Quantum dimer model with Z_2 liquid ground-state: interpolation between cylinder and disk topologies and toy model for a topological quantum-bit

We consider a quantum dimer model (QDM) on the kagome lattice which was introduced recently [Phys. Rev. Lett. 89, 137202 (2002)]. It realizes a Z_2 liquid phase and its spectrum was obtained exactly. It displays a topological degeneracy when the lattice has a non-trivial geometry (cylinder, torus, etc). We discuss and solve two extensions of the model where perturbations along lines are introduced: first the introduction of a potential energy term repelling (or attracting) the dimers along a line is added, second a perturbation allowing to create, move or destroy monomers. For each of these perturbations we show that there exists a critical value above which, in the thermodynamic limit, the degeneracy of the ground-state is lifted from 2 (on a cylinder) to 1. In both cases the exact value of the gap between the first two levels is obtained by a mapping to an Ising chain in transverse field. This model provides an example of solvable Hamiltonian for a topological quantum bit where the two perturbations act as a diagonal and a transverse operator in the two-dimensional subspace. We discuss how crossing the transitions may be used in the manipulation of the quantum bit to optimize simultaneously the frequency of operation and the losses due to decoherence.Comment: 11 pages, 7 (.eps) figures. Improved discussion of the destruction of the topological degeneracy and other minor corrections. Version to appear in Phys. Rev.

Temporal Variability of Ocean Colour Derived Products in the European Seas

The ten-year record of ocean colour data provided by the SeaWiFS mission is an important asset for monitoring and research activities conducted on the optically-complex European seas. This study mainly makes use of the SeaWiFS data set of normalized water leaving radiances LWN to study the major characteristics of temporal variability associated with optical properties across the entire European domain. Specifically, the time series of LWN, band ratios, diffuse attenuation coefficient Kd(490) and concentration of chlorophyll a Chla are decomposed into terms representing a fixed seasonal cycle, irregular variations and trends, and the contribution of these components to the total variance is described for the various basins. The diversity of the European waters is fully reflected by the range of results varying with regions and wavelengths. Generally, the Mediterranean and Baltic seas appear as two end-members with, respectively, high and low contributions of the seasonal component to the total variance. The existence of linear trends affecting the satellite products is also explored for each basin. The interpretation of the trends observed for LWN and band ratios is not straightforward, but it circumvents the limitations resulting from the levels of uncertainty, very variable in space and often high, that characterize derived products such as Chla in European waters. Results for Kd(490) and Chla are also analyzed. Statistically significant, and in some cases large, trends are detected in the Atlantic Ocean west of the European western shelf, the central North Sea, the English Channel, the Black Sea, the northern Adriatic, and various regions of the Mediterranean Sea and the northern Baltic Sea, revealing changes in the concentrations of optically significant constituents in these regions.JRC.H.3-Global environement monitorin

Multi-Year Analysis of Standard Ocean Colour Products for the European Seas

A 10-year time series of ocean colour products has been assembled for the European Seas from the SeaWiFS and MODIS full resolution satellite imagery. The JRC ocean colour archive is first briefly described. Then the study focuses on the analysis of the spatial and temporal variability of standard products such as the chlorophyll a pigment concentration and the diffuse attenuation coefficient. The European seas are partitioned into a set of specific regions for which average time series are derived and analysed in terms of seasonal and inter-annual variability. Finally, a statistical analysis yields a decomposition of the series into seasonal, irregular and linear trend components, thus providing a classification of the European waters on the basis of their temporal variations.JRC.H.3-Global environement monitorin

Selected UV Photochemical and Photobiological Impacts on Marine Ecosystems: General Characteristics and Sensitivity Analyses

In the recent years, numerous efforts have been performed in order to characterize the impacts of UVR on marine photobiology and photochemistry. The quantification of these UV-dependent processes through modelling approaches requires (i) an accurate description of UV underwater light field (ii) an adapted parameterization of the response of marine water compounds and/or organisms to spatio-temporal changes in solar radiations. The spatial and temporal variability of the absorption coefficient of the colored detrital material, which is a key element for studying undersea UV climate, has been characterized in the two basins selected for this study (the Mediterranean Sea and the Norwegian Seas) using the SeaWiFS products archive recently achieved for the period 1998-2006. Moreover, the various models currently available for the description of selected optical (CDOM photobleaching), photochemical (CO and DIC production) and photobiological (primary production inhibition) effects of UVR on marine waters have been described. Further, the general characteristics of these UV-dependent processes have been presented focusing, in particularly, on their variability along the daily, vertical and spectral dimensions. Several sensitivity analyses have been performed in order to define the relative importance of the various inputs of the spectral and depth resolved model on the final estimations. Finally, some of the straightforward models recently proposed in order to estimate some of the UV impacts at large temporal and or spatial scales have been tested and their limits of application have been discussed.JRC.H.3-Global environement monitorin

A Comparative Study of Time-Frequency Representations for Fault Detection in Wind Turbine

To reduce the cost of wind energy, minimization and prediction of maintenance operations in wind turbine is of key importance. In variable speed turbine generator, advanced signal processing tools are required to detect and diagnose the generator faults from the stator current. To detect a fault in non-stationary conditions, previous studies have investigated the use of time-frequency techniques such as the Spectrogram, the Wavelet transform, the Wigner-Ville representation and the Hilbert-Huang transform. In this paper, these techniques are presented and compared for broken-rotor bar detection in squirrel-cage generators. The comparison is based on several criteria such as the computational complexity, the readability of the representation and the easiness of interpretatio

Modelling of the ultrasonic propagation in polycrystalline materials

International audienceIn some polycrystalline materials, ultrasonic non destructive testing is affected by structural noise and attenuation. Those phenomena can cause significant loss in detection performances. Thus, the presence of a microstructure is a limiting factor of ultrasonic inspection capabilities and it must be accounted for when designing new NDE methods. Modelling work has been underway at CEA-LIST in the software CIVA to describe attenuation, structural noise and beam distortions appearing when a wave propagates in a heterogeneous medium such as a polycrystalline material. The objective is to develop new simulation tools based on metallurgical data input. To achieve this goal, a theoretical model relating ultrasonic scattering to properties of the microstructure and its integration to existing algorithms, allowing us to compute structural noise and attenuation from material properties is proposed. Further numerical study and comparisons with experimental results have been performed to study the influence of the type of structure (simple or duplex), the shape (equiaxed or elongated) and the size of grains

Continuous-discrete observers for time-varying nonlinear systems: A tutorial on recent results

Continuous-discrete systems can occur when the plant state evolves in continuous time but the output values are only available at discrete instants. Continuous-discrete observers have the valuable property that the observation error between the true state of the system and the observer state converges to zero in a uniform way. The design of continuous-discrete observers can often be done by building framers, which provide componentwise upper and lower bounds for the plant state. This paper is a tutorial on these approaches, highlighting recent results in the literature, and also providing previously unpublished, original results which are not being simultaneously submitted elsewhere

Homologie de contact des varietes toroidales

We show that contact homology distinguishes infinitely many tight contact structures on any orientable, toroidal, irreducible 3-manifold. As a consequence of the contact homology computations, on a very large class of toroidal manifolds, all known examples of universally tight contact structures with nonvanishing torsion satisfy the Weinstein conjecture. ----- On montre que l'homologie de contact distingue une infinite de structures de contact tendues sur toute variete toroidale irreductible et orientable de dimension trois. En consequence des calculs d'homologie de contact, sur une tres large classe de varietes toroidales, tous les exemples de structures de contact universellement tendues de torsion non nulle connus verifient la conjecture de Weinstein.Comment: Published by Geometry and Topology at http://www.maths.warwick.ac.uk/gt/GTVol9/paper9.abs.html Version 3: one minor correctio