The photoinduced $K\alpha$ x-ray spectra of elements between sodium and chlorine were measured by means of high-resolution crystal diffractometry. The $K{\alpha }_{1,2}{L}^{\mathrm{}\left(1\right)\mathrm{}}$ satellites were observed and resolved from the corresponding diagram lines. The probabilities for producing via shake processes $K^{\mathrm{}\left(1\right)\mathrm{}}{L}^{\mathrm{}\left(1\right)\mathrm{}}$ double-hole states were deduced from the satellite to parent diagram line yield ratios. The results are compared to theoretical sudden approximation predictions. A satisfactory agreement is found except for sodium and chlorine for which our experimental results are markedly underestimated by the calculations.

Total cross sections (TCSs) for electron scattering by polyatomic molecules ${\mathrm{C}}_2{\mathrm{F}}_4,$ ${\mathrm{C}}_2{\mathrm{F}}_6,$ ${\mathrm{C}}_3{\mathrm{F}}_6,$ ${\mathrm{C}}_3{\mathrm{F}}_8,$ ${\mathrm{C}}_6{\mathrm{F}}_6,$ and ${\mathrm{C}}_6{\mathrm{H}}_6$ are calculated in the incident electron energy range 30–3000 eV employing the 100% additivity rule and energy-dependent geometric additivity rule (EGAR) approaches. The EGAR, proposed by Jiang et al. [J. Phys. B 30, 5025 (1997); Phys. Lett. A 237, 53 (1997)], relates to molecular properties and the energy of incident electrons. Two approaches for the TCS of ${\mathrm{C}}_2{\mathrm{F}}_6,$ ${\mathrm{C}}_3{\mathrm{F}}_8,$ ${\mathrm{C}}_6{\mathrm{F}}_6,$ and ${\mathrm{C}}_6{\mathrm{H}}_6$ molecules are compared and the EGAR yields better accord with available experimental measurements in the whole energy region. The new results for ${\mathrm{C}}_2{\mathrm{F}}_4$ and ${\mathrm{C}}_3{\mathrm{F}}_6$ are also presented although no experimental data are available for comparison. The atoms are presented by spherical complex optical potential, which is composed of static, exchange, polarization, and absorption terms.

The relative contribution of double ionization versus single ionization of heavy atoms (with an atomic number $Z\gtrsim 20)$ by electrons is investigated. General expressions are derived that predict the contribution to the collisions where the outer shells of the target atoms are s or d shells with two electrons, a configuration of most atoms of the Periodic Table.

We report on an experiment to determine both the group and phase delays experienced by orthogonally polarized photon pairs traveling through a birefringent medium. Both types of delay are determined from the same set of coincidence-counting data. The experiment is based on an interference technique using two-photon multipath indistinguishability to produce an interference feature. Earlier work has shown that this interference feature can be used to measure the group velocity of single-photon wave packets in dielectric media. In the current work, the two-photon interferometer has been modified to produce an additional interference feature that is sensitive to the phase velocity of the light. We have used this technique to simultaneously measure the group delay in crystal quartz with a precision of 0.1 fs and the phase delay with a precision of 8 attoseconds. Our analysis clarifies the effects of group and phase delays and shows the unexpected result that dispersive temporal broadening, which is well known to be canceled for the original interferometer setup, is not canceled for this type of “postponed compensation” interferometer.

An algorithm based on local scaling transformations for electronic structure calculations that scales linearly with the size of the system is presented. The key feature of the method is the absence of the orthogonalization step during iterative minimization. We illustrate the feasibility and potential of the method by applying it to total energy calculations for a variety of small clusters, viz., ${\mathrm{Na}}_2,$ ${\mathrm{Na}}_7\mathrm{Al},$ ${\mathrm{Na}}_{20},$ ${\mathrm{Si}}_4,$ and ${\mathrm{Al}}_{13}.$ The method is easily parallelizable and therefore has the potential to deal with large real life systems.

Ideally, quantum teleportation should transfer a quantum state without distortion and without providing any information about that state. However, quantum teleportation of continuous electromagnetic-field variables introduces additional noise, limiting the fidelity of the quantum-state transfer. In this article, the operator describing the quantum-state transfer is derived. The transfer operator modifies the probability amplitudes of the quantum state in a shifted photon-number basis by enhancing low photon numbers and suppressing high photon numbers. This modification of the statistical weight corresponds to a measurement of finite resolution performed on the original quantum state. The limited fidelity of quantum teleportation is thus shown to be a direct consequence of the information obtained in the measurement.

We propose schemes that are efficient when each pair of qubits undergoes some imperfect collective decoherence with different baths. In the proposed scheme, each pair of qubits is first encoded in a decoherence-free subspace composed of two qubits. Leakage out of the encoding space generated by the imperfection is reduced by the quantum Zeno effect. Phase errors in the encoded bits generated by the imperfection are reduced by concatenation of the decoherence-free subspace with either a three-qubit quantum error correcting code that corrects only phase errors or a two-qubit quantum error detecting code that detects only phase errors connected with the quantum Zeno effect again.

We discuss a quantum key distribution scheme in which small phase and amplitude modulations of cw light beams carry the key information. The presence of EPR-type correlations provides the quantum protection. We identify universal constraints on the level of shared information between the intended receiver (Bob) and any eavesdropper (Eve) and use this to make a general evaluation of security. We identify teleportation as an optimum eavesdropping technique.

The role of avoided crossings for resonant x-ray Raman scattering is explored. The strong dependence of the electronic transition amplitudes on internuclear distance at avoided crossings causes new, and even anomalous, spectral features, and can bring about a breakdown of the generally accepted participator or spectator classification of the transitions. We report, and apply the theory on, an experimental observation of the electron decay of the N $1\overrightarrow{s}{\pi }^{*}$ core-excited level in ${\mathrm{N}}_2$ the $B^2{\Sigma }_u^{+}$ final state with peculiar, non-Franck-Condon, vibrational intensity distributions.

We have studied the exchange-correlation hole $n_{\mathrm{xc}}$ and pair-correlation function in the valence shell of the Si atom in its ${}^{3}P$ ground state, using highly accurate Slater-Jastrow wave functions and the variational Monte Carlo method. The exchange-correlation hole shows a number of interesting features caused by the open shell structure of Si, including a marked transition from efficient to poor screening behavior as a test majority-spin electron is moved from the center of a valence p orbital onto the axis perpendicular to the occupied p orbitals. This behavior results from the dramatic difference in the exchange hole in the two cases, which is partially compensated by a corresponding anisotropy in the correlation hole. In addition we observe an anisotropic change in the spin density induced by Coulomb correlation, reducing the spatial overlap between the different spin components of the density and contributing to the anisotropy of the correlation hole. The exclusion effect correlation, in which a ${3s}^2\to {3p}^2$ substitution excitation is allowed along the unoccupied axis and prohibited along the other axes, was found to have a noticeable effect on the correlation hole and is partially accountable for its anisotropy; however, it is inconsistent with the observed changes in spin density. In contrast to the longer-range features, we find that the “on-top” correlation hole is well described by linear density-functional theory for a large range of local density and magnetization.

The interaction energies for ${\mathrm{N}}_2-\mathrm{He}$ and ${\mathrm{N}}_2-{\mathrm{H}}_2$ are calculated by accurate ab initio methods. The virial coefficient and differential scattering cross section for ${\mathrm{N}}_2-{\mathrm{H}}_2$ are calculated; the theoretical results are compared with experimental data. The transport collision integrals for ${\mathrm{N}}_2-{\mathrm{H}}_2$ and ${\mathrm{N}}_2-{\mathrm{N}}_2$ interactions are calculated and tabulated; the results yield transport coefficients that compare well with measured data. Transport coefficients are found to be determined accurately from the interaction energies for a specific configuration of the molecule formed from the interaction partners. Comparisons with results of measurement and accurate calculations demonstrate that the transport properties of complex molecular interactions can be determined rapidly and fairly accurately from the interaction energies of simpler systems using combination rules for the short-range parameters of effective interaction energies and the coefficients for the long-range forces. The coefficients for a two-parameter temperature expansion of diffusion and viscosity are tabulated for a realistic universal potential-energy that is based primarily on the results of very accurate calculations of the He-He interaction energy.

Calculated elastic differential, integral, and momentum transfer cross sections as well as total (elastic + inelastic) cross sections for electron-${\mathrm{CH}}_4$ collisions are reported in the $(20\mathrm{}–500)-\mathrm{eV}$ energy range. Four model potentials of both a nonempirical and semiempirical nature are used to represent absorption effects. The Schwinger variational iterative method combined with the distorted-wave approximation is used to solve the scattering equations. Through the comparison of our calculated results with available experimental data, two of these model absorption potentials are recommended as more convenient for treating electron-molecule collision problems.

In this paper, we report on theoretical calculations of electron-impact excitation cross sections and radiative transition rates for ${\mathrm{Mg}}^{10+}.$ The excitation cross sections were calculated using semirelativistic close-coupling and fully relativistic distorted-wave theory and the radiative rates were determined using semirelativistic and fully relativistic atomic-structure theory. After the solution of the corresponding collisional-radiative equations, the $K{\alpha }_2/K{\alpha }_1$ $(1s2p{\hspace{0.5em}}^3{P}_1\to {1s}^2{\hspace{0.5em}}^1{S}_0$ over $1s2p{\hspace{0.5em}}^1{P}_1\to {1s}^2{\hspace{0.5em}}^1{S}_0)$ emission line ratio and the $K{\beta }_2/K{\beta }_1$ $(1s3p{\hspace{0.5em}}^3{P}_1\to {1s}^2{\hspace{0.5em}}^1{S}_0$ over $1s3p{\hspace{0.5em}}^1{P}_1\to {1s}^2{\hspace{0.5em}}^1{S}_0)$ emission line ratio were calculated as a function of electron temperature and density. The various scattering calculations involving different numbers of levels enabled us to study the influence of resonance structures and cascades from highly excited levels on the collisional-radiative modeling and we found that they have little effect on the level populations. However, even in this ten-times ionized species, the effects of orbital relaxation are found to be important in the determination of accurate electric-dipole radiative transition rates. Both line ratios were found to be strongly affected by whether the magnetic-dipole radiative transition from the $1s2s$ ${}^3{S}_1$ level to the ground state was included or not. At very low electron densities, the $1s2s$ ${}^1{S}_0$ two-photon transition to the ground state also has an effect on the $K{\alpha }_2/K{\alpha }_1$ line ratio. In addition, we found that the line ratios are enhanced at high temperatures by radiative and dielectronic recombination from the hydrogenic ${\mathrm{Mg}}^{11+}$ ion. However, the dielectronic satellite lines have no effect on the line ratios for the low-density astrophysical, solar, and magnetic-fusion plasmas considered in this paper.

Baym, Blaizot, and Zinn-Justin have recently used the large-$N$ approximation to calculate the effect of interactions on the transition temperature of dilute Bose gases. We extend their calculation to next-to-leading order in $1/N$ and find a relatively small correction of $-26\%$ to the leading order result. This suggests that the large-$N$ approximation works surprisingly well in this application.

We show that the reduction in photon-number fluctuations at laser threshold often cited as a fundamental laser property does not occur in small semiconductor lasers. The conventional theory of threshold noise is not valid in lasers with a spontaneous emission factor larger than ${10}^{-8}.$ If the spontaneous-emission factor is larger than ${10}^{-4},$ the photon-number statistics even remain thermal far above threshold. We therefore conclude that the reduction in photon-number fluctuations is not a fundamental laser property but rather a matter of size and the corresponding relative importance of quantum fluctuations above threshold.

Every measurement on a quantum system causes a state change from the system state just before the measurement to the system state just after the measurement conditional upon the outcome of measurement. This paper determines all the possible conditional state changes caused by measurements of nondegenerate discrete observables. For this purpose, the following conditions are shown to be equivalent for measurements of nondegenerate discrete observables: (i) The joint probability distribution of the outcomes of successive measurements depends affinely on the initial state. (ii) The apparatus has an indirect measurement model. (iii) The state change is described by a positive superoperator valued measure. (iv) The state change is described by a completely positive superoperator valued measure. (v) The output state is independent of the input state and the family of output states can be arbitrarily chosen by the choice of the apparatus. The implications to the measurement problem are discussed briefly.

We introduce the concepts of Grover operators and Grover kernels to systematically analyze Grover’s searching algorithms. Then we investigate a one-parameter family of quantum searching algorithms of Grover type and we show that the standard Grover algorithm is a distinguished member of this family. We show that all the algorithms of this class solve the searching problem with an efficiency of order $O\left(\sqrt{N}\right),$ with a coefficient which is class-dependent. The analysis of this dependence is a test of the stability and robustness of the algorithms. We show the stability of this constructions under perturbations of the initial conditions and extend them to a very general class of Grover operators.

Scattering of positronium (Ps) by a helium atom has been investigated in a three-Ps-state coupled-channel model including $\mathrm{Ps}\mathrm{}(1s,2s,2p)\mathrm{}$ states using a recently proposed time-reversal-symmetric regularized electron-exchange model potential. Specifically, we report results of differential cross sections for elastic scattering and target-elastic Ps excitations. We also present results for total and different partial cross sections and compare them with experiment and other calculations.

We have investigated the ground-state geometries of ${\mathrm{Li}}_n\mathrm{Be}$ and ${\mathrm{Li}}_n\mathrm{Mg}(n=1\mathrm{}–12)$ clusters using ab initio molecular dynamics. These divalent impurities Be and Mg induce different geometries and follow a different growth path for $n>\mathrm{}5.\mathrm{}$ ${\mathrm{Li}}_n\mathrm{Mg}$ clusters are significantly different from the host geometries while ${\mathrm{Li}}_n\mathrm{Be}$ clusters can be approximately viewed as Be occupying an interstitial site in the host. Our results indicate that Be gets trapped inside the Li cage, while Mg remains on the surface of the cluster. Mg-induced geometries become three-dimensional earlier at $n=4\mathrm{}$ as compared to the Be system. In spite of a distinct arrangement of atoms in both cases the character of the wave functions in the d manifold is remarkably similar. In both cases an eight valence electron system has been found to be the most stable, in conformity with the spherical jellium model.

Using superintense 46-fs Ti:sapphire laser pulses, high-order harmonic generation in argon gaseous medium is experimentally studied. When the driving laser intensity is pushed well above the ionization threshold (saturation intensity) of the medium applied, the double-peak spectral splitting of high harmonics has been clearly observed. This splitting is mainly due to the propagation effects in the rapidly ionizing medium, which is verified by means of one-dimensional calculations. The numerical simulations agree well with the experimental results.