265 research outputs found
Geometric phases in quantum control disturbed by classical stochastic processes
We describe the geometric (Berry) phases arising when some quantum systems
are driven by control classical parameters but also by outer classical
stochastic processes (as for example classical noises). The total geometric
phase is then divided into an usual geometric phase associated with the control
parameters and a second geometric phase associated with the stochastic
processes. The geometric structure in which these geometric phases take place
is a composite bundle (and not an usual principal bundle), which is explicitely
built in this paper. We explain why the composite bundle structure is the more
natural framework to study this problem. Finally we treat a very simple example
of a two level atom driven by a phase modulated laser field with a phase
instability described by a gaussian white noise. In particular we compute the
average geometric phase issued from the noise
Exotic Smoothness and Physics
The essential role played by differentiable structures in physics is reviewed
in light of recent mathematical discoveries that topologically trivial
space-time models, especially the simplest one, , possess a rich
multiplicity of such structures, no two of which are diffeomorphic to each
other and thus to the standard one. This means that physics has available to it
a new panoply of structures available for space-time models. These can be
thought of as source of new global, but not properly topological, features.
This paper reviews some background differential topology together with a
discussion of the role which a differentiable structure necessarily plays in
the statement of any physical theory, recalling that diffeomorphisms are at the
heart of the principle of general relativity. Some of the history of the
discovery of exotic, i.e., non-standard, differentiable structures is reviewed.
Some new results suggesting the spatial localization of such exotic structures
are described and speculations are made on the possible opportunities that such
structures present for the further development of physical theories.Comment: 13 pages, LaTe
Natural extensions and entropy of -continued fractions
We construct a natural extension for each of Nakada's -continued
fractions and show the continuity as a function of of both the entropy
and the measure of the natural extension domain with respect to the density
function . In particular, we show that, for all , the product of the entropy with the measure of the domain equals .
As a key step, we give the explicit relationship between the -expansion
of and of
Separation of trajectories and its Relation to Entropy for Intermittent Systems with a Zero Lyapunov exponent
One dimensional intermittent maps with stretched exponential separation of
nearby trajectories are considered. When time goes infinity the standard
Lyapunov exponent is zero. We investigate the distribution of
,
where is determined by the nonlinearity of the map in the vicinity of
marginally unstable fixed points. The mean of is determined
by the infinite invariant density. Using semi analytical arguments we calculate
the infinite invariant density for the Pomeau-Manneville map, and with it
obtain excellent agreement between numerical simulation and theory. We show
that \alpha \left is equal to Krengel's entropy and
to the complexity calculated by the Lempel-Ziv compression algorithm. This
generalized Pesin's identity shows that \left and
Krengel's entropy are the natural generalizations of usual Lyapunov exponent
and entropy for these systems.Comment: 12 pages, 10 figure
Pesin-type relation for subexponential instability
We address here the problem of extending the Pesin relation among positive
Lyapunov exponents and the Kolmogorov-Sinai entropy to the case of dynamical
systems exhibiting subexponential instabilities. By using a recent rigorous
result due to Zweim\"uller, we show that the usual Pesin relation can be
extended straightforwardly for weakly chaotic one-dimensional systems of the
Pomeau-Manneville type, provided one introduces a convenient subexponential
generalization of the Kolmogorov-Sinai entropy. We show, furthermore, that
Zweim\"uller's result provides an efficient prescription for the evaluation of
the algorithm complexity for such systems. Our results are confirmed by
exhaustive numerical simulations. We also point out and correct a misleading
extension of the Pesin relation based on the Krengel entropy that has appeared
recently in the literature.Comment: 10 pages, 4 figures. Final version to appear in Journal of
Statistical Mechanics (JSTAT
The entropy of alpha-continued fractions: numerical results
We consider the one-parameter family of interval maps arising from
generalized continued fraction expansions known as alpha-continued fractions.
For such maps, we perform a numerical study of the behaviour of metric entropy
as a function of the parameter. The behaviour of entropy is known to be quite
regular for parameters for which a matching condition on the orbits of the
endpoints holds. We give a detailed description of the set M where this
condition is met: it consists of a countable union of open intervals,
corresponding to different combinatorial data, which appear to be arranged in a
hierarchical structure. Our experimental data suggest that the complement of M
is a proper subset of the set of bounded-type numbers, hence it has measure
zero. Furthermore, we give evidence that the entropy on matching intervals is
smooth; on the other hand, we can construct points outside of M on which it is
not even locally monotone.Comment: 33 pages, 14 figure
Identification of the Major Expressed S-Layer and Cell Surface-Layer-Related Proteins in the Model Methanogenic Archaea: Methanosarcina barkeri Fusaro and Methanosarcina acetivorans C2A
Many archaeal cell envelopes contain a protein coat or sheath composed of one or more surface exposed proteins. These surface layer (S-layer) proteins contribute structural integrity and protect the lipid membrane from environmental challenges. To explore the species diversity of these layers in the Methanosarcinaceae, the major S-layer protein in Methanosarcina barkeri strain Fusaro was identified using proteomics. The Mbar_A1758 gene product was present in multiple forms with apparent sizes of 130, 120, and 100 kDa, consistent with post-translational modifications including signal peptide excision and protein glycosylation. A protein with features related to the surface layer proteins found in Methanosarcina acetivorans C2A and Methanosarcina mazei Goel was identified in the M. barkeri genome. These data reveal a distinct conserved protein signature with features and implied cell surface architecture in the Methanosarcinaceae that is absent in other archaea. Paralogous gene expression patterns in two Methanosarcina species revealed abundant expression of a single S-layer paralog in each strain. Respective promoter elements were identified and shown to be conserved in mRNA coding and upstream untranslated regions. Prior M. acetivorans genome annotations assigned S-layer or surface layer associated roles of eighty genes: however, of 68 examined none was significantly expressed relative to the experimentally determined S-layer gene
Pyrophosphate-Dependent ATP Formation from Acetyl Coenzyme A in Syntrophus aciditrophicus, a New Twist on ATP Formation.
UnlabelledSyntrophus aciditrophicus is a model syntrophic bacterium that degrades key intermediates in anaerobic decomposition, such as benzoate, cyclohexane-1-carboxylate, and certain fatty acids, to acetate when grown with hydrogen-/formate-consuming microorganisms. ATP formation coupled to acetate production is the main source for energy conservation by S. aciditrophicus However, the absence of homologs for phosphate acetyltransferase and acetate kinase in the genome of S. aciditrophicus leaves it unclear as to how ATP is formed, as most fermentative bacteria rely on these two enzymes to synthesize ATP from acetyl coenzyme A (CoA) and phosphate. Here, we combine transcriptomic, proteomic, metabolite, and enzymatic approaches to show that S. aciditrophicus uses AMP-forming, acetyl-CoA synthetase (Acs1) for ATP synthesis from acetyl-CoA. acs1 mRNA and Acs1 were abundant in transcriptomes and proteomes, respectively, of S. aciditrophicus grown in pure culture and coculture. Cell extracts of S. aciditrophicus had low or undetectable acetate kinase and phosphate acetyltransferase activities but had high acetyl-CoA synthetase activity under all growth conditions tested. Both Acs1 purified from S. aciditrophicus and recombinantly produced Acs1 catalyzed ATP and acetate formation from acetyl-CoA, AMP, and pyrophosphate. High pyrophosphate levels and a high AMP-to-ATP ratio (5.9 ± 1.4) in S. aciditrophicus cells support the operation of Acs1 in the acetate-forming direction. Thus, S. aciditrophicus has a unique approach to conserve energy involving pyrophosphate, AMP, acetyl-CoA, and an AMP-forming, acetyl-CoA synthetase.ImportanceBacteria use two enzymes, phosphate acetyltransferase and acetate kinase, to make ATP from acetyl-CoA, while acetate-forming archaea use a single enzyme, an ADP-forming, acetyl-CoA synthetase, to synthesize ATP and acetate from acetyl-CoA. Syntrophus aciditrophicus apparently relies on a different approach to conserve energy during acetyl-CoA metabolism, as its genome does not have homologs to the genes for phosphate acetyltransferase and acetate kinase. Here, we show that S. aciditrophicus uses an alternative approach, an AMP-forming, acetyl-CoA synthetase, to make ATP from acetyl-CoA. AMP-forming, acetyl-CoA synthetases were previously thought to function only in the activation of acetate to acetyl-CoA
Limit theorems for von Mises statistics of a measure preserving transformation
For a measure preserving transformation of a probability space
we investigate almost sure and distributional convergence
of random variables of the form where (called the \emph{kernel})
is a function from to and are appropriate normalizing
constants. We observe that the above random variables are well defined and
belong to provided that the kernel is chosen from the projective
tensor product with We establish a form of the individual ergodic theorem for such
sequences. Next, we give a martingale approximation argument to derive a
central limit theorem in the non-degenerate case (in the sense of the classical
Hoeffding's decomposition). Furthermore, for and a wide class of
canonical kernels we also show that the convergence holds in distribution
towards a quadratic form in independent
standard Gaussian variables . Our results on the
distributional convergence use a --\,invariant filtration as a prerequisite
and are derived from uni- and multivariate martingale approximations
Exotic Differentiable Structures and General Relativity
We review recent developments in differential topology with special concern
for their possible significance to physical theories, especially general
relativity. In particular we are concerned here with the discovery of the
existence of non-standard (``fake'' or ``exotic'') differentiable structures on
topologically simple manifolds such as , \R and
Because of the technical difficulties involved in the smooth case, we begin
with an easily understood toy example looking at the role which the choice of
complex structures plays in the formulation of two-dimensional vacuum
electrostatics. We then briefly review the mathematical formalisms involved
with differentiable structures on topological manifolds, diffeomorphisms and
their significance for physics. We summarize the important work of Milnor,
Freedman, Donaldson, and others in developing exotic differentiable structures
on well known topological manifolds. Finally, we discuss some of the geometric
implications of these results and propose some conjectures on possible physical
implications of these new manifolds which have never before been considered as
physical models.Comment: 11 pages, LaTe
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