The potential of Meyer and Frommhold describing the interaction between hydrogen and helium atoms is used to calculate the diffusion coefficients of hydrogen atoms in helium gas and helium atoms in hydrogen gas as functions of temperature, taking into account higher-order corrections of transport theory. Comparison with experiment suggests that the interaction potential should be steepened at small nuclear separations. Calculations of the diffusion coefficients for temperatures between zero and 1000 K with the modified potential are reported. The corresponding H-He scattering length resulting from the zero-temperature value of the diffusion coefficients has the magnitude ${0.359a}_0.$ Direct calculation shows it to be negative.

We show that a dense spectrum of chaotic multiply excited eigenstates can play a major role in collision processes involving many-electron multicharged ions. A statistical theory based on chaotic properties of the eigenstates enables one to obtain relevant energy-averaged cross sections in terms of sums over single-electron orbitals. Our calculation of low-energy electron recombination of ${\mathrm{Au}}^{25+}$ shows that the resonant process is 200 times more intense than direct radiative recombination, which explains the recent experimental results of Hoffknecht et al. [J. Phys. B $31,$ 2415 (1998)].

Most methods for experimentally reconstructing the quantum state of light involve determining a quasiprobability distribution such as the Wigner function. In this paper we present a scheme for measuring individual density matrix elements in the photon number state representation. Remarkably, the scheme is simple, involving two beam splitters and a reference field in a coherent state.

The stability of the ${(m}^{2+}{,e}^{-}{,e}^{-}{,e}^{+})$ system is studied as a function of the mass of the $m^{2+}$ particle. The system is found to be stable for all values of $m^{2+}<{0.68m}_e.$ At the smallest values of $m^{2+}$ the system evolves into a $m^{2+}+{\mathrm{Ps}}^{-}$ configuration, which is absolutely stable since both fragments are electrically charged. This suggests that a positron can be expected to bind to all the group II and IIB atoms with the possible exception of mercury.

The photoelectron angular distribution asymmetry parameter β for $3s$ electrons of atomic chlorine has been calculated using the multiconfiguration Hartree-Fock method for both bound and continuum wave functions. It is determined in the electric dipole approximation within the framework of the angular-momentum-transfer formulation. The effects of electron correlation are investigated, and it is shown that electron correlations are very important for β values corresponding to both ${}^{3}P$ and ${}^{1}P$ ionic states. Our results confirm the recent experimental prediction that β for s subshell photoionization in open-shell atoms, is, in general, term and photon energy dependent and, in addition, in the case of chlorine $3s$ photoionization, β has minima for both terms of ${\mathrm{Cl}}^{+}$ ${3s(}^{3}P,{}^{1}P),$ in qualitative agreement with experiment. However, our calculation of β for ${\mathrm{Cl}}^{+}$ ${3s(}^{3}P,{}^{1}P)$ channels demonstrates that interchannel coupling should be included in order to achieve quantitative agreement with experiment.

We study the pairing instability and mechanical collapse of a dilute homogeneous Bose gas with an attractive interaction. The pairing phase is found to be a saddle point and is unstable against pairing fluctuations. This pairing saddle point exists above a critical temperature. Below this critical temperature, the system is totally unstable in the pairing channel. Thus the system could collapse in the pairing channel in addition to mechanical collapse. The critical temperatures of pairing instability and mechanical collapse are higher than the Bose-Einstein condensation (BEC) temperature of an ideal Bose gas with the same density. When fluctuations are taken into account, we find that the critical temperature of mechanical collapse is even higher. The difference between the collapse temperature and the BEC temperature is proportional to $(n|a_s{|}^3{)}^{2/9},$ where n is the density and $a_s$ is the scattering length.

We consider the simulation of the dynamics of one nonlocal Hamiltonian by another, allowing arbitrary local resources but no entanglement or classical communication. We characterize notions of simulation, and proceed to focus on deterministic simulation involving one copy of the system. More specifically, two otherwise isolated systems A and B interact by a nonlocal Hamiltonian $H\ne H_A{+H}_B.$ We consider the achievable space of Hamiltonians $H^{\prime }$ such that the evolution $e^{-{\mathrm{iH}}^{\prime }t}$ can be simulated by the interaction H interspersed with local operations. For any dimensions of A and B, and any nonlocal Hamiltonians H and $H^{\prime },$ there exists a scale factor s such that for all times t the evolution $e^{-{\mathrm{iH}}^{\prime }\mathrm{st}}$ can be simulated by H acting for time t interspersed with local operations. For two-qubit Hamiltonians H and $H^{\prime },$ we calculate the optimal s and give protocols achieving it. The optimal protocols do not require local ancillas, and can be understood geometrically in terms of a polyhedron defined by a partial order on the set of two-qubit Hamiltonians.

Read-only-memory-based (ROM-based) quantum computation (QC) is an alternative to oracle-based QC. It has the advantages of being less “magical,” and being more suited to implementing space-efficient computation (i.e., computation using the minimum number of writable qubits). Here we consider a number of small (one- and two-qubit) quantum algorithms illustrating different aspects of ROM-based QC. They are: (a) a one-qubit algorithm to solve the Deutsch problem; (b) a one-qubit binary multiplication algorithm; (c) a two-qubit controlled binary multiplication algorithm; and (d) a two-qubit ROM-based version of the Deutsch-Jozsa algorithm. For each algorithm we present experimental verification using nuclear magnetic resonance ensemble QC. The average fidelities for the implementation were in the ranges 0.9–0.97 for the one-qubit algorithms, and 0.84–0.94 for the two-qubit algorithms. We conclude with a discussion of future prospects for ROM-based quantum computation. We propose a four-qubit algorithm, using Grover’s iterate, for solving a miniature “real-world” problem relating to the lengths of paths in a network.

The ideas underlying dynamical decoupling methods are revisited within the framework of quantum-information processing, and their potential for direct implementations in terms of encoded rather than physical degrees of freedom is examined. The usefulness of encoded decoupling schemes as a tool for engineering both closed- and open-system encoded evolutions is investigated based on simple examples.

In Li et al.’s work on optimal probabilistic teleportation in the two-level case [W. L. Li, C. F. Li, and G. C. Guo, Phys. Rev. A 61, 034301(2000)], the authors consider the extraction of the unknown qubit $|\phi 〉={\alpha }_0|0〉+{\alpha }_1|1〉$ from the qubit $|\psi 〉=(1/\sqrt{N})({\alpha }_0{\beta }_0|0〉+{\alpha }_1{\beta }_1|1〉).\mathrm{}$ In this paper, we consider the extraction in the n-level case. It is proved that, under some specific collective unitary transformation U on $|\psi 〉$ and auxiliary qubits as probe, the maximal probability of successfully extracting the wanted qubit is $\mathrm{}(1/N)\mathrm{}\min \{{\beta }_i^2\}.$ It is also shown that the entries of such U are independent of all unknown ${\alpha }_i.$ The result can also be used in the purification of the entanglement via entanglement swapping.

Coupled-cluster calculations starting from exchange-only local-density approximation (XLDA), Krieger-Li-Iafrate (KLI), and Kohn-Sham (KS) wave functions are compared with those using the Hartree-Fock (HF) determinant as a reference. The total energies are found to be close, the difference being maximally 2 mhartree in the systems studied (the first terms in the He, Be, Ne, Mg, Ar isoelectronic series). The convergence is, however, sensitive to the choice of the reference: KLI and KS converge, in general, faster than HF in spite of being a worse approximation in the first two orders of perturbation theory. The improvement of convergence due to the use of the KLI or KS references is more pronounced in the systems showing near degeneracy, such as in the Be series. For XLDA, the convergence properties are either comparable to those of KLI or oscillatory, depending on the system. In a second part, the numerical results are analyzed (in the HF and KLI cases) by using first-order developments with respect to nuclear charge Z at large $Z.\mathrm{}$

Low-temperature binary collision-induced light-scattering spectra from gaseous helium are reported. Both components, anisotropic and isotropic, are deduced from our measurements and calibrated on an absolute scale. Comparison of the experimental spectra with theoretical profiles computed quantum mechanically is made, by using as input up-to-date interaction potential and incremental polarizability functions. The present study confirms the tendencies we have previously observed at room temperature and strengthens our confidence in certain ab initio models.

We show that the resolvent $G\left(E\right)=1/\left(E-H\right)$ can be represented by a series that converges exponentially in the square root of the number of terms. The (arbitrary) Hamiltonian H can be constructed on a real basis, and as the basis is enlarged, the accuracy of the series improves. We illustrate the method by an application to the ac Stark shift and width of atomic hydrogen.

Ejected electrons from the decay of metastable ${2p}^{5}3s3p{}^4{D}_{7/2}$ autoionizing states of Na-like ${\mathrm{Ar}}^{7+},\hspace{0.5em}{\mathrm{Cl}}^{6+},$ and ${\mathrm{S}}^{5+}$ have been detected using an ion-beam delayed-coincidence technique. The previously calculated and measured ${\mathrm{Ar}}^{7+}$ Auger line at $114.10\pm 0.33\hspace{0.5em}\mathrm{eV}$ was used to calibrate a high-efficiency two-stage parallel-plate electron spectrometer. The energies of the corresponding Auger lines from decay of metastable ${\mathrm{Cl}}^{6+}$ and ${\mathrm{S}}^{5+}$ were subsequently determined to be $102.0\pm 1.2\hspace{0.5em}\mathrm{eV}$ and $86.6\pm 1.0\hspace{0.5em}\mathrm{eV},$ respectively. The results are consistent with published data for other members of the Na isoelectronic sequence, and they together exhibit a quadratic scaling with ionic charge q over the range $4<~q<~15.$

A computationally efficient analytic form of the Born-approximation electron-impact ionization amplitude is derived for general neutral-atom targets. High-quality Hartree-Fock Slater orbitals are used to model the target wave function. Full orthogonalization of the continuum Coulomb wave to all occupied orbitals of the target atom is enforced. Results are presented for noble gases (Ne, Ar, Kr, and Xe), selected transition metals (Fe, Cu, and Ag), and elements from the fourth, fifth, and sixth columns of the periodic table (Si, Ge, Sn, P, As, Sb, S, Se, and Te), where theoretical comparisons are lacking. Full orthogonalization significantly improves agreement with experimental data for the noble-gas series compared to previous Born models. Overall agreement with all elements is uniformly good and variations within each series are systematic.

Total ionization cross sections of neutral carbon, nitrogen, and oxygen atoms by electron impact are presented. In our theory we have included the possibilities that (a) some target atoms used in an experiment were in metastable states close to the ground state, (b) excitation-autoionization of ${2s2p}^m$ exited states may be substantial, and (c) ions produced in experiments may be in excited, low-lying metastable states. The binary-encounter Bethe (BEB) model of Kim and Rudd [Phys. Rev. A 50, 3954 (1994)] is used to calculate the cross sections for direct ionization. Plane-wave Born cross sections scaled by the method developed by Kim [Phys. Rev. 64, 032713 (2001)] are used to determine the contributions from excitation-autoionization. A sum of the BEB cross sections for direct ionization weighted according to the statistical weights of the final ion states is used to modify the direct ionization cross sections. The combination of the BEB model and the scaled Born cross sections is in excellent agreement with available experimental data. The present method can easily be extended to heavier open-shell atoms.

We have carried out an $\mathrm{}(e,2e)\mathrm{}$ experiment that investigates ionization in Xe over the autoionizing region between the ${}^2{P}_{3/2}$ and ${}^2{P}_{1/2}$ ionic thresholds. Manipulation of $\mathrm{}(e,2e)\mathrm{}$ energy spectra reveals interference effects between odd- and even-parity multipole ionization amplitudes. The data is analyzed in terms of a simple model that demonstrates that the data contains both magnitude and phase information about these complex ionization amplitudes.

We report the observation of pronounced, spectrally narrow laser-induced continuum structures (LICS) in the two ionization continua of xenon. A nanosecond laser pulse with nearly transform-limited bandwidth induces a LICS by coupling the initially unpopulated ${5p}^{5}9p[1/2{]}_0$ state to the continuum. The LICS is probed by one-photon ionization of the ${5p}^{5}6p[1/2{]}_0$ state. This state is populated in an independent excitation step by an additional laser pulse acting prior to the probe and dressing pulses. The two ionization channels that leave the xenon ion in either the ${}^2{P}_{1/2}$ or the ${}^2{P}_{3/2}$ fine-structure state are detected separately by monitoring the kinetic energy of the photoelectrons in a time-of-flight electron spectrometer. A suppression of the photoelectron signal of up to 80% as compared to the far off resonance value is achieved. The line shapes reveal a Fano parameter close to zero. The LICS is identical for the coupling into the two continua. Therefore the branching ratio into the two ionization channels does not vary with the dressing-laser detuning for the coupling scheme implemented here.

An interaction of a high-spin molecule with a bifrequency ac magnetic field, occurring at times much shorter than the molecule relaxation times, has been considered. The molecule is subjected to a dc magnetic field perpendicular to the easy anisotropy axis of the molecule. The bifrequency ac field is a superposition of two ac fields, one of which is perpendicular to the easy anisotropy axis and causes resonant transitions between the lower states of the fundamental and first excited doublets. The other ac field is parallel to the easy anisotropy axis and has a frequency much smaller than the frequency of the first ac field. It has been shown that, first, the molecule can absorb or emit energy, depending on the frequency of the low-frequency ac field, second, the bifrequency ac magnetic field induces tunneling of the molecule magnetization with the Rabi frequency. The conditions of observation of the effects predicted are discussed.

We consider the Josephson effect between two spatially separated Bose-Einstein condensates of atoms each of which can be in two hyperfine states. We derive simple equations of motion for this system closely analogous to the Bloch equations. We also map the dynamics of the system onto those of a classical particle in a well. We find density and spin modes of oscillation and stable equilibrium points of the motion that are unstable in the spinless case. Finally we analyze the oscillation modes in the spin-1 $\mathrm{}(F=1\mathrm{})\mathrm{}$ case.