426 research outputs found
Mean-field analysis of a dynamical phase transition in a cellular automaton model for collective motion
A cellular automaton model is presented for random walkers with biologically
motivated interactions favoring local alignment and leading to collective
motion or swarming behavior. The degree of alignment is controlled by a
sensitivity parameter, and a dynamical phase transition exhibiting spontaneous
breaking of rotational symmetry occurs at a critical parameter value. The model
is analyzed using nonequilibrium mean field theory: Dispersion relations for
the critical modes are derived, and a phase diagram is constructed. Mean field
predictions for the two critical exponents describing the phase transition as a
function of sensitivity and density are obtained analytically.Comment: 4 pages, 4 figures, final version as publishe
Monte Carlo simulations of random copolymers at a selective interface
We investigate numerically using the bond--fluctuation model the adsorption
of a random AB--copolymer at the interface between two solvents. From our
results we infer several scaling relations: the radius of gyration of the
copolymer in the direction perpendicular to the interface () scales
with , the interfacial selectivity strength, as
where is the usual Flory exponent and
is the copolymer's length; furthermore the monomer density at the interface
scales as for small . We also determine numerically the
monomer densities in the two solvents and discuss their dependence on the
distance from the interface.Comment: Latex text file appended with figures.tar.g
Preparation of distilled and purified continuous variable entangled states
The distribution of entangled states of light over long distances is a major
challenge in the field of quantum information. Optical losses, phase diffusion
and mixing with thermal states lead to decoherence and destroy the
non-classical states after some finite transmission-line length. Quantum
repeater protocols, which combine quantum memory, entanglement distillation and
entanglement swapping, were proposed to overcome this problem. Here we report
on the experimental demonstration of entanglement distillation in the
continuous-variable regime. Entangled states were first disturbed by random
phase fluctuations and then distilled and purified using interference on beam
splitters and homodyne detection. Measurements of covariance matrices clearly
indicate a regained strength of entanglement and purity of the distilled
states. In contrast to previous demonstrations of entanglement distillation in
the complementary discrete-variable regime, our scheme achieved the actual
preparation of the distilled states, which might therefore be used to improve
the quality of downstream applications such as quantum teleportation
Many worlds in one
A generic prediction of inflation is that the thermalized region we inhabit
is spatially infinite. Thus, it contains an infinite number of regions of the
same size as our observable universe, which we shall denote as \O-regions. We
argue that the number of possible histories which may take place inside of an
\O-region, from the time of recombination up to the present time, is finite.
Hence, there are an infinite number of \O-regions with identical histories up
to the present, but which need not be identical in the future. Moreover, all
histories which are not forbidden by conservation laws will occur in a finite
fraction of all \O-regions. The ensemble of \O-regions is reminiscent of
the ensemble of universes in the many-world picture of quantum mechanics. An
important difference, however, is that other \O-regions are unquestionably
real.Comment: 9 pages, 2 figures, comments and references adde
Critical exponents for random knots
The size of a zero thickness (no excluded volume) polymer ring is shown to
scale with chain length in the same way as the size of the excluded volume
(self-avoiding) linear polymer, as , where . The
consequences of that fact are examined, including sizes of trivial and
non-trivial knots.Comment: 4 pages, 0 figure
Do non-philosophers think epistemic consequentialism is counterintuitive?
Direct epistemic consequentialism is the idea that X is epistemically permissible iff X maximizes epistemic value. It has received lots of attention in recent years and is widely accepted by philosophers to have counterintuitive implications. There are various reasons one might suspect that the relevant intuitions will not be widely shared among non-philosophers. This paper presents an initial empirical study of ordinary intuitions. The results of two experiments demonstrate that the counterintuitiveness of epistemic consequentialism is more than a philosophers' worry---the folk seem to agree
Who Produces for Whom in the World Economy?
International audienceFor two decades, the share of trade in inputs, also called vertical trade, has been dramatically increasing. In reallocating trade flows to their original input-producing industries and countries, this paper suggests a new measure of international trade: "value-added trade" and makes it possible to answer the question "who produces for whom?". In 2004, 27% of international trade was vertical trade. The industrial and geographic patterns of value-added trade are very different from those of standard trade. Value-added trade is relatively less important in regional trade but the difference is not more important for Asia than for AmericaLa part du commerce en produits intermédiaires dans le commerce international, appelé aussi "commerce vertical", n'a cessé d'augmenter depuis vingt ans. Cet article propose une nouvelle mesure du commerce international "le commerce en valeur ajoutée" qui réalloue les flux commerciaux aux pays et aux secteurs produisant les intrants. Les répartitions géographique et sectorielle du commerce en valeur ajoutée sont très différentes de celles du commerce " standard ". La différence entre le commerce en valeur ajoutée et le commerce standard est plus importante dans le cas du commerce régional mais ce n'est pas plus le cas en Asie qu'en Amérique
Mixtures of Bosonic and Fermionic Atoms in Optical Lattices
We discuss the theory of mixtures of Bosonic and Fermionic atoms in periodic
potentials at zero temperature. We derive a general Bose--Fermi Hubbard
Hamiltonian in a one--dimensional optical lattice with a superimposed harmonic
trapping potential. We study the conditions for linear stability of the mixture
and derive a mean field criterion for the onset of a Bosonic superfluid
transition. We investigate the ground state properties of the mixture in the
Gutzwiller formulation of mean field theory, and present numerical studies of
finite systems. The Bosonic and Fermionic density distributions and the onset
of quantum phase transitions to demixing and to a Bosonic Mott--insulator are
studied as a function of the lattice potential strength. The existence is
predicted of a disordered phase for mixtures loaded in very deep lattices. Such
a disordered phase possessing many degenerate or quasi--degenerate ground
states is related to a breaking of the mirror symmetry in the lattice.Comment: 11 pages, 8 figures; added discussions; conclusions and references
expande
The pursuit of happiness: the social and scientific origins of Hans Selye’s natural philosophy of life
© The Author(s) 2012. Open access article.In 1956, Hans Selye tentatively suggested that the scientific study of stress could ‘help us
to formulate a precise program of conduct’ and ‘teach us the wisdom to live a rich and
meaningful life’. Nearly two decades later, Selye expanded this limited vision of social
order into a full-blown philosophy of life. In Stress without Distress, first published in 1974,
he proposed an ethical code of conduct designed to mitigate personal and social
problems. Basing his arguments on contemporary understandings of the biological
processes involved in stress reactions, Selye referred to this code as ‘altruistic egotism’.
This article explores the origins and evolution of Selye’s ‘natural philosophy of life’,
analysing the links between his theories and adjacent intellectual developments in
biology, psychosomatic and psychosocial medicine, cybernetics and socio-biology, and
situating his work in the broader cultural framework of modern western societies.Wellcome Trus
Quantum Communication
Quantum communication, and indeed quantum information in general, has changed
the way we think about quantum physics. In 1984 and 1991, the first protocol
for quantum cryptography and the first application of quantum non-locality,
respectively, attracted a diverse field of researchers in theoretical and
experimental physics, mathematics and computer science. Since then we have seen
a fundamental shift in how we understand information when it is encoded in
quantum systems. We review the current state of research and future directions
in this new field of science with special emphasis on quantum key distribution
and quantum networks.Comment: Submitted version, 8 pg (2 cols) 5 fig
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