Developments in electromagnetic tomography instrumentation.

A new EMT sensor and instrumentation is described which combines the best features of previous systems and has a modular structure to allow for future system expansion and development

Oxygen isotopes implanted in the LDEF spacecraft

Secondary ion mass spectrometry was used to study oxygen implanted in the surface of copper from the Long Duration Exposure Facility (LDEF). Oxidation that occurred in orbit shows a characteristic oxygen isotope composition, depleted in O-18. The measured depletion is comparable to the predicted depletion (45 percent) based on a model of the gravitational separation of the oxygen isotopes. The anomalous oxygen was contained within 10nm of the surface. Tray E10 was calculated to have received 5.14 x 10(exp 21) atoms of oxygen cm(sup -2) during the LDEF mission and so there is sufficient anomalous implanted oxygen present in the surface to obtain a reliable isotopic profile

Magnetospheric Cavity Modes Driven by Solar Wind Dynamic Pressure Fluctuations

We present results from Lyon-Fedder-Mobarry (LFM) global, three-dimensional magnetohydrodynamic (MHD) simulations of the solar wind-magnetosphere interaction. We use these simulations to investigate the role that solar wind dynamic pressure fluctuations play in the generation of magnetospheric ultra-low frequency (ULF) pulsations. The simulations presented in this study are driven with idealized solar wind input conditions. In four of the simulations, we introduce monochromatic ULF fluctuations in the upstream solar wind dynamic pressure. In the fifth simulation, we introduce a continuum of ULF frequencies in the upstream solar wind dynamic pressure fluctuations. In this numerical experiment, the idealized nature of the solar wind driving conditions allows us to study the magnetospheric response to only a fluctuating upstream dynamic pressure, while holding all other solar wind driving parameters constant. The simulation results suggest that ULF fluctuations in the solar wind dynamic pressure can drive magnetospheric ULF pulsations in the electric and magnetic fields on the dayside. Moreover, the simulation results suggest that when the driving frequency of the solar wind dynamic pressure fluctuations matches one of the natural frequencies of the magnetosphere, magnetospheric cavity modes can be energized.Comment: 2 figure

Measuring errors in single qubit rotations by pulsed electron paramagnetic resonance

The ability to measure and reduce systematic errors in single-qubit logic gates is crucial when evaluating quantum computing implementations. We describe pulsed electron paramagnetic resonance (EPR) sequences that can be used to measure precisely even small systematic errors in rotations of electron-spin-based qubits. Using these sequences we obtain values for errors in rotation angle and axis for single-qubit rotations using a commercial EPR spectrometer. We conclude that errors in qubit operations by pulsed EPR are not limiting factors in the implementation of electron-spin based quantum computers

Measurement of the Charge Transfer Efficiency of Electrons Clocked on Superfluid Helium

Electrons floating on the surface of liquid helium are possible qubits for quantum information processing. Varying electric potentials do not modify spin states, which allows their transport on helium using a charge-coupled device (CCD)-like array of underlying gates. This scheme depends upon efficient inter-gate electron transfer and on the absence of electron traps. We will present a measurement of the charge transfer efficiency (CTE) of electrons clocked back and forth above a short CCD-like structure. The CTE obtained at low clocking frequencies is 0.999 with an electron density of about 4 electrons per square micrometer. We find no evidence for deep electron trapping.Comment: 8 pages, 3 figures (corrected typos and added details on experimental procedure), submitted to AP

High Fidelity Single Qubit Operations using Pulsed EPR

Systematic errors in spin rotation operations using simple RF pulses place severe limitations on the usefulness of the pulsed magnetic resonance methods in quantum computing applications. In particular, the fidelity of quantum logic operations performed on electron spin qubits falls well below the threshold for the application of quantum algorithms. Using three independent techniques, we demonstrate the use of composite pulses to improve this fidelity by several orders of magnitude. The observed high-fidelity operations are limited by pulse phase errors, but nevertheless fall within the limits required for the application of quantum error correction.Comment: 4 pages, 3 figures To appear in Phys. Rev. Let

Comparison of Birkeland current observations during two magnetic cloud events with MHD simulations

Low altitude field-aligned current densities ob- tained from global magnetospheric simulations are compared with two-dimensional distributions of Birkeland currents at the topside ionosphere derived from magnetic field observa- tions by the constellation of Iridium satellites. We present the analysis of two magnetic cloud events, 17–19 August 2003 and 19–21 March 2001, where the interplanetary magnetic field (IMF) rotates slowly (∼10◦/h) to avoid time-aliasing in the magnetic perturbations used to calculate the Birkeland currents. In the August 2003 event the IMF rotates from southward to northward while maintaining a negative IMF By during much of the interval. During the March 2001 event the IMF direction varies from dawnward to southward to duskward. We find that the distributions of the Birkeland current densities in the simulations agree qualitatively with the observations for northward IMF. For southward IMF, the dayside Region-1 currents are reproduced in the simu- ◦ the ionospheric grids in the simulations and the observations is shown to have only secondary effect on the magnitudes of the Birkeland currents. The electric potentials in the simu- lation for southward IMF periods are twice as large as those obtained from measurements of the plasma drift velocities by DMSP, implying that the reconnection rates in the simulation are too large. Keywords. Ionosphere (Electric fields and currents; Ionosphere-magnetosphere interactions; Modeling and forecasting) 1 Introduction Global magnetohydrodynamic (MHD) models are the most comprehensive numerical tool for studying the coupling of energy and momentum of the solar wind into the Earth’s magnetosphere and ionosphere. A particular advantage of global MHD simulations is the ability to provide continu- ous temporal and spatial coverage of key physical parame- ters over the entire simulation volume. For this reason, MHD simulations have become one of the principal tools for study- ing space weather events such as the interaction of the Earth’s magnetosphere with coronal mass ejections (CMEs) (Ridley et al., 2002) as well as magnetic storms (Slinker et al., 1998; Goodrich et al., 1998) and substorms (Lyon et al., 1998; Lopez et al., 1998; Wiltberger et al., 2000). Since the simula- tion results are frequently used to interpret physical processes in the magnetosphere–ionosphere system, assessing their ac- curacy by comparison with observations is an important task. A number of such studies have been carried out in the past us- ing space-based (Frank et al., 1995; Raeder et al., 1997) and ground-based observations (Ridley et al., 2001), or a com- bination thereof (Fedder et al., 1998; Slinker et al., 1999). However, interpreting the discrepancies between model and observations is not straightforward because the observational lation, but appear on average 5 served location, while the nightside Region-1 currents and the Region-2 currents are largely under-represented. Com- parison of the observed and simulated Birkeland current dis- tributions, which are intimately related to the plasma drifts at the ionosphere, shows that the ionospheric convection pat- tern in the MHD model and its dependence on the IMF ori- entation is essentially correct. The Birkeland total currents in the simulations are about a factor of 2 larger than observed during southward IMF. For Bz\u3e0 the disparity in the total current is reduced and the simulations for purely northward IMF agree with the observations to within 10%. The dispar- ities in the magnitudes of the Birkeland currents between the observations and the simulation results are a combined effect of the simulation overestimating the ionospheric electric field and of the Iridium fits underestimating the magnetic pertur- bations

Environmental effects on electron spin relaxation in N@C60

We examine environmental effects of surrounding nuclear spins on the electron spin relaxation of the N@C60 molecule (which consists of a nitrogen atom at the centre of a fullerene cage). Using dilute solutions of N@C60 in regular and deuterated toluene, we observe and model the effect of translational diffusion of nuclear spins of the solvent molecules on the N@C60 electron spin relaxation times. We also study spin relaxation in frozen solutions of N@C60 in CS2, to which small quantities of a glassing agent, S2Cl2 are added. At low temperatures, spin relaxation is caused by spectral diffusion of surrounding nuclear 35Cl and 37Cl spins in the S2Cl2, but nevertheless, at 20 K, T2 times as long as 0.23 ms are observed.Comment: 7 pages, 6 figure

Predicting magnetopause crossings at geosynchronous orbit during the Halloween storms

[1] In late October and early November of 2003, the Sun unleashed a powerful series of events known as the Halloween storms. The coronal mass ejections launched by the Sun produced several severe compressions of the magnetosphere that moved the magnetopause inside of geosynchronous orbit. Such events are of interest to satellite operators, and the ability to predict magnetopause crossings along a given orbit is an important space weather capability. In this paper we compare geosynchronous observations of magnetopause crossings during the Halloween storms to crossings determined from the Lyon-Fedder-Mobarry global magnetohydrodynamic simulation of the magnetosphere as well to predictions of several empirical models of the magnetopause position. We calculate basic statistical information about the predictions as well as several standard skill scores. We find that the current Lyon-Fedder-Mobarry simulation of the storm provides a slightly better prediction of the magnetopause position than the empirical models we examined for the extreme conditions present in this study. While this is not surprising, given that conditions during the Halloween storms were well outside the parameter space of the empirical models, it does point out the need for physics-based models that can predict the effects of the most extreme events that are of significant interest to users of space weather forecasts

Coherence of Spin Qubits in Silicon

Given the effectiveness of semiconductor devices for classical computation one is naturally led to consider semiconductor systems for solid state quantum information processing. Semiconductors are particularly suitable where local control of electric fields and charge transport are required. Conventional semiconductor electronics is built upon these capabilities and has demonstrated scaling to large complicated arrays of interconnected devices. However, the requirements for a quantum computer are very different from those for classical computation, and it is not immediately obvious how best to build one in a semiconductor. One possible approach is to use spins as qubits: of nuclei, of electrons, or both in combination. Long qubit coherence times are a prerequisite for quantum computing, and in this paper we will discuss measurements of spin coherence in silicon. The results are encouraging - both electrons bound to donors and the donor nuclei exhibit low decoherence under the right circumstances. Doped silicon thus appears to pass the first test on the road to a quantum computer.Comment: Submitted to J Cond Matter on Nov 15th, 200