725,352 research outputs found
A contextual extension of Spekkens' toy model
Quantum systems show contextuality. More precisely, it is impossible to
reproduce the quantum-mechanical predictions using a non-contextual realist
model, i.e., a model where the outcome of one measurement is independent of the
choice of compatible measurements performed in the measurement context. There
has been several attempts to quantify the amount of contextuality for specific
quantum systems, for example, in the number of rays needed in a KS proof, or
the number of terms in certain inequalities, or in the violation, noise
sensitivity, and other measures. This paper is about another approach: to use a
simple contextual model that reproduces the quantum-mechanical contextual
behaviour, but not necessarily all quantum predictions. The amount of
contextuality can then be quantified in terms of additional resources needed as
compared with a similar model without contextuality. In this case the
contextual model needs to keep track of the context used, so the appropriate
measure would be memory. Another way to view this is as a memory requirement to
be able to reproduce quantum contextuality in a realist model. The model we
will use can be viewed as an extension of Spekkens' toy model [Phys. Rev. A 75,
032110 (2007)], and the relation is studied in some detail. To reproduce the
quantum predictions for the Peres-Mermin square, the memory requirement is more
than one bit in addition to the memory used for the individual outcomes in the
corresponding noncontextual model.Comment: 10 page
Modeling the Singlet State with Local Variables
A local-variable model yielding the statistics from the singlet state is
presented for the case of inefficient detectors and/or lowered visibility. It
has independent errors and the highest efficiency at perfect visibility is
77.80%, while the highest visibility at perfect detector-efficiency is 63.66%.
Thus, the model cannot be refuted by measurements made to date.Comment: 15 pages, 13 figure
Superconducting d-wave junctions: The disappearance of the odd ac components
We study voltage-biased superconducting planar d-wave junctions for arbitrary
transmission and arbitrary orientation of the order parameters of the
superconductors. For a certain orientation of the superconductors the odd ac
components disappear, resulting in a doubling of the Josephson frequency. We
study the sensitivity of this disappearance to orientation and compare with
experiments on grain boundary junctions. We also discuss the possibility of a
current flow parallel to the junction.Comment: 5 pages, 3 figure
Anomalously strong pinning of the filling factor nu=2 in epitaxial graphene
We explore the robust quantization of the Hall resistance in epitaxial
graphene grown on Si-terminated SiC. Uniquely to this system, the dominance of
quantum over classical capacitance in the charge transfer between the substrate
and graphene is such that Landau levels (in particular, the one at exactly zero
energy) remain completely filled over an extraordinarily broad range of
magnetic fields. One important implication of this pinning of the filling
factor is that the system can sustain a very high nondissipative current. This
makes epitaxial graphene ideally suited for quantum resistance metrology, and
we have achieved a precision of 3 parts in 10^10 in the Hall resistance
quantization measurements
Spectrum of pi electrons in bilayer graphene nanoribbons and nanotubes: an analytical approach
We present an analytical description of pi electrons of a finite size bilayer
graphene within a framework of the tight-binding model. The bilayered
structures considered here are characterized by a rectangular geometry and have
a finite size in one or both directions with armchair- and zigzag-shaped edges.
We provide an exact analytical description of the spectrum of pi electrons in
the zigzag and armchair bilayer graphene nanoribbons and nanotubes. We analyze
the dispersion relations, the density of states, and the conductance
quantization.Comment: 8 figure
Constraining New Physics with a Positive or Negative Signal of Neutrino-less Double Beta Decay
We investigate numerically how accurately one could constrain the strengths
of different short-range contributions to neutrino-less double beta decay in
effective field theory. Depending on the outcome of near-future experiments
yielding information on the neutrino masses, the corresponding bounds or
estimates can be stronger or weaker. A particularly interesting case, resulting
in strong bounds, would be a positive signal of neutrino-less double beta decay
that is consistent with complementary information from neutrino oscillation
experiments, kinematical determinations of the neutrino mass, and measurements
of the sum of light neutrino masses from cosmological observations. The keys to
more robust bounds are improvements of the knowledge of the nuclear physics
involved and a better experimental accuracy.Comment: 23 pages, 3 figures. Minor changes. Matches version published in JHE
On Understanding Catastrophe — The Case of Highly Severe Influenza-Like Illness
Computational epidemiology is a form of spatiotemporal
reasoning in which social link structures
are employed, and spatially explicit models
are specified and executed. We point to issues thus
far addressed neither by engineers, nor scientists, in
the light of a use case focusing on catastrophic scenarios
that assume the emergence of a highly unlikely
but lethal and contagious strain of influenza.
Our conclusion is that important perspectives are
missing when dealing with policy issues resulting
from scenario execution and analyses in computational
epidemiology
Hypervisor Integrity Measurement Assistant
An attacker who has gained access to a computer may want to upload or modify configuration files, etc., and run arbitrary programs of his choice. We can severely restrict the power of the attacker by having a white-list of approved file checksums and preventing the kernel from loading loading any file with a bad checksum. The check may be placed in the kernel, but that requires a kernel that is prepared for it. The check may also be placed in a hypervisor which intercepts and prevents the kernel from loading a bad file.
We describe the implementation of and give performance results for two systems. In one the checksumming, or integrity measurement, and decision is performed by the hypervisor instead of the OS. In the other only the final integrity decision is done in the hypervisor. By moving the integrity check out from the VM kernel it becomes harder for the intruder to bypass the check.
We conclude that it is technically possible to put file integrity control into the hypervisor, both for kernels without and with pre-compiled support for integrity measurement
Directional antennas for wireless sensor networks
Directional antennas provide angle-of-arrival information, which can be used for localization and routing algorithms in wireless sensor networks. We briefly describe three classical, major types of antennas: 1) the Adcock-pair antenna, 2) the pseudo-Doppler antenna, and 3) the electronically switched parasitic element antenna. We have found the last type to be the most suitable for wireless sensor networks, and we present here the early design details and beam pattern measurements of a prototype antenna for the
2.4-GHz ISM band, the SPIDA: SICS Parasitic Interference Directional Antenna
CHORUS Deliverable 4.5: Report of the 3rd CHORUS Conference
The third and last CHORUS conference on Multimedia Search Engines took place from the 26th to the 27th of May 2009 in Brussels, Belgium. About 100 participants from 15 European countries, the US, Japan and Australia learned about the latest developments in the domain. An exhibition of 13 stands presented 16 research projects currently ongoing around the
world
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