Lower bound for electron spin entanglement from beamsplitter current correlations

We determine a lower bound for the entanglement of pairs of electron spins injected into a mesoscopic conductor. The bound can be expressed in terms of experimentally accessible quantities, the zero-frequency current correlators (shot noise power or cross-correlators) after transmission through an electronic beam splitter. The effect of spin relaxation (T_1 processes) and decoherence (T_2 processes) during the ballistic coherent transmission of the carriers in the wires is taken into account within Bloch theory. The presence of a variable inhomogeneous magnetic field allows the determination of a useful lower bound for the entanglement of arbitrary entangled states. The decrease in entanglement due to thermally mixed states is studied. Both the entanglement of the output of a source (entangler) and the relaxation (T_1) and decoherence (T_2) times can be determined.Comment: 4 pages, 3 figure

A discontinuous Galerkin method for the Vlasov-Poisson system

A discontinuous Galerkin method for approximating the Vlasov-Poisson system of equations describing the time evolution of a collisionless plasma is proposed. The method is mass conservative and, in the case that piecewise constant functions are used as a basis, the method preserves the positivity of the electron distribution function and weakly enforces continuity of the electric field through mesh interfaces and boundary conditions. The performance of the method is investigated by computing several examples and error estimates associated system's approximation are stated. In particular, computed results are benchmarked against established theoretical results for linear advection and the phenomenon of linear Landau damping for both the Maxwell and Lorentz distributions. Moreover, two nonlinear problems are considered: nonlinear Landau damping and a version of the two-stream instability are computed. For the latter, fine scale details of the resulting long-time BGK-like state are presented. Conservation laws are examined and various comparisons to theory are made. The results obtained demonstrate that the discontinuous Galerkin method is a viable option for integrating the Vlasov-Poisson system.Comment: To appear in Journal for Computational Physics, 2011. 63 pages, 86 figure

Optical readout of charge and spin in a self-assembled quantum dot in a strong magnetic field

We present a theory and experiment demonstrating optical readout of charge and spin in a single InAs/GaAs self-assembled quantum dot. By applying a magnetic field we create the filling factor 2 quantum Hall singlet phase of the charged exciton. Increasing or decreasing the magnetic field leads to electronic spin-flip transitions and increasing spin polarization. The increasing total spin of electrons appears as a manifold of closely spaced emission lines, while spin flips appear as discontinuities of emission lines. The number of multiplets and discontinuities measures the number of carriers and their spin. We present a complete analysis of the emission spectrum of a single quantum dot with N=4 electrons and a single hole, calculated and measured in magnetic fields up to 23 Tesla.Comment: 9 pages, 3 figures, submitted to Europhysics Letter

Fracture strength and Young's modulus of ZnO nanowires

The fracture strength of ZnO nanowires vertically grown on sapphire substrates was measured in tensile and bending experiments. Nanowires with diameters between 60 and 310 nm and a typical length of 2 um were manipulated with an atomic force microscopy tip mounted on a nanomanipulator inside a scanning electron microscope. The fracture strain of (7.7 +- 0.8)% measured in the bending test was found close to the theoretical limit of 10% and revealed a strength about twice as high as in the tensile test. From the tensile experiments the Young's modulus could be measured to be within 30% of that of bulk ZnO, contrary to the lower values found in literature.Comment: 5 pages, 3 figures, 1 tabl