333 research outputs found
Communication Complexity and Intrinsic Universality in Cellular Automata
The notions of universality and completeness are central in the theories of
computation and computational complexity. However, proving lower bounds and
necessary conditions remains hard in most of the cases. In this article, we
introduce necessary conditions for a cellular automaton to be "universal",
according to a precise notion of simulation, related both to the dynamics of
cellular automata and to their computational power. This notion of simulation
relies on simple operations of space-time rescaling and it is intrinsic to the
model of cellular automata. Intrinsinc universality, the derived notion, is
stronger than Turing universality, but more uniform, and easier to define and
study. Our approach builds upon the notion of communication complexity, which
was primarily designed to study parallel programs, and thus is, as we show in
this article, particulary well suited to the study of cellular automata: it
allowed to show, by studying natural problems on the dynamics of cellular
automata, that several classes of cellular automata, as well as many natural
(elementary) examples, could not be intrinsically universal
Magnetic dephasing in mesoscopic spin glasses
We have measured Universal Conductance Fluctuations in the metallic spin
glass Ag:Mn as a function of temperature and magnetic field. From this
measurement, we can access the phase coherence time of the electrons in the
spin glass. We show that this phase coherence time increases with both the
inverse of the temperature and the magnetic field. From this we deduce that
decoherence mechanisms are still active even deep in the spin glass phase
Modélisations numériques des pertes en régime variable dans des tubes supraconducteurs
Les pertes AC dans les câbles supraconducteurs générées par un environnement variable dans le temps impactent la cryogénie et donc la faisabilité industrielle des dispositifs supraconducteurs. Nexans est aujourd'hui sur le point de réaliser des fils cylindriques supraconducteurs pour des câbles de forte puissance. Aucune étude numérique n'a pour l'instant porté sur le calcul des pertes AC dans un ou plusieurs tubes. Cet article présente les étapes de création d'un modèle de calcul de pertes à  l'aide d'un logiciel d'éléments finis pour une nouvelle géométrie :tube ou cylindre supraconducteur. La non-linéarité des formules E-J ainsi que les problèmes de convergence ont été traités par l'implémentation d'une formulation en H pour la résolution numérique. Les résultats ont été comparés aux formules analytiques. Dans le but de vérifier l'exactitude du modèle, une série de mesures expérimentales a aussi été réalisée sur un ruban supraconducteur industriel.</p
Intrinsic universality in tile self-assembly requires cooperation
We prove a negative result on the power of a model of algorithmic
self-assembly for which it has been notoriously difficult to find general
techniques and results. Specifically, we prove that Winfree's abstract Tile
Assembly Model, when restricted to use noncooperative tile binding, is not
intrinsically universal. This stands in stark contrast to the recent result
that, via cooperative binding, the abstract Tile Assembly Model is indeed
intrinsically universal. Noncooperative self-assembly, also known as
"temperature 1", is where tiles bind to each other if they match on one or more
sides, whereas cooperative binding requires binding on multiple sides. Our
result shows that the change from single- to multi-sided binding qualitatively
improves the kinds of dynamics and behavior that these models of nanoscale
self-assembly are capable of. Our lower bound on simulation power holds in both
two and three dimensions; the latter being quite surprising given that
three-dimensional noncooperative tile assembly systems simulate Turing
machines. On the positive side, we exhibit a three-dimensional noncooperative
self-assembly tile set capable of simulating any two-dimensional noncooperative
self-assembly system.
Our negative result can be interpreted to mean that Turing universal
algorithmic behavior in self-assembly does not imply the ability to simulate
arbitrary algorithmic self-assembly processes.Comment: Added references. Improved presentation of definitions and proofs.
This article uses definitions from arXiv:1212.4756. arXiv admin note: text
overlap with arXiv:1006.2897 by other author
GI-1.0: A Fast and Scalable Two-level Radiance Caching Scheme for Real-time Global Illumination
Real-time global illumination is key to enabling more dynamic and physically
realistic worlds in performance-critical applications such as games or any
other applications with real-time constraints.Hardware-accelerated ray tracing
in modern GPUs allows arbitrary intersection queries against the geometry,
making it possible to evaluate indirect lighting entirely at runtime. However,
only a small number of rays can be traced at each pixel to maintain high
framerates at ever-increasing image resolutions. Existing solutions, such as
probe-based techniques, approximate the irradiance signal at the cost of a few
rays per frame but suffer from a lack of details and slow response times to
changes in lighting. On the other hand, reservoir-based resampling techniques
capture much more details but typically suffer from poorer performance and
increased amounts of noise, making them impractical for the current generation
of hardware and gaming consoles. To find a balance that achieves high lighting
fidelity while maintaining a low runtime cost, we propose a solution that
dynamically estimates global illumination without needing any content
preprocessing, thus enabling easy integration into existing real-time rendering
pipelines
Root system markup language: toward an unified root architecture description language
The number of image analysis tools supporting the extraction of architectural features of root systems has increased over the last years. These tools offer a handy set of complementary facilities, yet it is widely accepted that none of these software tool is able to extract in an efficient way growing array of static and dynamic features for different types of images and species.
We describe the Root System Markup Language (RSML) that has been designed to overcome two major challenges: (i) to enable portability of root architecture data between different software tools in an easy and interoperable manner allowing seamless collaborative work, and (ii) to provide a standard format upon which to base central repositories which will soon arise following the expanding worldwide root phenotyping effort.
RSML follows the XML standard to store 2D or 3D image metadata, plant and root properties and geometries, continuous functions along individual root paths and a suite of annotations at the image, plant or root scales, at one or several time points. Plant ontologies are used to describe botanical entities that are relevant at the scale of root system architecture. An xml-schema describes the features and constraints of RSML and open-source packages have been developed in several languages (R, Excel, Java, Python, C#) to enable researchers to integrate RSML files into popular research workflow
Cross-Over between universality classes in a magnetically disordered metallic wire
In this article we present numerical results of conduction in a disordered
quasi-1D wire in the possible presence of magnetic impurities. Our analysis
leads us to the study of universal properties in different conduction regimes
such as the localized and metallic ones. In particular, we analyse the
cross-over between universality classes occurring when the strength of magnetic
disorder is increased. For this purpose, we use a numerical Landauer approach,
and derive the scattering matrix of the wire from electron's Green's function.Comment: Final version, accepted for publication in New Journ. of Physics, 27
pages, 28 figures. Replaces the earlier shorter preprint arXiv:0910.427
Tests with a Carlina-type diluted telescope; Primary coherencing
Studies are under way to propose a new generation of post-VLTI
interferometers. The Carlina concept studied at the Haute- Provence Observatory
is one of the proposed solutions. It consists in an optical interferometer
configured like a diluted version of the Arecibo radio telescope: above the
diluted primary mirror made of fixed cospherical segments, a helium balloon (or
cables suspended between two mountains), carries a gondola containing the focal
optics. Since 2003, we have been building a technical demonstrator of this
diluted telescope. First fringes were obtained in May 2004 with two
closely-spaced primary segments and a CCD on the focal gondola. We have been
testing the whole optical train with three primary mirrors. The main aim of
this article is to describe the metrology that we have conceived, and tested
under the helium balloon to align the primary mirrors separate by 5-10 m on the
ground with an accuracy of a few microns. The servo loop stabilizes the mirror
of metrology under the helium balloon with an accuracy better than 5 mm while
it moves horizontally by 30 cm in open loop by 10-20 km/h of wind. We have
obtained the white fringes of metrology; i.e., the three mirrors are aligned
(cospherized) with an accuracy of {\approx} 1 micron. We show data proving the
stability of fringes over 15 minutes, therefore providing evidence that the
mechanical parts are stabilized within a few microns. This is an important step
that demonstrates the feasibility of building a diluted telescope using cables
strained between cliffs or under a balloon. Carlina, like the MMT or LBT, could
be one of the first members of a new class of telescopes named diluted
telescopes.Comment: 18 pages, 17 figures, A&A, accepte
The Carlina-type diluted telescope: Stellar fringes on Deneb
Context. The performance of interferometers has largely been increased over
the last ten years. But the number of observable objects is still limited due
to the low sensitivity and imaging capability of the current facilities.
Studies have been done to propose a new generation of interferometers. Aims.
The Carlina concept studied at the Haute-Provence Observatory consists in an
optical interferometer configured as a diluted version of the Arecibo radio
telescope: above the diluted primary mirror made of fixed co-spherical
segments, a helium balloon or cables suspended between two mountains and/or
pylons, carries a gondola containing the focal optics. This concept does not
require delay lines. Methods. Since 2003, we have been building a technical
demonstrator of this diluted telescope. The main goals of this project were to
find the opto-mechanical solutions to stabilize the optics attached under
cables at several tens of meters above the ground, and to characterize this
diluted telescope under real conditions. In 2012, we have obtained metrology
fringes, and co-spherized the primary mirrors within one micron accuracy. In
2013, we have tested the whole optical train: servo loop, metrology, and the
focal gondola. Results. We obtained stellar fringes on Deneb in September 2013.
In this paper, we present the characteristics of these observations: quality of
the guiding, S /N reached, and possible improvements for a future system.
Conclusions. It is an important step that demonstrates the feasibility of
building a diluted telescope using cables strained between cliffs or pylons.
Carlina, like the MMT or LBT, could be one of the first members of a new class
of telescopes named Large Diluted Telescopes. Its optical architecture has many
advantages for future projects: Planet Formation Imager, Post-ELTs,
Interferometer in space.Comment: 8 pages, 7 figures, Astronomy & Astrophysic
Communications in cellular automata
The goal of this paper is to show why the framework of communication
complexity seems suitable for the study of cellular automata. Researchers have
tackled different algorithmic problems ranging from the complexity of
predicting to the decidability of different dynamical properties of cellular
automata. But the difference here is that we look for communication protocols
arising in the dynamics itself. Our work is guided by the following idea: if we
are able to give a protocol describing a cellular automaton, then we can
understand its behavior
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