44,513 research outputs found
MCViNE -- An object oriented Monte Carlo neutron ray tracing simulation package
MCViNE (Monte-Carlo VIrtual Neutron Experiment) is a versatile Monte Carlo
(MC) neutron ray-tracing program that provides researchers with tools for
performing computer modeling and simulations that mirror real neutron
scattering experiments. By adopting modern software engineering practices such
as using composite and visitor design patterns for representing and accessing
neutron scatterers, and using recursive algorithms for multiple scattering,
MCViNE is flexible enough to handle sophisticated neutron scattering problems
including, for example, neutron detection by complex detector systems, and
single and multiple scattering events in a variety of samples and sample
environments. In addition, MCViNE can take advantage of simulation components
in linear-chain-based MC ray tracing packages widely used in instrument design
and optimization, as well as NumPy-based components that make prototypes useful
and easy to develop. These developments have enabled us to carry out detailed
simulations of neutron scattering experiments with non-trivial samples in
time-of-flight inelastic instruments at the Spallation Neutron Source. Examples
of such simulations for powder and single-crystal samples with various
scattering kernels, including kernels for phonon and magnon scattering, are
presented. With simulations that closely reproduce experimental results,
scattering mechanisms can be turned on and off to determine how they contribute
to the measured scattering intensities, improving our understanding of the
underlying physics.Comment: 34 pages, 14 figure
Quantification of abnormal repetitive behaviour in captive European starlings (Sturnus vulgaris).
Stereotypies are repetitive, unvarying and goalless behaviour patterns that are often considered indicative of poor welfare in captive animals. Quantifying stereotypies can be difficult, particularly during the early stages of their development when behaviour is still flexible. We compared two methods for objectively quantifying the development of route-tracing stereotypies in caged starlings. We used Markov chains and T-pattern analysis (implemented by the software package, Theme) to identify patterns in the sequence of locations a bird occupied within its cage. Pattern metrics produced by both methods correlated with the frequency of established measures of stereotypic behaviour and abnormal behaviour patterns counted from video recordings, suggesting that both methods could be useful for identifying stereotypic individuals and quantifying stereotypic behaviour. We discuss the relative benefits and disadvantages of the two approaches
Coarse grained belief propagation for simulation of interacting quantum systems at all temperatures
We continue our numerical study of quantum belief propagation initiated in
[Phys. Rev. A, 77 (2008), p. 052318]. We demonstrate how the method can be
expressed in terms of an effective thermal potential that materializes when the
system presents quantum correlations, but is insensitive to classical
correlations. The thermal potential provides an efficient means to assess the
precision of belief propagation on graphs with no loops. We illustrate these
concepts using the one-dimensional quantum Ising model and compare our results
with exact solutions. We also use the method to study the transverse field
quantum Ising spin glass for which we obtain a phase diagram that is largely in
agreement with the one obtained in [arXiv:0706.4391] using a different
approach. Finally, we introduce the coarse grained belief propagation (CGBP)
algorithm to improve belief propagation at low temperatures. This method
combines the reliability of belief propagation at high temperatures with the
ability of entanglement renormalization to efficiently describe low energy
subspaces of quantum systems with local interactions. With CGBP, thermodynamic
properties of quantum systems can be calculated with a high degree of accuracy
at all temperatures.Comment: updated references and acknowledgement
Differentiable Programming Tensor Networks
Differentiable programming is a fresh programming paradigm which composes
parameterized algorithmic components and trains them using automatic
differentiation (AD). The concept emerges from deep learning but is not only
limited to training neural networks. We present theory and practice of
programming tensor network algorithms in a fully differentiable way. By
formulating the tensor network algorithm as a computation graph, one can
compute higher order derivatives of the program accurately and efficiently
using AD. We present essential techniques to differentiate through the tensor
networks contractions, including stable AD for tensor decomposition and
efficient backpropagation through fixed point iterations. As a demonstration,
we compute the specific heat of the Ising model directly by taking the second
order derivative of the free energy obtained in the tensor renormalization
group calculation. Next, we perform gradient based variational optimization of
infinite projected entangled pair states for quantum antiferromagnetic
Heisenberg model and obtain start-of-the-art variational energy and
magnetization with moderate efforts. Differentiable programming removes
laborious human efforts in deriving and implementing analytical gradients for
tensor network programs, which opens the door to more innovations in tensor
network algorithms and applications.Comment: Typos corrected, discussion and refs added; revised version accepted
for publication in PRX. Source code available at
https://github.com/wangleiphy/tensorgra
Quantum Belief Propagation
We present an accurate numerical algorithm, called quantum belief propagation
(QBP), for simulation of one-dimensional quantum systems at non-zero
temperature. The algorithm exploits the fact that quantum effects are
short-range in these systems at non-zero temperature, decaying on a length
scale inversely proportional to the temperature. We compare to exact results on
a spin-1/2 Heisenberg chain. Even a very modest calculation, requiring
diagonalizing only 10-by-10 matrices, reproduces the peak susceptibility with a
relative error of less than , while more elaborate calculations
further reduce the error.Comment: 4 pages, 1 figure; revised time estimates due to improved
implementation. Typographical corrections to Eq. 7 made; thanks to David
Poulin for pointing out the mistak
The Computation of Perfect and Proper Equilibrium for Finite Games via Simulated Annealing
This paper exploits an analogy between the “trembles” that underlie the functioning of simulated annealing and the player “trembles” that underlie the Nash refinements known as perfect and proper equilibrium. This paper shows that this relationship can be used to provide a method for computing perfect and proper equilibria of n-player strategic games. This paper also shows, by example, that simulated annealing can be used to locate a perfect equilibrium in an extensive form game.Game Theory
Chaos and Complexity of quantum motion
The problem of characterizing complexity of quantum dynamics - in particular
of locally interacting chains of quantum particles - will be reviewed and
discussed from several different perspectives: (i) stability of motion against
external perturbations and decoherence, (ii) efficiency of quantum simulation
in terms of classical computation and entanglement production in operator
spaces, (iii) quantum transport, relaxation to equilibrium and quantum mixing,
and (iv) computation of quantum dynamical entropies. Discussions of all these
criteria will be confronted with the established criteria of integrability or
quantum chaos, and sometimes quite surprising conclusions are found. Some
conjectures and interesting open problems in ergodic theory of the quantum many
problem are suggested.Comment: 45 pages, 22 figures, final version, at press in J. Phys. A, special
issue on Quantum Informatio
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