58 research outputs found
Scanning tunnelling spectroscopy of electron resonators
The electronic structure of artificial Mn atom arrays on Ag(111) is
characterized in detail with scanning tunnelling spectroscopy and spectroscopic
imaging at low temperature. We demonstrate the degree to which variations in
geometry may be used to control spatial and spectral distributions of surface
state electrons confined within the arrays, how these are influenced by atoms
placed within the structure and how the ability to induce spectral features at
specific energies may be exploited through lineshape analyses to deduce
quasiparticle lifetimes near the Fermi level. Through extensive comparison of
maps and spectra we demonstrate the utility of a model based upon
two-dimensional s-wave scatterers for describing and predicting the
characteristics of specific resonators
On Secure Network Coding with Nonuniform or Restricted Wiretap Sets
The secrecy capacity of a network, for a given collection of permissible
wiretap sets, is the maximum rate of communication such that observing links in
any permissible wiretap set reveals no information about the message. This
paper considers secure network coding with nonuniform or restricted wiretap
sets, for example, networks with unequal link capacities where a wiretapper can
wiretap any subset of links, or networks where only a subset of links can
be wiretapped. Existing results show that for the case of uniform wiretap sets
(networks with equal capacity links/packets where any can be wiretapped),
the secrecy capacity is given by the cut-set bound, and can be achieved by
injecting random keys at the source which are decoded at the sink along
with the message. This is the case whether or not the communicating users have
information about the choice of wiretap set. In contrast, we show that for the
nonuniform case, the cut-set bound is not achievable in general when the
wiretap set is unknown, whereas it is achievable when the wiretap set is made
known. We give achievable strategies where random keys are canceled at
intermediate non-sink nodes, or injected at intermediate non-source nodes.
Finally, we show that determining the secrecy capacity is a NP-hard problem.Comment: 24 pages, revision submitted to IEEE Transactions on Information
Theor
Connecting Multiple-unicast and Network Error Correction: Reduction and Unachievability
We show that solving a multiple-unicast network coding problem can be reduced
to solving a single-unicast network error correction problem, where an
adversary may jam at most a single edge in the network. Specifically, we
present an efficient reduction that maps a multiple-unicast network coding
instance to a network error correction instance while preserving feasibility.
The reduction holds for both the zero probability of error model and the
vanishing probability of error model. Previous reductions are restricted to the
zero-error case. As an application of the reduction, we present a constructive
example showing that the single-unicast network error correction capacity may
not be achievable, a result of separate interest.Comment: ISIT 2015. arXiv admin note: text overlap with arXiv:1410.190
Lossy Compression with Privacy Constraints: Optimality of Polar Codes
A lossy source coding problem with privacy constraint is studied in which two
correlated discrete sources and are compressed into a reconstruction
with some prescribed distortion . In addition, a privacy
constraint is specified as the equivocation between the lossy reconstruction
and . This models the situation where a certain amount of source
information from one user is provided as utility (given by the fidelity of its
reconstruction) to another user or the public, while some other correlated part
of the source information must be kept private. In this work, we show that
polar codes are able, possibly with the aid of time sharing, to achieve any
point in the optimal rate-distortion-equivocation region identified by
Yamamoto, thus providing a constructive scheme that obtains the optimal
tradeoff between utility and privacy in this framework.Comment: Submitted for publicatio
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