7,861 research outputs found
An optimised scalable synthesis of H2O@C60and a new synthesis of H2@C60
New high-yielding synthetic routes to the small-molecule endofullerenes H2O@C60, D2O@C60 and H2@C60 are described. The use of high temperatures and pressures for the endohedral molecule incorporation are avoided. A new partial closure step using PPh3, and final suturing using a novel DielsâAlder/retro-DielsâAlder sequence are amongst the advances reported
Fault tolerant architectures for integrated aircraft electronics systems
Work into possible architectures for future flight control computer systems is described. Ada for Fault-Tolerant Systems, the NETS Network Error-Tolerant System architecture, and voting in asynchronous systems are covered
Fault tolerant architectures for integrated aircraft electronics systems, task 2
The architectural basis for an advanced fault tolerant on-board computer to succeed the current generation of fault tolerant computers is examined. The network error tolerant system architecture is studied with particular attention to intercluster configurations and communication protocols, and to refined reliability estimates. The diagnosis of faults, so that appropriate choices for reconfiguration can be made is discussed. The analysis relates particularly to the recognition of transient faults in a system with tasks at many levels of priority. The demand driven data-flow architecture, which appears to have possible application in fault tolerant systems is described and work investigating the feasibility of automatic generation of aircraft flight control programs from abstract specifications is reported
ELSA: An Integrated, Semi-Automated Nebular Abundance Package
We present ELSA, a new modular software package, written in C, to analyze and
manage spectroscopic data from emission-line objects. In addition to
calculating plasma diagnostics and abundances from nebular emission lines, the
software provides a number of convenient features including the ability to
ingest logs produced by IRAF's splot task, to semi-automatically merge spectra
in different wavelength ranges, and to automatically generate various data
tables in machine-readable or LaTeX format. ELSA features a highly
sophisticated interstellar reddening correction scheme that takes into account
temperature and density effects as well as He II contamination of the hydrogen
Balmer lines. Abundance calculations are performed using a 5-level atom
approximation with recent atomic data, based on R. Henry's ABUN program.
Improvements planned in the near future include use of a three-region
ionization model, similar to IRAF's nebular package, error propagation, and the
addition of ultraviolet and infrared line analysis capability. Detailed
documentation for all aspects of ELSA are available at
http://www.williams.edu/Astronomy/research/PN .Comment: 2 pages, contributed paper, IAU Symp. 234, Planetary Nebulae in Our
Galaxy and Beyon
Geometric Aspects of Composite Pulses
Unitary operations acting on a quantum system must be robust against
systematic errors in control parameters for reliable quantum computing.
Composite pulse technique in nuclear magnetic resonance (NMR) realises such a
robust operation by employing a sequence of possibly poor quality pulses. In
this article, we demonstrate that two kinds of composite pulses, one
compensates for a pulse length error in a one-qubit system and the other
compensates for a J-coupling error in a twoqubit system, have vanishing
dynamical phase and thereby can be seen as geometric quantum gates, which
implement unitary gates by the holonomy associated with dynamics of cyclic
vectors defined in the text.Comment: 20 pages, 4 figures. Accepted for publication in Philosophical
Transactions of the Royal Society
Nuclear Spins as Quantum Memory in Semiconductor Nanostructures
We theoretically consider solid state nuclear spins in a semiconductor
nanostructure environment as long-lived, high-fidelity quantum memory. In
particular, we calculate, in the limit of a strong applied magnetic field, the
fidelity versus time of P donor nuclear spins in random bath environments of Si
and GaAs, and the lifetime of excited intrinsic spins in polarized Si and GaAs
environments. In the former situation, the nuclear spin dephases due to
spectral diffusion induced by the dipolar interaction among nuclei in the bath.
We calculate the decay of nuclear spin quantum memory in the context of Hahn
and Carr-Purcell-Meiboom-Gill (CPMG) refocused spin echoes using a formally
exact cluster expansion technique which has previously been successful in
dealing with electron spin dephasing in a solid state nuclear spin bath. With
decoherence dominated by transverse dephasing (T2), we find it feasible to
maintain high fidelity (losses of less than 10^{-6}) quantum memory on nuclear
spins for times of the order of 100 microseconds (GaAs:P) and 1 to 2
milliseconds (natural Si:P) using CPMG pulse sequences of just a few (~2-4)
applied pulses. We also consider the complementary situation of a central
flipped intrinsic nuclear spin in a bath of completely polarized nuclear spins
where decoherence is caused by the direct flip-flop of the central spin with
spins in the bath. Exact numerical calculations that include a sufficiently
large neighborhood of surrounding nuclei show lifetimes on the order of 1-5 ms
for both GaAs and natural Si. Our calculated nuclear spin coherence times may
have significance for solid state quantum computer architectures using
localized electron spins in semiconductors where nuclear spins have been
proposed for quantum memory storage
Quantum information processing using strongly-dipolar coupled nuclear spins
Dipolar coupled homonuclear spins present challenging, yet useful systems for
quantum information processing. In such systems, eigenbasis of the system
Hamiltonian is the appropriate computational basis and coherent control can be
achieved by specially designed strongly modulating pulses. In this letter we
describe the first experimental implementation of the quantum algorithm for
numerical gradient estimation on the eigenbasis of a four spin system.Comment: 5 pages, 5 figures, Accepted in PR
Phase transition in the globalization of trade
Globalization processes interweave economic structures at a worldwide scale,
trade playing a central role as one of the elemental channels of interaction
among countries. Despite the significance of such phenomena, measuring economic
globalization still remains an open problem. More quantitative treatments could
improve the understanding of globalization at the same time that help a formal
basis for comparative economic history. In this letter, we investigate the time
evolution of the statistical properties of bilateral trade imbalances between
countries in the trade system. We measure their cumulative probability
distribution at different moments in time to discover a sudden transition circa
1960 from a regime where the distribution was always represented by a steady
characteristic function to a new state where the distribution dilates as time
goes on. This suggests that the rule that was governing the statistical
behavior of bilateral trade imbalances until the 60's abruptly changed to a new
form persistent in the last decades. In the new regime, the figures for the
different years collapse into a universal master curve when rescaled by the
corresponding global gross domestic product value. This coupling points to an
increased interdependence of world economies and its onset corresponds in time
with the starting of the last globalization wave.Comment: Final versio
Phase space spinor amplitudes for spin 1/2 systems
The concept of phase space amplitudes for systems with continuous degrees of
freedom is generalized to finite-dimensional spin systems. Complex amplitudes
are obtained on both a sphere and a finite lattice, in each case enabling a
more fundamental description of pure spin states than that previously given by
Wigner functions. In each case the Wigner function can be expressed as the star
product of the amplitude and its conjugate, so providing a generalized Born
interpretation of amplitudes that emphasizes their more fundamental status. The
ordinary product of the amplitude and its conjugate produces a (generalized)
spin Husimi function. The case of spin-\half is treated in detail, and it is
shown that phase space amplitudes on the sphere transform correctly as spinors
under under rotations, despite their expression in terms of spherical
harmonics. Spin amplitudes on a lattice are also found to transform as spinors.
Applications are given to the phase space description of state superposition,
and to the evolution in phase space of the state of a spin-\half magnetic
dipole in a time-dependent magnetic field.Comment: 19 pages, added new results, fixed typo
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