28,560 research outputs found
Security bounds for continuous variables quantum key distribution
Security bounds for key distribution protocols using coherent and squeezed
states and homodyne measurements are presented. These bounds refer to (i)
general attacks and (ii) collective attacks where Eve interacts individually
with the sent states, but delays her measurement until the end of the
reconciliation process. For the case of a lossy line and coherent states, it is
first proven that a secure key distribution is possible up to 1.9 dB of losses.
For the second scenario, the security bounds are the same as for the completely
incoherent attack.Comment: See also F. Grosshans, quant-ph/040714
Gaussian Operations and Privacy
We consider the possibilities offered by Gaussian states and operations for
two honest parties, Alice and Bob, to obtain privacy against a third
eavesdropping party, Eve. We first extend the security analysis of the protocol
proposed in M. Navascues et al., Phys. Rev. Lett. 94, 010502 (2005). Then, we
prove that a generalized version of this protocol does not allow to distill a
secret key out of bound entangled Gaussian states
Quasiparticle Density of States, Localization, and Distributed Disorder in the Cuprate Superconductors
We explore the effects of various kinds of random disorder on the
quasiparticle density of states of two-dimensional d-wave superconductors using
an exact real-space method, incorporating realistic details known about the
cuprates. Random on-site energy and pointlike unitary impurity models are found
to give rise to a vanishing DOS at the Fermi energy for narrow distributions
and low concentrations, respectively, and lead to a finite, but suppressed, DOS
at unrealistically large levels of disorder. Smooth disorder arising from
impurities located away from the copper-oxide planes meanwhile gives rise to a
finite DOS at realistic impurity concentrations. For the case of smooth
disorder whose average potential is zero, a resonance is found at zero energy
for the quasiparticle DOS at large impurity concentrations. We discuss the
implications of these results on the computed low-temperature specific heat,
the behavior of which we find is strongly affected by the amount of disorder
present in the system. We also compute the localization length as a function of
disorder strength for various types of disorder and find that intermediate- and
high-energy states are quasi-extended for low disorder, and that states near
the Fermi energy are strongly localized and have a localization length that
exhibits an unusual dependence on the amount of disorder. We comment on the
origin of disorder in the cuprates and provide constraints on these based on
known results from scanning tunneling spectroscopy and specific heat
experiments.Comment: 29 pages, 19 figures, published version, includes minor change
Parametrizing growth in dark energy and modified gravity models
It is well-known that an extremely accurate parametrization of the growth
function of matter density perturbations in CDM cosmology, with errors
below , is given by with . In this work, we show that a simple modification of this
expression also provides a good description of growth in modified gravity
theories. We consider the model-independent approach to modified gravity in
terms of an effective Newton constant written as and show
that provides fits to the numerical
solutions with similar accuracy to that of CDM. In the
time-independent case with , simple analytic expressions for
and are presented. In the time-dependent (but
scale-independent) case , we show that has the same time
dependence as . As an example, explicit formalae are provided in the
DGP model. In the general case, for theories with , we obtain a
perturbative expansion for around the General Relativity case
which, for theories, reaches an accuracy below . Finally,
as an example we apply the obtained fitting functions in order to forecast the
precision with which future galaxy surveys will be able to measure the
parameter.Comment: 12 pages, 12 figures. New section on applications to forecasts for
galaxy surveys and new references included. Matches version published in PR
Bounding the set of quantum correlations
We introduce a hierarchy of conditions necessarily satisfied by any
distribution P(ab) representing the probabilities for two separate observers to
obtain outcomes a and b when making local measurements on a shared quantum
state. Each condition in this hierarchy is formulated as a semidefinite
program. Our approach can be used to obtain upper-bounds on the quantum
violation of an arbitrary Bell inequality. It yields, for instance, tight
bounds for the violations of the Collins et al. inequalities.Comment: 5 pages, no figures. v2: minor modification
Regional productivity variation and the impact of public capital stock: an analysis with spatial interaction, with reference to Spain
In this paper we examine whether variations in the level of public capital across Spain?s Provinces affected productivity levels over the period 1996-2005. The analysis is motivated by contemporary urban economics theory, involving a production function for the competitive sector of the economy („industry?) which includes the level of composite services derived from 'service' firms under monopolistic competition. The outcome is potentially increasing returns to scale resulting from pecuniary externalities deriving from internal increasing returns in the monopolistic competition sector. We extend the production function by also making (log) labour efficiency a function of (log) total public capital stock and (log) human capital stock, leading to a simple and empirically tractable reduced form linking productivity level to density of employment, human capital and public capital stock. The model is further extended to include technological externalities or spillovers across provinces. Using panel data methodology, we find significant elasticities for total capital stock and for human capital stock, and a significant impact for employment density. The finding that the effect of public capital is significantly different from zero, indicating that it has a direct effect even after controlling for employment density, is contrary to some of the earlier research findings which leave the question of the impact of public capital unresolved.Public capital, urban economics, spatial econometrics.
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