1,960 research outputs found
Soliton creation during a Bose-Einstein condensation
We use stochastic Gross-Pitaevskii equation to study dynamics of
Bose-Einstein condensation. We show that cooling into a Bose-Einstein
condensate (BEC) can create solitons with density given by the cooling rate and
by the critical exponents of the transition. Thus, counting solitons left in
its wake should allow one to determine the critical exponents z and nu for a
BEC phase transition. The same information can be extracted from two-point
correlation functions.Comment: 4 pages, 3 figures, improved version to appear in PRL: scalings
discussed more extensively, fitting scheme for determination of z and nu
critical exponents is explaine
Topological Schr\"odinger cats: Non-local quantum superpositions of topological defects
Topological defects (such as monopoles, vortex lines, or domain walls) mark
locations where disparate choices of a broken symmetry vacuum elsewhere in the
system lead to irreconcilable differences. They are energetically costly (the
energy density in their core reaches that of the prior symmetric vacuum) but
topologically stable (the whole manifold would have to be rearranged to get rid
of the defect). We show how, in a paradigmatic model of a quantum phase
transition, a topological defect can be put in a non-local superposition, so
that - in a region large compared to the size of its core - the order parameter
of the system is "undecided" by being in a quantum superposition of conflicting
choices of the broken symmetry. We demonstrate how to exhibit such a
"Schr\"odinger kink" by devising a version of a double-slit experiment suitable
for topological defects. Coherence detectable in such experiments will be
suppressed as a consequence of interaction with the environment. We analyze
environment-induced decoherence and discuss its role in symmetry breaking.Comment: 7 pages, 4 figure
Sub-Planck spots of Schroedinger cats and quantum decoherence
Heisenberg's principle states that the product of uncertainties of
position and momentum should be no less than Planck's constant . This is
usually taken to imply that phase space structures associated with sub-Planck
() scales do not exist, or, at the very least, that they do not
matter. I show that this deeply ingrained prejudice is false: Non-local
"Schr\"odinger cat" states of quantum systems confined to phase space volume
characterized by `the classical action' develop spotty structure
on scales corresponding to sub-Planck . Such
structures arise especially quickly in quantum versions of classically chaotic
systems (such as gases, modelled by chaotic scattering of molecules), that are
driven into nonlocal Schr\"odinger cat -- like superpositions by the quantum
manifestations of the exponential sensitivity to perturbations. Most
importantly, these sub-Planck scales are physically significant: determines
sensitivity of a quantum system (or of a quantum environment) to perturbations.
Therefore sub-Planck controls the effectiveness of decoherence and
einselection caused by the environment. It may also be relevant in
setting limits on sensitivity of Schr\"odinger cats used as detectors.Comment: Published in Nature 412, 712-717 (2001
The Chandra Fornax Survey - I: The Cluster Environment
We present the first results of a deep Chandra survey of the inner 1 degree
of the Fornax cluster of galaxies. Ten 50 ksec pointings were obtained in a
mosaic centered on the giant elliptical galaxy NGC 1399 at the nominal cluster
center. Emission and temperature maps of Fornax are presented, and an initial
study of 771 detected X-ray point sources is made. Regions as small as 100pc
are resolved. The intra-cluster gas in Fornax exhibits a highly asymmetric
morphology and temperature structure, dominated by a 180 kpc extended ``plume''
of low surface brightness, cool, ~1 keV) gas to the North-East of NGC 1399 with
a sharper edge to the South West. The elliptical galaxy NGC 1404 also exhibits
a cool halo of X-ray gas within the cluster, with a highly sharpened leading
edge as it presumably falls into the cluster, and a cometary-like tail. We
estimate that some ~200-400 point sources are physically associated with
Fornax. Confirming earlier works, we find that the globular cluster population
in NGC 1399 is highly X-ray active, extending to globulars which may in fact be
intra-cluster systems. We have also found a remarkable correlation between the
location of giant and dwarf cluster galaxies and the presence of X-ray
counterparts, such that systems inhabiting regions of low gas density are more
likely to show X-ray activity. Not only does this correlate with the asymmetry
of the intra-cluster gas but also with the axis joining the center of Fornax to
an infalling group 1 Mpc to the South-West. We suggest that Fornax may be
experiencing an intergalactic ``headwind'' due to motion relative to the
surrounding large-scale structure.Comment: 35 pages, 15 figures, submitted to ApJ. Most figures not included
owing to severe compression degradation - we strongly recommend downloading
the full resolution paper from
http://www.astro.columbia.edu/~caleb/ms_highres.pdf (1.9Mb
Decoherence, Re-coherence, and the Black Hole Information Paradox
We analyze a system consisting of an oscillator coupled to a field. With the
field traced out as an environment, the oscillator loses coherence on a very
short {\it decoherence timescale}; but, on a much longer {\it relaxation
timescale}, predictably evolves into a unique, pure (ground) state. This
example of {\it re-coherence} has interesting implications both for the
interpretation of quantum theory and for the loss of information during black
hole evaporation. We examine these implications by investigating the
intermediate and final states of the quantum field, treated as an open system
coupled to an unobserved oscillator.Comment: 23 pages, 2 figures included, figures 3.1 - 3.3 available at
http://qso.lanl.gov/papers/Papers.htm
Physical Complexity of Symbolic Sequences
A practical measure for the complexity of sequences of symbols (``strings'')
is introduced that is rooted in automata theory but avoids the problems of
Kolmogorov-Chaitin complexity. This physical complexity can be estimated for
ensembles of sequences, for which it reverts to the difference between the
maximal entropy of the ensemble and the actual entropy given the specific
environment within which the sequence is to be interpreted. Thus, the physical
complexity measures the amount of information about the environment that is
coded in the sequence, and is conditional on such an environment. In practice,
an estimate of the complexity of a string can be obtained by counting the
number of loci per string that are fixed in the ensemble, while the volatile
positions represent, again with respect to the environment, randomness. We
apply this measure to tRNA sequence data.Comment: 12 pages LaTeX2e, 3 postscript figures, uses elsart.cls.
Substantially improved and clarified version, includes application to EMBL
tRNA sequence dat
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