5,923 research outputs found
NMR Knight shifts and linewidths in the NiâPdâP and NiâPtâP metallic glasses: Composition and temperature dependences
NMR Knight shift and linewidth measurements are reported for the ^(31)P nuclei in the metallic glasses (Ni_(0.50)Pd_(0.50))100â_xP_x (where x=16 to 26.5) and (Ni_yPd_(1ây))_(80)P_(20) (where y=0.20 to 0.80), and both the ^(31)P and 195Pt nuclei in the metallic glass (Ni_yPt_(1ây))_(75)P_(25) (where y=0.20 to 0.68). The results are discussed in terms of the amorphous structure, electronic structure, and stability of transition metal + metalloid metallic glasses
Fault-tolerant quantum computation versus Gaussian noise
We study the robustness of a fault-tolerant quantum computer subject to
Gaussian non-Markovian quantum noise, and we show that scalable quantum
computation is possible if the noise power spectrum satisfies an appropriate
"threshold condition." Our condition is less sensitive to very-high-frequency
noise than previously derived threshold conditions for non-Markovian noise.Comment: 30 pages, 6 figure
Young stars and dust in AFGL437: NICMOS/HST polarimetric imaging of an outflow source
We present near infrared broad band and polarimetric images of the compact
star forming cluster AFGL437 obtained with the NICMOS instrument aboard HST.
Our high resolution images reveal a well collimated bipolar reflection
nebulosity in the cluster and allow us to identify WK34 as the illuminating
source. The scattered light in the bipolar nebulosity centered on this source
is very highly polarized (up to 79%). Such high levels of polarization implies
a distribution of dust grains lacking large grains, contrary to the usual dust
models of dark clouds. We discuss the geometry of the dust distribution giving
rise to the bipolar reflection nebulosity and make mass estimates for the
underlying scattering material. We find that the most likely inclination of the
bipolar nebulosity, south lobe inclined towards Earth, is consistent with the
inclination of the large scale CO molecular outflow associated with the
cluster, strengthening the identification of WK34 as the source powering it.Comment: 26 pages, 10 figues. Accepted for publication in the Astrophysical
Journa
Do topological models provide good information about vulnerability in electric power networks?
In order to identify the extent to which results from topological graph
models are useful for modeling vulnerability in electricity infrastructure, we
measure the susceptibility of power networks to random failures and directed
attacks using three measures of vulnerability: characteristic path lengths,
connectivity loss and blackout sizes. The first two are purely topological
metrics. The blackout size calculation results from a model of cascading
failure in power networks. Testing the response of 40 areas within the Eastern
US power grid and a standard IEEE test case to a variety of attack/failure
vectors indicates that directed attacks result in larger failures using all
three vulnerability measures, but the attack vectors that appear to cause the
most damage depend on the measure chosen. While our topological and power grid
model results show some trends that are similar, there is only a mild
correlation between the vulnerability measures for individual simulations. We
conclude that evaluating vulnerability in power networks using purely
topological metrics can be misleading
Reply to "Comment on 'Decoherence and dissipation of a quantum harmonic oscillator coupled to two-level systems'"
Contrary to the assertion by Mogilevtsev and Shatokhin [preceding paper, Phys. Rev. A 78, 016101 (2008)], we show that the applicability of the Born-Markov master-equation approach in our treatment of the oscillator-spin model depends on the physical situation under study. Heating effects do occur although they may not be accurately captured by second-order perturbation theory inherent in the Born-Markov scheme
Quantum Walks, Quantum Gates and Quantum Computers
The physics of quantum walks on graphs is formulated in Hamiltonian language,
both for simple quantum walks and for composite walks, where extra discrete
degrees of freedom live at each node of the graph. It is shown how to map
between quantum walk Hamiltonians and Hamiltonians for qubit systems and
quantum circuits; this is done for both a single- and multi-excitation coding,
and for more general mappings. Specific examples of spin chains, as well as
static and dynamic systems of qubits, are mapped to quantum walks, and walks on
hyperlattices and hypercubes are mapped to various gate systems. We also show
how to map a quantum circuit performing the quantum Fourier transform, the key
element of Shor's algorithm, to a quantum walk system doing the same. The
results herein are an essential preliminary to a Hamiltonian formulation of
quantum walks in which coupling to a dynamic quantum environment is included.Comment: 17 pages, 10 figure
A comparison of optical and radar measurements of mesospheric winds and tides
Optical measurements of mesospheric winds by FabryâPerot spectrometers, FPSs, at Mawson, 67.6°S 62.9°E, and Davis, 68.6°S 78.0°E, Antarctica are compared with similar measurements obtained using a spacedâantenna MF radar at Davis. The FPSs observed the OH emission. Different analysis procedures, used to determine the mean wind, and amplitude and phase of the semidiurnal tide, have been compared. At these latitudes the diurnal tide is weak and the semiâdiurnal tide, although highly variable in amplitude, is usually the dominant periodicity. When comparing the amplitude and phase of the semidiurnal tide good agreement is obtained between measurements by the two instruments
The HR 4796A Debris System: Discovery of Extensive Exo-Ring Dust Material
The optically and IR bright, and starlight-scattering, HR 4796A ring-like
debris disk is one of the most (and best) studied exoplanetary debris systems.
The presence of a yet-undetected planet has been inferred (or suggested) from
the narrow width and inner/outer truncation radii of its r = 1.05" (77 au)
debris ring. We present new, highly sensitive, Hubble Space Telescope (HST)
visible-light images of the HR 4796A circumstellar debris system and its
environment over a very wide range of stellocentric angles from 0.32" (23 au)
to ~ 15" (1100 au). These very high contrast images were obtained with the
Space Telescope Imaging Spectrograph (STIS) using 6-roll PSF-template
subtracted coronagraphy suppressing the primary light of HR 4796A and using
three image plane occulters and simultaneously subtracting the background light
from its close angular proximity M2.5V companion. The resulting images
unambiguously reveal the debris ring embedded within a much larger,
morphologically complex, and bi-axially asymmetric exoring scattering
structure. These images at visible wavelengths are sensitive to, and map, the
spatial distribution, brightness, and radial surface density of micron size
particles over 5 dex in surface brightness. These particles in the exo-ring
environment may be unbound from the system and interacting with the local ISM.
Herein we present a new morphological and photometric view of the larger than
prior seen HR 4796A exoplanetary debris system with sensitivity to small
particles at stellocentric distances an order of magnitude greater than has
previously been observed.Comment: 28 pages, 17 figures, accepted for publication in the Astronomical
Journal 21 December 201
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