Gated combo nanodevice for sequential operations on single electron spin

An idea for a nanodevice in which an arbitrary sequence of three basic quantum single qubit gates - negation, Hadamard and phase shift - can be performed on a single electron spin. The spin state is manipulated using the spin-orbit coupling and the electron trajectory is controlled by the electron wave function self-focusing mechanism due to the electron interaction with the charge induced on metal gates. We present results of simulations based on iterative solution of the time dependent Schr\"odinger equation in which the subsequent operations on the electron spin can be followed and controlled. Description of the moving electron wave packet requires evaluation of the electric field within the entire nanodevice in each time step

Accuracy of the Hartree-Fock method for Wigner molecules at high magnetic fields

Few-electron systems confined in two-dimensional parabolic quantum dots at high magnetic fields are studied by the Hartree-Fock (HF) and exact diagonalization methods. A generalized multicenter Gaussian basis is proposed in the HF method. A comparison of the HF and exact results allows us to discuss the relevance of the symmetry of the charge density distribution for the accuracy of the HF method. It is shown that the energy estimates obtained with the broken-symmetry HF wave functions become exact in the infinite magnetic-field limit. In this limit the charge density of the broken-symmetry solution can be identified with the classical charge distribution.Comment: to appear in EPJ

Magnetic-field asymmetry of electron wave packet transmission in bent channels capacitively coupled to a metal gate

We study the electron wave packet moving through a bent channel. We demonstrate that the packet transmission probability becomes an uneven function of the magnetic field when the electron packet is capacitively coupled to a metal plate. The coupling occurs through a non-linear potential which translates a different kinetics of the transport for opposite magnetic field orientations into a different potential felt by the scattered electron

Magnetic-field-induced binding of few-electron systems in shallow quantum dots

Binding of few-electron systems in two-dimensional potential cavities in the presence of an external magnetic field is studied with the exact diagonalization approach. We demonstrate that for shallow cavities the few-electron system becomes bound only under the application of a strong magnetic field. The critical value of the depth of the cavity allowing the formation of a bound state decreases with magnetic field in a non-smooth fashion, due to the increasing angular momentum of the first bound state. In the high magnetic field limit the binding energies and the critical values for the depth of the potential cavity allowing the formation of a bound system tend to the classical values

Stark effect on the exciton spectra of vertically coupled quantum dots: horizontal field orientation and non-aligned dots

We study the effect of an electric-field on an electron-hole pair in an asymmetric system of vertically coupled self-assembled quantum dots taking into account their non-perfect alignment. We show that the non-perfect alignment does not qualitatively influence the exciton Stark effect for the electric field applied in the growth direction, but can be detected by application of a perpendicular electric field. We demonstrate that the direction of the shift between the axes of non-aligned dots can be detected by rotation of a weak electric field within the plane of confinement. Already for a nearly perfect alignment the two-lowest energy bright exciton states possess antilocked extrema as function of the orientation angle of the horizontal field which appear when the field is parallel to the direction of the shift between the dot centers

Electron spin and charge switching in a coupled quantum dot quantum ring system

Few-electron systems confined in a quantum dot laterally coupled to a surrounding quantum ring in the presence of an external magnetic field are studied by exact diagonalization. The distribution of electrons between the dot and the ring is influenced by the relative strength of the dot and ring confinement, the gate voltage and the magnetic field which induces transitions of electrons between the two parts of the system. These transitions are accompanied by changes in the periodicity of the Aharonov-Bohm oscillations of the ground-state angular momentum. The singlet-triplet splitting for a two electron system with one electron confined in the dot and the other in the ring exhibits piecewise linear dependence on the external field due to the Aharonov-Bohm effect for the ring-confined electron, in contrast to smooth oscillatory dependence of the exchange energy for laterally coupled dots in the side-by-side geometry.Comment: to appear in PRB in August 200

One-dimensional pair cascade emission in gamma-ray binaries

In gamma-ray binaries such as LS 5039 a large number of electron-positron pairs are created by the annihilation of primary very high energy (VHE) gamma-rays with photons from the massive star. The radiation from these particles contributes to the total high energy gamma-ray flux and can initiate a cascade, decreasing the effective gamma-ray opacity in the system. The aim of this paper is to model the cascade emission and investigate if it can account for the VHE gamma-ray flux detected by HESS from LS 5039 at superior conjunction, where the primary gamma-rays are expected to be fully absorbed. A one-dimensional cascade develops along the line-of-sight if the deflections of pairs induced by the surrounding magnetic field can be neglected. A semi-analytical approach can then be adopted, including the effects of the anisotropic seed radiation field from the companion star. Cascade equations are numerically solved, yielding the density of pairs and photons. In LS 5039, the cascade contribution to the total flux is large and anti-correlated with the orbital modulation of the primary VHE gamma-rays. The cascade emission dominates close to superior conjunction but is too strong to be compatible with HESS measurements. Positron annihilation does not produce detectable 511 keV emission. This study provides an upper limit to cascade emission in gamma-ray binaries at orbital phases where absorption is strong. The pairs are likely to be deflected or isotropized by the ambient magnetic field, which will reduce the resulting emission seen by the observer. Cascade emission remains a viable explanation for the detected gamma-rays at superior conjunction in LS 5039.Comment: 8 pages, 7 figures, 1 table, accepted for publication in Astronomy and Astrophysic