25,367 research outputs found
Comment on "Secure direct communication with a quantum one-time pad"
In the paper [Phys. Rev. A \textbf{69}, 052319 (2004)], a quantum direct
communication protocol is proposed which is claimed to be unconditionally
secure even for the case of a noisy channel. We show that this is not the case
by giving an undetectable attack scheme
The state space of short-range Ising spin glasses: the density of states
The state space of finite square and cubic Ising spin glass models is
analysed in terms of the global and the local density of states. Systems with
uniform and gaussian probability distribution of interactions are compared.
Different measures for the local state density are presented and discussed. In
particular the question whether the local density of states grows exponentially
or not is considered. The direct comparison of global and local densities leads
to consequences for the structure of the state space.Comment: 18 pages (including 6 figures); submitted to Z.f.Physik
Quantum Lubrication: Suppression of Friction in a First Principle Four Stroke Heat Engine
A quantum model of a heat engine resembling the Otto cycle is employed to
explore strategies to suppress frictional losses. These losses are caused by
the inability of the engine's working medium to follow adiabatically the change
in the Hamiltonian during the expansion and compression stages. By adding
external noise to the engine, frictional losses can be suppressed.Comment: references added some minor change
KWISP: an ultra-sensitive force sensor for the Dark Energy sector
An ultra-sensitive opto-mechanical force sensor has been built and tested in
the optics laboratory at INFN Trieste. Its application to experiments in the
Dark Energy sector, such as those for Chameleon-type WISPs, is particularly
attractive, as it enables a search for their direct coupling to matter. We
present here the main characteristics and the absolute force calibration of the
KWISP (Kinetic WISP detection) sensor. It is based on a thin Si3N4
micro-membrane placed inside a Fabry-Perot optical cavity. By monitoring the
cavity characteristic frequencies it is possible to detect the tiny membrane
displacements caused by an applied force. Far from the mechanical resonant
frequency of the membrane, the measured force sensitivity is 5.0e-14
N/sqrt(Hz), corresponding to a displacement sensitivity of 2.5e-15 m/sqrt(Hz),
while near resonance the sensitivity is 1.5e-14 N/sqrt(Hz), reaching the
estimated thermal limit, or, in terms of displacement, 7.5e-16 N/sqrt(Hz).
These displacement sensitivities are comparable to those that can be achieved
by large interferometric gravitational wave detectors.Comment: 9 pages, 8 figures in colo
Recent Developments in Finite Time Thermodynamics
Finite time thermodynamics is a non-equilibrium theory. Its aim is to provide performance bounds and extremes for irreversible thermodynamic processes. Recent developments in difierent areas of this theory are presented. First it is shown how irreversible processes between reversible systems can be described by the endoreversible theory. Then maximum power and minimum entropy production processes are introduced. And finally the extension of finite time thermodynamics to the realm of quantum theory is demonstrated
Detecting solar chameleons through radiation pressure
Light scalar fields can drive the accelerated expansion of the universe.
Hence, they are obvious dark energy candidates. To make such models compatible
with tests of General Relativity in the solar system and "fifth force" searches
on Earth, one needs to screen them. One possibility is the so-called
"chameleon" mechanism, which renders an effective mass depending on the local
matter density. If chameleon particles exist, they can be produced in the sun
and detected on Earth exploiting the equivalent of a radiation pressure. Since
their effective mass scales with the local matter density, chameleons can be
reflected by a dense medium if their effective mass becomes greater than their
total energy. Thus, under appropriate conditions, a flux of solar chameleons
may be sensed by detecting the total instantaneous momentum transferred to a
suitable opto-mechanical force/pressure sensor. We calculate the solar
chameleon spectrum and the reach in the chameleon parameter space of an
experiment using the preliminary results from a force/pressure sensor,
currently under development at INFN Trieste, to be mounted in the focal plane
of one of the X-Ray telescopes of the CAST experiment at CERN. We show, that
such an experiment signifies a pioneering effort probing uncharted chameleon
parameter space.Comment: revised versio
Two-Electron Photon Emission From Metallic Quantum Wells
Unusual emission of visible light is observed in scanning tunneling
microscopy of the quantum well system Na on Cu(111). Photons are emitted at
energies exceeding the energy of the tunneling electrons. Model calculations of
two-electron processes which lead to quantum well transitions reproduce the
experimental fluorescence spectra, the quantum yield, and the power-law
variation of the intensity with the excitation current.Comment: revised version, as published; 4 pages, 3 figure
Desiccation of a clay film: Cracking versus peeling
Cracking and peeling of a layer of clay on desiccation has been simulated
using a spring model. A vertical section through the layer with finite
thickness is represented by a rectangular array of nodes connected by linear
springs on a square lattice. The effect of reduction of the natural length of
the springs, which mimics the drying is studied. Varying the strength of
adhesion between sample and substrate and the rate of penetration of the drying
front produces an interesting phase diagram, showing cross-over from peeling to
cracking behavior. Changes in the number and width of cracks on varying the
layer thickness is observed to reproduce experimental reports.Comment: 4 figure
Approximation Algorithms for Scheduling with Resource and Precedence Constraints
We study non-preemptive scheduling problems on identical parallel machines and uniformly related machines under both resource constraints and general precedence constraints between jobs. Our first result is an O(logn)-approximation algorithm for the objective of minimizing the makespan on parallel identical machines under resource and general precedence constraints. We then use this result as a subroutine to obtain an O(logn)-approximation algorithm for the
more general objective of minimizing the total weighted completion time on parallel identical machines under both constraints. Finally, we present an O(logm logn)-approximation algorithm for scheduling under these constraints on uniformly related machines. We show that these results can all be generalized to include the case where each job has a release time. This is the first upper bound on the approximability of this class of scheduling problems where both resource and general precedence constraints must be satisfied simultaneously
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