4,301 research outputs found
Free-electron lasers : echoes of photons past
High-harmonic generation is an established method to significantly upshift laser photon energies. Now, researchers at the SLAC National Accelerator Laboratory have used echo concepts to generate coherent high-harmonic output from an electron-beam light source
Bias-tailored quantum LDPC codes
Bias-tailoring allows quantum error correction codes to exploit qubit noise
asymmetry. Recently, it was shown that a modified form of the surface code, the
XZZX code, exhibits considerably improved performance under biased noise. In
this work, we demonstrate that quantum low density parity check codes can be
similarly bias-tailored. We introduce a bias-tailored lifted product code
construction that provides the framework to expand bias-tailoring methods
beyond the family of 2D topological codes. We present examples of bias-tailored
lifted product codes based on classical quasi-cyclic codes and numerically
assess their performance using a belief propagation plus ordered statistics
decoder. Our Monte Carlo simulations, performed under asymmetric noise, show
that bias-tailored codes achieve several orders of magnitude improvement in
their error suppression relative to depolarising noise.Comment: 21 Pages, 13 Figures. Comments welcome
Critical currents in vicinal YBaCuO films
Most measurements of critical current densities in
YBaCuO thin films to date have been performed on films
where the \textit{c}-axis is grown normal to the film surface. With such films,
the analysis of the dependence of on the magnetic field angle is complex.
The effects of extrinsic contributions to the angular field dependence of
, such as the measurement geometry and disposition of pinning centres, are
convoluted with those intrinsically due to the anisotropy of the material. As a
consequence of this, it is difficult to distinguish between proposed FLL
structure models on the basis of angular critical current density measurements
on \textit{c}-axis films. Films grown on mis-cut (vicinal) substrates have a
reduced measurement symmetry and thus provide a greater insight into the
critical current anisotropy. In this paper previous descriptions of the
magnetic field angle dependence of in YBaCuO are
reviewed. Measurements on YBaCuO thin films grown on a
range of vicinal substrates are presented and the results interpreted in terms
of the structure and dimensionality of the FLL in YBaCuO.
There is strong evidence for a transition in the structure of the flux line
lattice depending on magnetic field magnitude, orientation and temperature. As
a consequence, a simple scaling law can not, by itself, describe the observed
critical current anisotropy in YBaCuO. The experimentally
obtained behaviour of YBCO is successfully described in terms of
a kinked vortex structure for fields applied near parallel to the \textit{a-b}
planes.Comment: 10 pages, 12 figures, Submitted to PR
Focused laser Doppler velocimeter
A system for remotely measuring velocities present in discrete volumes of air is described. A CO2 laser beam is focused by a telescope at such a volume, a focal volume, and within the focusable range, near field, of the telescope. The back scatter, or reflected light, principally from the focal volume, passes back through the telescope and is frequency compared with the original frequency of the laser, and the difference frequency or frequencies represent particle velocities in that focal volume
MaizeGDB becomes âsequence-centricâ
MaizeGDB is the maize research communityâs central repository for genetic and genomic information about the crop plant and research model Zea mays ssp. mays. The MaizeGDB team endeavors to meet research needs as they evolve based on researcher feedback and guidance. Recent work has focused on better integrating existing data with sequence information as it becomes available for the B73, Mo17 and Palomero Toluqueño genomes. Major endeavors along these lines include the implementation of a genome browser to graphically represent genome sequences; implementation of POPcorn, a portal ancillary to MaizeGDB that offers access to independent maize projects and will allow BLAST similarity searches of participating projectsâ data sets from a single point; and a joint MaizeGDB/PlantGDB project to involve the maize community in genome annotation. In addition to summarizing recent achievements and future plans, this article also discusses specific examples of community involvement in setting priorities and design aspects of MaizeGDB, which should be of interest to other database and resource providers seeking to better engage their users. MaizeGDB is accessible online at http://www.maizegdb.org
Hypercharge and the Cosmological Baryon Asymmetry
Stringent bounds on baryon and lepton number violating interactions have been
derived from the requirement that such interactions, together with electroweak
instantons, do not destroy a cosmological baryon asymmetry produced at an
extremely high temperature in the big bang. While these bounds apply in
specific models, we find that they are generically evaded. In particular, the
only requirement for a theory to avoid these bounds is that it contain charged
particles which, during a certain cosmological epoch, carry a non-zero
hypercharge asymmetry. Hypercharge neutrality of the universe then dictates
that the remaining particles must carry a compensating hypercharge density,
which is necessarily shared amongst them so as to give a baryon asymmetry.
Hence the generation of a hypercharge density in a sector of the theory forces
the universe to have a baryon asymmetry.Comment: 12 pages plus 1 Postscript figure available upon request. LBL 3482
Hot-spot model for accretion disc variability as random process
Theory of random processes provides an attractive mathematical tool to
describe the fluctuating signal from accreting sources, such as active galactic
nuclei and Galactic black holes observed in X-rays. These objects exhibit
featureless variability on different timescales, probably originating from an
accretion disc. We study the basic features of the power spectra in terms of a
general framework, which permits semi-analytical determination of the power
spectral density (PSD) of the resulting light curve. We consider the expected
signal generated by an ensemble of spots randomly created on the accretion disc
surface. Spot generation is governed by Poisson or by Hawkes processes. We
include general relativity effects shaping the signal on its propagation to a
distant observer. We analyse the PSD of a spotted disc light curve and show the
accuracy of our semi-analytical approach by comparing the obtained PSD with the
results of Monte Carlo simulations. The asymptotic slopes of PSD are 0 at low
frequencies and they drop to -2 at high frequencies, usually with a single
frequency break. More complex two-peak solutions also occur. The amplitude of
the peaks and their frequency difference depend on the inherent timescales of
the model. At intermediate frequencies, the intrinsic PSD is influenced by the
individual light curve profile as well as by the type of the underlying
process. However, even in cases when two Lorentzians seem to dominate the PSD,
it does not necessarily imply that two single oscillation mechanisms operate
simultaneously. Instead, it may well be the manifestation of the avalanche
mechanism. The main advantage of our approach is an insight in the model
functioning and the fast evaluation of the PSD.Comment: 18 pages, 9 figures; Astronomy & Astrophysics accepte
2003 Manifesto on the California Electricity Crisis
The authors, an ad-hocgroup of professionals with experience in regulatory and energy economics, share a common concern with the continuing turmoil facing the electricity industry ("the industry") in California. Most ofthe authorsendorsed the first California Electricity Manifesto issued on January 25, 2001. Almost two years have passed since that first Manifesto. While wholesale electric prices have moderated and California no longer faces the risk of blackouts, in many ways the industry is in worse shape now than it was at the start of 2001. As a result, the group of signatories continues to have a deep concern with the conflicting policy directions being pursued for the industry at both the State and Federal levels of government and the impact the uncertainties associated with these conflicting policies will have, long term, on the economy of California. Theauthorshave once again convened under the auspices of the Institute of Management, Innovation and Organization at the University of California, Berkeley, to put forward ourtheir ideas on a basic set of necessary policies to move the industry forward for the benefit of all Californians and the nation. The authors point out that theydo not pretend to be "representative." They do bring, however, a very diverse range of backgrounds and expertise.Technology and Industry, Regulatory Reform
Matrix metalloproteinase-3 (MMP-3)-mediated gene therapy for glaucoma.
Approximately 80 million people globally are affected by glaucoma, with a projected increase to over 110 million by 2040. Substantial issues surrounding patient compliance remain with topical eye drops, and up to 10% of patients become treatment resistant, putting them at risk of permanent vision loss. The major risk factor for glaucoma is elevated intraocular pressure, which is regulated by the balance between the secretion of aqueous humor and the resistance to its flow across the conventional outflow pathway. Here, we show that adeno-associated virus 9 (AAV9)-mediated expression of matrix metalloproteinase-3 (MMP-3) can increase outflow in two murine models of glaucoma and in nonhuman primates. We show that long-term AAV9 transduction of the corneal endothelium in the nonhuman primate is safe and well tolerated. Last, MMP-3 increases outflow in donor human eyes. Collectively, our data suggest that glaucoma can be readily treated with gene therapy-based methods, paving the way for deployment in clinical trials
Ultraviolet-Optical observations of the Seyfert 2 Galaxies NGC 7130, NGC 5135 and IC 3639: Implications for the Starburst-AGN Connection
We present and discuss HST (WFPC2 and FOC) images and UV GHRS spectra plus
ground-based near UV through to near IR spectra of three Seyfert 2 nuclei (NGC
7130, NGC 5135 and IC 3639). These galaxies, together to Mrk 477, were selected
from a bigger sample that comprises the 20 brightest Seyfert 2 nuclei, with the
goal to study the origin of the UV-optical-near IR featureless continuum in
Seyfert 2 nuclei. These four galaxies have bolometric luminosities, as computed
with the four IRAS bands, of 10^11 Lsol. They are close enough to be resolved
with HST the nuclear zone. This makes these Seyfert 2 galaxies benchmarks to
study the Starburst-AGN connection in more distant galaxies.
The data provide direct evidence of the existence of a central nuclear
starburst that dominates the UV light, and that seem to be responsible for the
origin of the so called featureless continuum. These starbursts are dusty and
compact. They have sizes (from less than 100 pc to a few hundred pc) much
smaller and closer to the nucleus than that seen in the prototype Seyfert 2
galaxy NGC 1068. The bolometric luminosity of these starbursts is similar to
the estimated bolometric luminosities of their obscured Seyfert 1 nuclei, and
thus they contribute in the same amount to the overall energetics of these
galaxies.Comment: to be published in ApJ 505, September issue. The figures are in a tar
files at: http://www.iaa.es/~rosa/Seyfert
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