiseparability” that arises in elliptic coordinates at certain internuclear separations and give a semiclassical interpretation of this in terms of quantized Einstein-Brillouin-Keller orbits.