5,391 research outputs found
The Rich-Club Phenomenon In The Internet Topology
We show that the Internet topology at the Autonomous System (AS) level has a
rich--club phenomenon. The rich nodes, which are a small number of nodes with
large numbers of links, are very well connected to each other. The rich--club
is a core tier that we measured using the rich--club connectivity and the
node--node link distribution. We obtained this core tier without any heuristic
assumption between the ASes. The rich--club phenomenon is a simple qualitative
way to differentiate between power law topologies and provides a criterion for
new network models. To show this, we compared the measured rich--club of the AS
graph with networks obtained using the Barab\'asi--Albert (BA) scale--free
network model, the Fitness BA model and the Inet--3.0 model.Comment: To be appeared in the IEEE Communications Letter
System size stochastic resonance in a model for opinion formation
We study a model for opinion formation which incorporates three basic
ingredients for the evolution of the opinion held by an individual: imitation,
influence of fashion and randomness. We show that in the absence of fashion,
the model behaves as a bistable system with random jumps between the two stable
states with a distribution of times following Kramer's law. We also demonstrate
the existence of system size stochastic resonance, by which there is an optimal
value for the number of individuals N for which the average opinion follows
better the fashion.Comment: 10 pages, to appear in Physica
Accurately modeling the Internet topology
Based on measurements of the Internet topology data, we found out that there
are two mechanisms which are necessary for the correct modeling of the Internet
topology at the Autonomous Systems (AS) level: the Interactive Growth of new
nodes and new internal links, and a nonlinear preferential attachment, where
the preference probability is described by a positive-feedback mechanism. Based
on the above mechanisms, we introduce the Positive-Feedback Preference (PFP)
model which accurately reproduces many topological properties of the AS-level
Internet, including: degree distribution, rich-club connectivity, the maximum
degree, shortest path length, short cycles, disassortative mixing and
betweenness centrality. The PFP model is a phenomenological model which
provides a novel insight into the evolutionary dynamics of real complex
networks.Comment: 20 pages and 17 figure
Continuous-variable quantum key distribution protocols over noisy channels
A continuous-variable quantum key distribution protocol based on squeezed
states and heterodyne detection is introduced and shown to attain higher secret
key rates over a noisy line than any other one-way Gaussian protocol. This
increased resistance to channel noise can be understood as resulting from
purposely adding noise to the signal that is converted into the secret key.
This notion of noise-enhanced tolerance to noise also provides a better
physical insight into the poorly understood discrepancies between the
previously defined families of Gaussian protocols.Comment: Minor modifications to match published manuscrip
Unconditional optimality of Gaussian attacks against continuous-variable QKD
A fully general approach to the security analysis of continuous-variable
quantum key distribution (CV-QKD) is presented. Provided that the quantum
channel is estimated via the covariance matrix of the quadratures, Gaussian
attacks are shown to be optimal against all eavesdropping strategies, including
collective and coherent attacks. The proof is made strikingly simple by
combining a physical model of measurement, an entanglement-based description of
CV-QKD, and a recent powerful result on the extremality of Gaussian states
[Phys. Rev. Lett. 96, 080502 (2006)].Comment: 4 pages, 4 figure
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