1,340 research outputs found
Robustness of the European power grids under intentional attack
The power grid defines one of the most important technological networks of
our times and sustains our complex society. It has evolved for more than a
century into an extremely huge and seemingly robust and well understood system.
But it becomes extremely fragile as well, when unexpected, usually minimal,
failures turn into unknown dynamical behaviours leading, for example, to sudden
and massive blackouts. Here we explore the fragility of the European power grid
under the effect of selective node removal. A mean field analysis of fragility
against attacks is presented together with the observed patterns. Deviations
from the theoretical conditions for network percolation (and fragmentation)
under attacks are analysed and correlated with non topological reliability
measures.Comment: 7 pages, 4 figure
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The status of the ICF target physics program at Lawrence Livermore National Laboratory
Calculations of x-ray driven igniting implosions require several critical parameters which have been separately tested on Nova, viz., acceptable levels of SBS and SRS from plasmas equivalent to the plasmas in igniting hohlraums, quantitative understanding of radiation temperature in gas-filled hohlraums, demonstration of control of drive symmetry in gas-filled hohlraums, low levels of seeding of hydrodynamic instabilities from surfaces, especially cryogenic deuterium tritium ice, and quantitative understanding of the mix of cold fuel into a hot spot in high growth factor implosions. 14 refs
Beating noise with abstention in state estimation
We address the problem of estimating pure qubit states with non-ideal (noisy)
measurements in the multiple-copy scenario, where the data consists of a number
N of identically prepared qubits. We show that the average fidelity of the
estimates can increase significantly if the estimation protocol allows for
inconclusive answers, or abstentions. We present the optimal such protocol and
compute its fidelity for a given probability of abstention. The improvement
over standard estimation, without abstention, can be viewed as an effective
noise reduction. These and other results are exemplified for small values of N.
For asymptotically large N, we derive analytical expressions of the fidelity
and the probability of abstention, and show that for a fixed fidelity gain the
latter decreases with N at an exponential rate given by a Kulback-Leibler
(relative) entropy. As a byproduct, we obtain an asymptotic expression in terms
of this very entropy of the probability that a system of N qubits, all prepared
in the same state, has a given total angular momentum. We also discuss an
extreme situation where noise increases with N and where estimation with
abstention provides a most significant improvement as compared to the standard
approach
Topological reversibility and causality in feed-forward networks
Systems whose organization displays causal asymmetry constraints, from
evolutionary trees to river basins or transport networks, can be often
described in terms of directed paths (causal flows) on a discrete state space.
Such a set of paths defines a feed-forward, acyclic network. A key problem
associated with these systems involves characterizing their intrinsic degree of
path reversibility: given an end node in the graph, what is the uncertainty of
recovering the process backwards until the origin? Here we propose a novel
concept, \textit{topological reversibility}, which rigorously weigths such
uncertainty in path dependency quantified as the minimum amount of information
required to successfully revert a causal path. Within the proposed framework we
also analytically characterize limit cases for both topologically reversible
and maximally entropic structures. The relevance of these measures within the
context of evolutionary dynamics is highlighted.Comment: 9 pages, 3 figure
Nonlinear Jaynes-Cummings model of atom-field interaction
Interaction of a two-level atom with a single mode of electromagnetic field
including Kerr nonlinearity for the field and intensity-dependent atom-field
coupling is discussed. The Hamiltonian for the atom-field system is written in
terms of the elements of a closed algebra, which has
SU(1,1) and Heisenberg-Weyl algebras as limiting cases. Eigenstates and
eigenvalues of the Hamiltonian are constructed.
With the field being in a coherent state initially, the dynamical behaviour
of atomic-inversion, field-statistics and uncertainties in the field
quadratures are studied. The appearance of nonclassical features during the
evolution of the field is shown. Further, we explore the overlap of initial and
time-evolved field states.Comment: 14 pages, 6 figures is PS forma
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Scalable Dynamic Instrumentation for BlueGene/L
Dynamic binary instrumentation for performance analysis on new, large scale architectures such as the IBM Blue Gene/L system (BG/L) poses new challenges. Their scale--with potentially hundreds of thousands of compute nodes--requires new, more scalable mechanisms to deploy and to organize binary instrumentation and to collect the resulting data gathered by the inserted probes. Further, many of these new machines don't support full operating systems on the compute nodes; rather, they rely on light-weight custom compute kernels that do not support daemon-based implementations. We describe the design and current status of a new implementation of the DPCL (Dynamic Probe Class Library) API for BG/L. DPCL provides an easy to use layer for dynamic instrumentation on parallel MPI applications based on the DynInst dynamic instrumentation mechanism for sequential platforms. Our work includes modifying DynInst to control instrumentation from remote I/O nodes and porting DPCL's communication to use MRNet, a scalable data reduction network for collecting performance data. We describe extensions to the DPCL API that support instrumentation of task subsets and aggregation of collected performance data. Overall, our implementation provides a scalable infrastructure that provides efficient binary instrumentation on BG/L
Near-infrared Linear Polarization of Ultracool Dwarfs
We report on near-infrared J- and H-band linear polarimetric photometry of
eight ultracool dwarfs (two late-M, five L0-L7.5, and one T2.5) with known
evidence for photometric variability due to dust clouds, anomalous red infrared
colors, or low-gravity atmospheres. The polarimetric data were acquired with
the LIRIS instrument on the William Herschel Telescope. We also provide
mid-infrared photometry in the interval 3.4-24 micron for some targets obtained
with Spitzer and WISE, which has allowed us to confirm the peculiar red colors
of five sources in the sample. We can impose modest upper limits of 0.9% and
1.8% on the linear polarization degree for seven targets with a confidence of
99%. Only one source, 2MAS, J02411151-0326587 (L0), appears to be strongly
polarized (P ~ 3%) in the J-band with a significance level of P/sigma_P ~ 10.
The likely origin of its linearly polarized light and rather red infrared
colors may reside in a surrounding disk with an asymmetric distribution of
grains. Given its proximity (66 +/- 8 pc), this object becomes an excellent
target for the direct detection of the disk.Comment: Accepted for publication in Ap
Expansion history and f(R) modified gravity
We attempt to fit cosmological data using modified Lagrangians
containing inverse powers of the Ricci scalar varied with respect to the
metric. While we can fit the supernova data well, we confirm the behaviour at medium to high redshifts reported elsewhere and argue
that the easiest way to show that this class of models are inconsistent with
the data is by considering the thickness of the last scattering surface. For
the best fit parameters to the supernova data, the simplest 1/R model gives
rise to a last scattering surface of thickness , inconsistent
with observations.Comment: accepted in JCAP, presentation clarified, results and conclusions
unchange
myKaryoView: A Light-Weight Client for Visualization of Genomic Data
The Distributed Annotation System (DAS) is a protocol for easy sharing and integration of biological annotations. In order to visualize feature annotations in a genomic context a client is required. Here we present myKaryoView, a simple light-weight DAS tool for visualization of genomic annotation. myKaryoView has been specifically configured to help analyse data derived from personal genomics, although it can also be used as a generic genome browser visualization. Several well-known data sources are provided to facilitate comparison of known genes and normal variation regions. The navigation experience is enhanced by simultaneous rendering of different levels of detail across chromosomes. A simple interface is provided to allow searches for any SNP, gene or chromosomal region. User-defined DAS data sources may also be added when querying the system. We demonstrate myKaryoView capabilities for adding user-defined sources with a set of genetic profiles of family-related individuals downloaded directly from 23andMe. myKaryoView is a web tool for visualization of genomic data specifically designed for direct-to-consumer genomic data that uses publicly available data distributed throughout the Internet. It does not require data to be held locally and it is capable of rendering any feature as long as it conforms to DAS specifications. Configuration and addition of sources to myKaryoView can be done through the interface. Here we show a proof of principle of myKaryoView's ability to display personal genomics data with 23andMe genome data sources. The tool is available at: http://mykaryoview.com
Nanosecond spin lifetimes in single- and few-layer graphene-hBN heterostructures at room temperature
We present a new fabrication method of graphene spin-valve devices which
yields enhanced spin and charge transport properties by improving both the
electrode-to-graphene and graphene-to-substrate interface. First, we prepare
Co/MgO spin injection electrodes onto Si/SiO. Thereafter, we
mechanically transfer a graphene-hBN heterostructure onto the prepatterned
electrodes. We show that room temperature spin transport in single-, bi- and
trilayer graphene devices exhibit nanosecond spin lifetimes with spin diffusion
lengths reaching 10m combined with carrier mobilities exceeding 20,000
cm/Vs.Comment: 15 pages, 5 figure
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