Définition et identification d'objets sur une image à haute résolution spatiale. Application à la différenciation de types de châtaigneraies

Les données spatiales à haute résolution, ainsi que les capacités de traitement sont et seront de plus en plus accessibles aux différents utilisateurs. Face à cette abondance d'informations, nous proposons de concentrer les efforts sur l'identification d'objets pertinents, tant au niveau thématique qu'informatique. L'objectif est d'automatiser l'extraction d'objets chargés de sens thématiques. Cette démarche a été testée pour la distinction de types de vergers et de taillis de châtaigniers sur une photographie aérienne en infrarouge couleur de l'IFN (Inventaire forestier national). Une typologie de châtaigneraies regroupant cinq classes est construite dans l'objectif de relier formes et pratiques agricoles. Les objets se distinguent, non seulement par leur couleur, mais aussi par l'arrangement spatial de leurs constituants (les houppiers de châtaigniers). Nous avons retenu le module CLAPAS (Classement de paysages et segmentation) : le processus de classification tient compte du voisinage des points ce qui permet de différencier les types de même couleur mais de texture différente. Les classes de référence sont choisies sur l'image à l'aide de connaissances de terrain (classification assistée). L'analyse montre que, malgré des confusions entre types dans certaines zones, les sorties générées par la procédure correspondent bien aux relevés de terrain. / High resolution spatial data are bound to be more and more reachable for all users. Facing so many data, we propose to define pertinent object, both on thematic and methodological levels. The aim is to automate the extraction of objects that are suitable for research and management. This process is used to distinguish classes of chestnut tree orchards and copses with infrared aerial photos of IFN. A chestnut tree classification is elaborated so as to associate shapes with agricultural practices. Not only do objects differ in colour but also in component (chestnut tree crowns) spatial distribution. CLAPAS is selected: the classification integrates points proximity which allows to distinguish classes with the same colour but differing in texture. Reference classes are chosen on the image thanks to vegetation observation (assisted classification). The analysis shows that, in spite of confusions, output fit with sites checking

Spin-wave instabilities in spin-transfer-driven magnetization dynamics

We study the stability of magnetization precessions induced in spin-transfer devices by the injection of spin-polarized electric currents. Instability conditions are derived by introducing a generalized, far-from-equilibrium interpretation of spin-waves. It is shown that instabilities are generated by distinct groups of magnetostatically coupled spin-waves. Stability diagrams are constructed as a function of external magnetic field and injected spin-polarized current. These diagrams show that applying larger fields and currents has a stabilizing effect on magnetization precessions. Analytical results are compared with numerical simulations of spin-transfer-driven magnetization dynamics.Comment: 4 pages, 2 figure

Colossal magnetoresistance in an ultra-clean weakly interacting 2D Fermi liquid

We report the observation of a new phenomenon of colossal magnetoresistance in a 40 nm wide GaAs quantum well in the presence of an external magnetic field applied parallel to the high-mobility 2D electron layer. In a strong magnetic field, the magnetoresistance is observed to increase by a factor of ~300 from 0 to 45T without the system undergoing any metal-insulator transition. We discuss how this colossal magnetoresistance effect cannot be attributed to the spin degree-of-freedom or localization physics, but most likely emanates from strong magneto-orbital coupling between the two-dimensional electron gas and the magnetic field. Our observation is consistent with a field-induced 2D-to-3D transition in the confined electronic system

A lattice of double wells for manipulating pairs of cold atoms

We describe the design and implementation of a 2D optical lattice of double wells suitable for isolating and manipulating an array of individual pairs of atoms in an optical lattice. Atoms in the square lattice can be placed in a double well with any of their four nearest neighbors. The properties of the double well (the barrier height and relative energy offset of the paired sites) can be dynamically controlled. The topology of the lattice is phase stable against phase noise imparted by vibrational noise on mirrors. We demonstrate the dynamic control of the lattice by showing the coherent splitting of atoms from single wells into double wells and observing the resulting double-slit atom diffraction pattern. This lattice can be used to test controlled neutral atom motion among lattice sites and should allow for testing controlled two-qubit gates.Comment: 9 pages, 11 figures Accepted for publication in Physical Review

Dynamique d'acteurs ( agriculteurs et institutions) et innovation agro-écologique pour la gestion des risques environnementaux en Guadeloupe

Dans le contexte de crise économique et environnementale qui touche la filière banane à la Guadeloupe, l'objectif de cette contribution est d'engager une réflexion sur l'organisation d'acteurs à l'origine des innovations agro-écologiques. Nous interrogeons les dynamiques d'acteurs face aux crises et perturbations qui touchent l'agriculture antillaise, par le biais de l'organisation qu'ils mettent en place pour accompagner le changement. Des entretiens menés d'une part auprès d'un groupe de planteurs de bananes, d'autre part auprès des institutions en charge de la question environnementale sur l'île, ont permis de faire émerger trois principales conclusions sur la nature des innovations mises en oeuvre face aux crises environnementale et économique, leur échelle d'application et les forces motrices qui les sous-tendent : du point de vue technique, la mise en oeuvre de nouvelles pratiques indépendamment des échanges entre agriculteurs ; du point de vue stratégique, des changements d'orientation productive en liaison avec une forte dynamique d'échanges ; au niveau des institutions, de nouveaux lieux de dialogue. D'une manière générale, il apparaît qu'il y a un décalage dans la dynamique de concertation selon les niveaux d'organisation (agriculteurs vs institutions) et le lien entre ces deux niveaux d'organisation semble peu existant. / The aim of our research is to urge a reflexion on the agro-ecological innovations to manage the environmental risks. We analyse the organization that the stakeholders (farmers and institutions) set up facing the crises and disturbances which touch the agriculture of French West Indies. To do this, we enquired a group of farmers of banana production in Guadeloupe, and also the agricultural and environmental institutions of this island. This made it possible to identify three principal conclusions on the nature of the innovations, their scale of application and the driving forces which underlie them: from a technical point of view, new practices are implemented independently of the exchanges between farmers; from a strategic point of view, changes of production are made, depending on exchanges dynamic; on the level of the institutions, new places of dialogue are created. In conclusion, there is a shift in the dynamics of dialogue according to the levels of organization (farmers versus institutions) and the bond between these two levels of organization seems to be very thin.FILIERE;BANANE;INNOVATION;AGRICULTEUR;INSTITUTION;GUADELOUPE;RESEAU DE DIALOGUE ENTRE AGRICULTEURS;INNOVATION AGRO-ECOLOGIQUE

Attosecond tracking of light absorption and refraction in fullerenes

The collective response of matter is ubiquitous and widely exploited, e.g. in plasmonic, optical and electronic devices. Here we trace on an attosecond time scale the birth of collective excitations in a finite system and find distinct new features in this regime. Combining quantum chemical computation with quantum kinetic methods we calculate the time-dependent light absorption and refraction in fullerene that serve as indicators for the emergence of collective modes. We explain the numerically calculated novel transient features by an analytical model and point out the relevance for ultra-fast photonic and electronic applications. A scheme is proposed to measure the predicted effects via the emergent attosecond metrology.Comment: 11 pages, 3 figures, accepted in Phys. Rev.

On Binary Matroid Minors and Applications to Data Storage over Small Fields

Locally repairable codes for distributed storage systems have gained a lot of interest recently, and various constructions can be found in the literature. However, most of the constructions result in either large field sizes and hence too high computational complexity for practical implementation, or in low rates translating into waste of the available storage space. In this paper we address this issue by developing theory towards code existence and design over a given field. This is done via exploiting recently established connections between linear locally repairable codes and matroids, and using matroid-theoretic characterisations of linearity over small fields. In particular, nonexistence can be shown by finding certain forbidden uniform minors within the lattice of cyclic flats. It is shown that the lattice of cyclic flats of binary matroids have additional structure that significantly restricts the possible locality properties of $\mathbb{F}_{2}$-linear storage codes. Moreover, a collection of criteria for detecting uniform minors from the lattice of cyclic flats of a given matroid is given, which is interesting in its own right.Comment: 14 pages, 2 figure

Watershed regulation and local action: analysis of the Senegal River watershed management by a regional organisation and public participation

International audienceSeveral social scientists have dealt with the usefulness of a participative approach in development plans. The call for sustainable development has increased the focus on this type of approach in a very classical way, which is the case for the creation of new water tanks. Most of these scientists have also pinpointed the major difficulties and failures faced during the execution of this new approach in developing countries. This study is a concrete example which underlines the lack of this type of approach as far as water management in the Senegal River is concerned, mainly in relation to watershed. We base our study on the analysis and criticism of the regional organization OMVS (Organization for the Development of the Senegal River) which is in charge of water management in the Senegal River. The results of the study can, therefore, be summed up as follows: (i) An on-site direct observation, individual interviews, group discussion and information analysis point out the lack of participation of local people in water management in the Senegal River and, in general, the harmful socio-economic impacts resulting from it. (ii) The reasons for this lack of participative approach are mainly due to the model set up by the OMVS in terms of water management in the Senegal River, a model that has excluded or tackled in a very light way the issue of public participation in decision-making through out its juridical and regulation instruments. (iii) Elements of consideration on some measures, which could possibly improve the level of participation of local people in river water management