371 research outputs found
Exponential Decay of Correlations for Strongly Coupled Toom Probabilistic Cellular Automata
We investigate the low-noise regime of a large class of probabilistic
cellular automata, including the North-East-Center model of Toom. They are
defined as stochastic perturbations of cellular automata belonging to the
category of monotonic binary tessellations and possessing a property of
erosion. We prove, for a set of initial conditions, exponential convergence of
the induced processes toward an extremal invariant measure with a highly
predominant spin value. We also show that this invariant measure presents
exponential decay of correlations in space and in time and is therefore
strongly mixing.Comment: 21 pages, 0 figure, revised version including a generalization to a
larger class of models, structure of the arguments unchanged, minor changes
suggested by reviewers, added reference
A Toom rule that increases the thickness of sets
Toom's north-east-self voting cellular automaton rule R is known to suppress
small minorities. A variant which we call R^+ is also known to turn an
arbitrary initial configuration into a homogenous one (without changing the
ones that were homogenous to start with). Here we show that R^+ always
increases a certain property of sets called thickness. This result is intended
as a step towards a proof of the fast convergence towards consensus under R^+.
The latter is observable experimentally, even in the presence of some noise.Comment: 16 pages, 8 figure
Bracketing off population does not advance ethical reflection on EVCs: a reply to Kayser and Schneider
Involvement of BMP-2, TGF-Ăź2 and TGF-Ăź3 Signaling in Initial and Early Stages of Heterotopic Ossification in a rat Experimental Model
This study focused on the localization and expression of bone morphogenetic protein 2 (BMP-2) and different isoforms of transforming growth factor s (TGF-s1, TGF-s2 and TGF-s3) in the initial and early stages of heterotopic ossification (HO) employing an animal model mimicking the situation after total hip arthroplasty (THA). Bone growth was induced in rats using s-tricalcium phosphate implants immersed either in osteoinductive rhBMP-2 solution or in saline and implanted at the site where the HO is usually expected to develop after THA. Implants were removed at 3 or 21 days after the operation and handled according to stereology principles. mRNA expression and protein staining of growth factors in different types of tissues was determined by in situ hybridization and immunohistochemistry, respectively. After three days, TGF-s3 content in the undifferentiated mesenchymal-like cells in the rhBMP-2 treated implants was, as assessed by immunohistochemistry, 49.6% higher compared to the saline treated group (p=0.024). This was also supported by in situ hybridization of mRNA of TGF-s3, which showed stronger expression in rhBMP-2 treated group. Immunohistochemical investigation showed that after 21 days the connective tissue in the rhBMP-2 treated implants contained more TGF-s1, TGF-s2 and TGF-s3, compared to BMP-2 and osteoblasts contained significantly (27.2%) more TGF-s3 compared to TGF-s1 (p=0.045). In the formed HO the proportion of the TGF-s2 and TGF-s3 producing bone tissue was increased by 32.1% and 47.8% respectively, compared to the TGF-s1 producing bone tissue (p=0.007 and p=0.006) and although this difference was not so clear at mRNA level, this suggests that TGF-s2 and TGF-s3 signaling seem to play an important role during initial and early stages of HO formation.
Invariant Measures and Decay of Correlations for a Class of Ergodic Probabilistic Cellular Automata
We give new sufficient ergodicity conditions for two-state probabilistic
cellular automata (PCA) of any dimension and any radius. The proof of this
result is based on an extended version of the duality concept. Under these
assumptions, in the one dimensional case, we study some properties of the
unique invariant measure and show that it is shift-mixing. Also, the decay of
correlation is studied in detail. In this sense, the extended concept of
duality gives exponential decay of correlation and allows to compute
explicitily all the constants involved
Harness processes and harmonic crystals
In the Hammersley harness processes the real-valued height at each site i in
Z^d is updated at rate 1 to an average of the neighboring heights plus a
centered random variable (the noise). We construct the process "a la Harris"
simultaneously for all times and boxes contained in Z^d. With this
representation we compute covariances and show L^2 and almost sure time and
space convergence of the process. In particular, the process started from the
flat configuration and viewed from the height at the origin converges to an
invariant measure. In dimension three and higher, the process itself converges
to an invariant measure in L^2 at speed t^{1-d/2} (this extends the convergence
established by Hsiao). When the noise is Gaussian the limiting measures are
Gaussian fields (harmonic crystals) and are also reversible for the process.Comment: 21 pages. Revised version with minor changes. Version almost
identical to the one to be published in SP
Quantum memories based on engineered dissipation
Storing quantum information for long times without disruptions is a major
requirement for most quantum information technologies. A very appealing
approach is to use self-correcting Hamiltonians, i.e. tailoring local
interactions among the qubits such that when the system is weakly coupled to a
cold bath the thermalization process takes a long time. Here we propose an
alternative but more powerful approach in which the coupling to a bath is
engineered, so that dissipation protects the encoded qubit against more general
kinds of errors. We show that the method can be implemented locally in four
dimensional lattice geometries by means of a toric code, and propose a simple
2D set-up for proof of principle experiments.Comment: 6 +8 pages, 4 figures, Includes minor corrections updated references
and aknowledgement
Stochastic theory of non-equilibrium wetting
We study a Langevin equation describing non-equilibrium depinning and wetting
transitions. Attention is focused on short-ranged attractive
substrate-interface potentials. We confirm the existence of first order
depinning transitions, in the temperature-chemical potential diagram, and a
tricritical point beyond which the transition becomes a non-equilibrium
complete wetting transition. The coexistence of pinned and depinned interfaces
occurs over a finite area, in line with other non-equilibrium systems that
exhibit first order transitions. In addition, we find two types of phase
coexistence, one of which is characterized by spatio-temporal intermittency
(STI). A finite size analysis of the depinning time is used to characterize the
different coexisting regimes. Finally, a stationary distribution of
characteristic triangles or facets was shown to be responsible for the
structure of the STI phase.Comment: To appear in Europhys. Lett. // 3 figure
Influence of transport and ocean ice extent on biogenic aerosol sulfur in the Arctic atmosphere
The recent decline in sea ice cover in the Arctic Ocean could affect the regional radiative forcing via changes in sea ice-atmosphere exchange of dimethyl sulfide (DMS) and biogenic aerosols formed from its atmospheric oxidation, such as methanesulfonic acid (MSA). This study examines relationships between changes in total sea ice extent north of 70 degrees N and atmospheric MSA measurement at Alert, Nunavut, during 1980-2009; at Barrow, Alaska, during 1997-2008; and at Ny-Alesund, Svalbard, for 1991-2004. During the 1980-1989 and 1990-1997 periods, summer (July-August) and June MSA concentrations at Alert decreased. In general, MSA concentrations increased at all locations since 2000 with respect to 1990 values, specifically during June and summer at Alert and in summer at Barrow and Ny-Alesund. Our results show variability in MSA at all sites is related to changes in the source strengths of DMS, possibly linked to changes in sea ice extent as well as to changes in atmospheric transport patterns. Since 2000, a late spring increase in atmospheric MSA at the three sites coincides with the northward migration of the marginal ice edge zone where high DMS emissions from ocean to atmosphere have previously been reported. Significant negative correlations are found between sea ice extent and MSA concentrations at the three sites during the spring and June. These results suggest that a decrease in seasonal ice cover influencing other mechanisms of DMS production could lead to higher atmospheric MSA concentrations
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