289 research outputs found
Thermally-activated Non-Schmid Glide of Screw Dislocations in W using Atomistically-informed Kinetic Monte Carlo Simulations
Thermally-activated \small{\nicefrac{1}{2}} screw dislocation motion
is the controlling plastic mechanism at low temperatures in body-centered cubic
(bcc) crystals. Motion proceeds by the nucleation and propagation of
atomic-sized kink pairs susceptible of being studied using molecular dynamics
(MD). However, MD's natural inability to properly sample thermally-activated
processes as well as to capture screw dislocation glide calls for the
development of other methods capable of overcoming these limitations. Here we
develop a kinetic Monte Carlo (kMC) approach to study single screw dislocation
dynamics from room temperature to and at stresses
, where and are the melting point and
the Peierls stress. The method is entirely parameterized with atomistic
simulations using an embedded atom potential for tungsten. To increase the
physical fidelity of our simulations, we calculate the deviations from Schmid's
law prescribed by the interatomic potential used and we study single
dislocation kinetics using both projections. We calculate dislocation
velocities as a function of stress, temperature, and dislocation line length.
We find that considering non-Schmid effects has a strong influence on both the
magnitude of the velocities and the trajectories followed by the dislocation.
We finish by condensing all the calculated data into effective stress and
temperature dependent mobilities to be used in more homogenized numerical
methods
Ladder proof of nonlocality for two spin-half particles revisited
In this paper we extend the ladder proof of nonlocality without inequalities
for two spin-half particles given by Boschi et al [PRL 79, 2755 (1997)] to the
case in which the measurement settings of the apparatus measuring one of the
particles are different from the measurement settings of the apparatus
measuring the other particle. It is shown that, in any case, the proportion of
particle pairs for which the contradiction with local realism goes through is
maximized when the measurement settings are the same for each apparatus. Also
we write down a Bell inequality for the experiment in question which is
violated by quantum mechanics by an amount which is twice as much as the amount
by which quantum mechanics violates the Bell inequality considered in the above
paper by Boschi et al.Comment: LaTeX, 7 pages, 1 figure, journal versio
A feasible quantum optical experiment capable of refuting noncontextuality for single photons
Elaborating on a previous work by Simon et al. [PRL 85, 1783 (2000)] we
propose a realizable quantum optical single-photon experiment using standard
present day technology, capable of discriminating maximally between the
predictions of quantum mechanics (QM) and noncontextual hidden variable
theories (NCHV). Quantum mechanics predicts a gross violation (up to a factor
of 2) of the noncontextual Bell-like inequality associated with the proposed
experiment. An actual maximal violation of this inequality would demonstrate
(modulo fair sampling) an all-or-nothing type contradiction between QM and
NCHV.Comment: LaTeX file, 8 pages, 1 figur
Two-particle entanglement as a property of three-particle entangled states
In a recent article [Phys. Rev. A 54, 1793 (1996)] Krenn and Zeilinger
investigated the conditional two-particle correlations for the subensemble of
data obtained by selecting the results of the spin measurements by two
observers 1 and 2 with respect to the result found in the corresponding
measurement by a third observer. In this paper we write out explicitly the
condition required in order for the selected results of observers 1 and 2 to
violate Bell's inequality for general measurement directions. It is shown that
there are infinitely many sets of directions giving the maximum level of
violation. Further, we extend the analysis by the authors to the class of
triorthogonal states |Psi> = c_1 |z_1>|z_2>|z_3> + c_2 |-z_1>|-z_2>|-z_3>. It
is found that a maximal violation of Bell's inequality occurs provided the
corresponding three-particle state yields a direct ("all or nothing")
nonlocality contradiction.Comment: REVTeX, 7 pages, no figure
Independent Set Reconfiguration in Cographs
We study the following independent set reconfiguration problem, called
TAR-Reachability: given two independent sets and of a graph , both
of size at least , is it possible to transform into by adding and
removing vertices one-by-one, while maintaining an independent set of size at
least throughout? This problem is known to be PSPACE-hard in general. For
the case that is a cograph (i.e. -free graph) on vertices, we show
that it can be solved in time , and that the length of a shortest
reconfiguration sequence from to is bounded by , if such a
sequence exists.
More generally, we show that if is a graph class for which (i)
TAR-Reachability can be solved efficiently, (ii) maximum independent sets can
be computed efficiently, and which satisfies a certain additional property,
then the problem can be solved efficiently for any graph that can be obtained
from a collection of graphs in using disjoint union and complete join
operations. Chordal graphs are given as an example of such a class
Reconfiguring Independent Sets in Claw-Free Graphs
We present a polynomial-time algorithm that, given two independent sets in a
claw-free graph , decides whether one can be transformed into the other by a
sequence of elementary steps. Each elementary step is to remove a vertex
from the current independent set and to add a new vertex (not in )
such that the result is again an independent set. We also consider the more
restricted model where and have to be adjacent
Three-particle entanglement versus three-particle nonlocality
The notions of three-particle entanglement and three-particle nonlocality are
discussed in the light of Svetlichny's inequality [Phys. Rev. D 35, 3066
(1987)]. It is shown that there exist sets of measurements which can be used to
prove three-particle entanglement, but which are nevertheless useless at
proving three-particle nonlocality. In particular, it is shown that the quantum
predictions giving a maximal violation of Mermin's three-particle Bell
inequality [Phys. Rev. Lett. 65, 1838 (1990)] can be reproduced by a hybrid
hidden variables model in which nonlocal correlations are present only between
two of the particles. It should be possible, however, to test the existence of
both three-particle entanglement and three-particle nonlocality for any given
quantum state via Svetlichny's inequality.Comment: REVTeX4, 4 pages, journal versio
Bayesian Nash Equilibria and Bell Inequalities
Games with incomplete information are formulated in a multi-sector
probability matrix formalism that can cope with quantum as well as classical
strategies. An analysis of classical and quantum strategy in a multi-sector
extension of the game of Battle of Sexes clarifies the two distinct roles of
nonlocal strategies, and establish the direct link between the true quantum
gain of game's payoff and the breaking of Bell inequalities.Comment: 6 pages, LaTeX JPSJ 2 column format, changes in sections 1, 3 and 4,
added reference
Multiparty multilevel Greenberger-Horne-Zeilinger states
The proof of Bell's theorem without inequalities by Greenberger, Horne, and
Zeilinger (GHZ) is extended to multiparticle multilevel systems. The proposed
procedure generalizes previous partial results and provides an operational
characterization of the so-called GHZ states for multiparticle multilevel
systems.Comment: REVTeX, 5 pages, 1 figur
A Simulated Environment for Testing 4D Detect See and Avoid Scenarios for UAVs
A synthetic environment has been developed which permits the realistic simulation of encounter scenarios of two small aircraft. This may be used to develop and test Detect Sense and Avoid (DSA) strategies for Unmanned Aerial Vehicles (UAVs). A novel maneuver has been invented that simplifies the generation of air-to-air encounters. This âPhi Maneuverâ guarantees a variety of
encounter geometries, and at least one encounter for each pass of the Intruder aircraft through the loiter zone of the Target aircraft. This provides an effect way to test multiple DSA scenarios and generate statistics on the relative merits of a particular DSA method
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