Andrei Y. Istomin

Perturbative calculation of the triply differential cross section for photo-double-ionization of He

Single-photon, two-electron ionization of He is analysed, taking into account electron correlation using lowest-order perturbation theory and including all individual electron angular momenta in the final two-electron continuum. Perturbative account of electron correlation in the final state, which describes the so-called TS-1 mechanism of double photoionization, combined with a variational account of electron screening, is found to provide results for the triply differential cross section at an excess energy of 20 eV that are in excellent agreement with both absolute experimental data and results of non-perturbative calculations, for all kinematics of the process in which the TS-1 mechanism is expected to dominate.

Nondipole effects in double photoionization of He at 450 eV excess energy

Convergent close-coupling results for the triply differential cross section for double photoionization of He that include dipole-quadrupole terms are shown to have improved agreement (as compared to dipole approximation results) with recent experiments using linearly polarized light (Knapp A et al 2005 J. Phys. B: At. Mol. Opt. Phys. 38 615) for a number of kinematical configurations.

Magnetic materials for finite-temperature quantum computing

The potential use of interacting magnetic nanodots for quantum computing (qubit) operations is investigated by model calculations. The quantum entanglement of the low-lying ferromagnetic states, as quantified by the concurrence, exhibits a resonant peak whose position and width depend on parameters such as dot anisotropy, interdot exchange, and external field gradient. The maximum operation temperature is proportional to the magnetocrystalline anisotropy of the dot material. A specific condition is that the dots are sufficiently small so that the interatomic exchange ensures a coherent magnetization state and quantum coherence at finite temperatures. From a material point of view, there is a quite rigid upper limit of about 100 K, but to avoid decoherence it will be necessary to sacrifice a substantial fraction of this temperature, probably at least one order of magnitude.

Nondipole Effects in Double Photoionization of He

Lowest‐order nondipole effects are studied in double photoionization (DPI) of the He atom. Ab initio parametrizations of the quadrupole transition amplitude for DPI from the 1S0‐state are presented in terms of the exact two‐electron reduced matrix elements. Parametrizations for the dipole‐quadrupole triply differential cross section (TDCS) and doubly differential cross section (DDCS) are presented in terms of polarization‐independent amplitudes for the case of an elliptically polarized photon. Expressions for the DDCS in terms of the reduced two‐electron matrix elements are also given. A general analysis of retardation‐induced asymmetries of the TDCS including the circular dichroism effect at equal energy sharing is presented. Our numerical results exhibit a nondipole forward‐backward asymmetry in the TDCS for DPI of He at an excess energy of 450 eV that is in qualitative agreement with existing experimental data.