7,346 research outputs found
Formalisation and Implementation of the XACML Access Control Mechanism
We propose a formal account of XACML, an OASIS standard adhering to the Policy Based Access Control model for the specifica- tion and enforcement of access control policies. To clarify all ambiguous and intricate aspects of XACML, we provide it with a more manageable alternative syntax and with a solid semantic ground. This lays the basis
for developing tools and methodologies which allow software engineers to easily and precisely regulate access to resources using policies. To demonstrate feasibility and effectiveness of our approach, we provide a software tool, supporting the specification and evaluation of policies and access requests, whose implementation fully relies on our formal development
On the dimension of subspaces with bounded Schmidt rank
We consider the question of how large a subspace of a given bipartite quantum
system can be when the subspace contains only highly entangled states. This is
motivated in part by results of Hayden et al., which show that in large d x
d--dimensional systems there exist random subspaces of dimension almost d^2,
all of whose states have entropy of entanglement at least log d - O(1). It is
also related to results due to Parthasarathy on the dimension of completely
entangled subspaces, which have connections with the construction of
unextendible product bases. Here we take as entanglement measure the Schmidt
rank, and determine, for every pair of local dimensions dA and dB, and every r,
the largest dimension of a subspace consisting only of entangled states of
Schmidt rank r or larger. This exact answer is a significant improvement on the
best bounds that can be obtained using random subspace techniques. We also
determine the converse: the largest dimension of a subspace with an upper bound
on the Schmidt rank. Finally, we discuss the question of subspaces containing
only states with Schmidt equal to r.Comment: 4 pages, REVTeX4 forma
Algebraic varieties with automorphism groups of maximal rank
We confirm, to some extent, the belief that a projective variety X has the
largest number (relative to the dimension of X) of independent commuting
automorphisms of positive entropy only when X is birational to a complex torus
or a quotient of a torus. We also include an addendum to an early paper though
it is not used in the present paper.Comment: Mathematische Annalen (to appear
Selection rules for J^PC Exotic Hybrid Meson Decay in Large-N_c
The coupling of a neutral hybrid {1,3,5...}^-+ exotic particle (or current)
to two neutral (hybrid) meson particles with the same J^PC and J=0 is proved to
be sub-leading to the usual large-N_c QCD counting. The coupling of the same
exotic particle to certain two - (hybrid) meson currents with the same J^PC and
J=0 is also sub-leading. The decay of a {1,3,5...}^-+ hybrid to eta pi^0, eta'
pi^0, eta' eta, eta(1295) pi^0, pi(1300)^0 pi0, eta(1440) pi^0, a_0(980)^0
sigma or f_0(980) sigma is sub-leading, assuming that these final state
particles are (hybrid) mesons in the limit of large N_c.Comment: 16 pages, LaTeX. Main paper shortened/rewritten and appendices
expanded. Implications for phenomenology of exotic hybrid mesons clarifie
Diffuse export of nutrients under different land uses in the irrigation area of lower Beiyunhe River (China)
AbstractNon-point source pollution is serious in the agriculture watershed of China. Understanding the characteristics of rainfall-runoff from agriculture watershed can provide theoretical support for controlling non-point source pollution. In this study, we investigated runoff characteristics of eight indices (dissolved total N, NO3--N, NH4+-N, total phosphorus, dissolved total phosphorus, particulate phosphorus, total organic carbon, COD) from three types of land uses, including farmland, forest and village in the downstream irrigation area of the Beiyunhe River basin. The results showed that the event mean concentrations (EMCs) of total dissolved N in village, farmland and forestland were 17.81mg/L, 12.68mg/L and 3.14mg/L, respectively. EMC of total phosphorus in the order: farmland (0.44mg/L) > village (0.22mg/L) > forestland (0.17mg/L). EMC of COD in the order: farmland (45.07mg/L) > forestland (27.06mg/L) > village (18.03mg/L). The changes in the nutrients concentrations of the runoff water over a rainfall event indicated that the transports of the nutrients are similar among various land use types. The instantaneous concentrations of TN, NH4+-N, and NO3--N were high in the initial period of runoff, tend to decreasing with rainfall continuing, and increase in later period. Phosphorus concentration with time variation was not obvious among three land use types. The phosphorus species with high proportion in the total phosphorus was particle P (accounting for 75%) in forestland, dissolve P (79%) in farmland, and particle P (48%) and dissolve P (52%) in village. The curves of COD and TOC had been shown as high in the initial period of runoff, tending to increasing with rainfall continuing, and decrease in the later period. First-flush of all the indices were obvious in all three land use types with the rank of village > forestland > farmland. In village, all of the pollutions have taken place the phenomenon of first flush, while in farmland, pollutions tended to uniformly distribute or dilution throughout the storm event
Exact Master Equation and Non-Markovian Decoherence for Quantum Dot Quantum Computing
In this article, we report the recent progress on decoherence dynamics of
electrons in quantum dot quantum computing systems using the exact master
equation we derived recently based on the Feynman-Vernon influence functional
approach. The exact master equation is valid for general nanostructure systems
coupled to multi-reservoirs with arbitrary spectral densities, temperatures and
biases. We take the double quantum dot charge qubit system as a specific
example, and discuss in details the decoherence dynamics of the charge qubit
under coherence controls. The decoherence dynamics risen from the entanglement
between the system and the environment is mainly non-Markovian. We further
discuss the decoherence of the double-dot charge qubit induced by quantum point
contact (QPC) measurement where the master equation is re-derived using the
Keldysh non-equilibrium Green function technique due to the non-linear coupling
between the charge qubit and the QPC. The non-Markovian decoherence dynamics in
the measurement processes is extensively discussed as well.Comment: 15 pages, Invited article for the special issue "Quantum Decoherence
and Entanglement" in Quantum Inf. Proces
Entanglement-Assisted Communication Surpassing the Ultimate Classical Capacity
Entanglement underpins a variety of quantum-enhanced communication, sensing,
and computing capabilities. Entanglement-assisted communication (EACOMM)
leverages entanglement pre-shared by communication parties to boost the rate of
classical information transmission. Pioneering theory works showed that EACOMM
can enable a communication rate well beyond the ultimate classical capacity of
optical communications, but an experimental demonstration of any EACOMM
advantage remains elusive. Here, we report the implementation of EACOMM
surpassing the classical capacity over lossy and noisy bosonic channels. We
construct a high-efficiency entanglement source and a phase-conjugate quantum
receiver to reap the benefit of pre-shared entanglement, despite entanglement
being broken by channel loss and noise. We show that EACOMM beats the
Holevo-Schumacher-Westmoreland capacity of classical communication by up to
14.6%, when both protocols are subject to the same power constraint at the
transmitter. As a practical performance benchmark, a classical communication
protocol without entanglement assistance is implemented, showing that EACOMM
can reduce the bit-error rate by up to 69% over the same bosonic channel. Our
work opens a route to provable quantum advantages in a wide range of quantum
information processing tasks.Comment: 12 pages, 5 figures. Comments are welcom
Quantitative tract-based white matter heritability in 1- and 2-year-old twins
White matter (WM) microstructure, as determined by diffusion tensor imaging (DTI), is increasingly recognized as an important determinant of cognitive function and is also altered in neuropsychiatric disorders. Little is known about genetic and environmental influences on WM microstructure, especially in early childhood, an important period for cognitive development and risk for psychiatric disorders. We studied the heritability of DTI parameters, fractional anisotropy (FA), radial diffusivity (RD) and axial diffusivity (AD) along 34 tracts, including 10 bilateral fiber pathways and the respective subdivision, using quantitative tractography in a longitudinal sample of healthy children at 1 year (N = 215) and 2 years (N = 165) of age. We found that heritabilities for whole brain AD, RD, and FA were 0.48, 0.69, and 0.72 at age 1, and 0.59, 0.77, and 0.76 at age 2 and that mean heritabilities of tract-averaged AD, RD, and FA for individual bundles were moderate (over 0.4). However, the heritability of DTI change between 1 and 2 years of age was not significant for most tracts. We also demonstrated that point-wise heritability tended to be significant in the central portions of the tracts and was generally spatially consistent at ages 1 and 2 years. These results, especially when compared to heritability patterns in neonates, indicate that the heritability of WM microstructure is dynamic in early childhood and likely reflect heterogeneous maturation of WM tracts and differential genetic and environmental influences on maturation patterns
The Energy-dependent Checkerboard Patterns in Cuprate Superconductors
Motivated by the recent scanning tunneling microscopy (STM) experiments [J.
E. Hoffman {\it et al.}, Science {\bf 297}, 1148 (2002); K. McElroy {\it et
al.}, Nature (to be published)], we investigate the real space local density of
states (LDOS) induced by weak disorder in a d-wave superconductor. We first
present the energy dependent LDOS images around a single weak defect at several
energies, and then point out that the experimentally observed checkerboard
pattern in the LDOS could be understood as a result of quasiparticle
interferences by randomly distributed defects. It is also shown that the
checkerboard pattern oriented along to the Cu-O bonds at low energies
would transform to that oriented parallel to the Cu-O bonds at higher energies.
This result is consistent with the experiments.Comment: 3 pages, 3 figure
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