Direct imaging with highly diluted apertures. II. Properties of the point spread function of a hypertelescope

In the future, optical stellar interferometers will provide true images thanks to larger number of telescopes and to advanced cophasing subsystems. These conditions are required to have sufficient resolution elements (resel) in the image and to provide direct images in the hypertelescope mode. It has already been shown that hypertelescopes provide snapshot images with a significant gain in sensitivity without inducing any loss of the useful field of view for direct imaging applications. This paper aims at studying the properties of the point spread functions of future large arrays using the hypertelescope mode. Numerical simulations have been performed and criteria have been defined to study the image properties. It is shown that the choice of the configuration of the array is a trade-off between the resolution, the halo level and the field of view. A regular pattern of the array of telescopes optimizes the image quality (low halo level and maximum encircled energy in the central peak), but decreases the useful field of view. Moreover, a non-redundant array is less sensitive to the space aliasing effect than a redundant array.Comment: 10 pages paper with referee in A&

Light self-trapping in a large cloud of cold atoms

We show that, for a near-resonant propagating beam, a large cloud of cold 87Rb atoms acts as a saturable Kerr medium and produces self-trapping of light. By side fluorescence imaging we monitor the transverse size of the beam and, depending on the sign of the laser detuning with respect to the atomic transition, we observe self-focusing or -defocusing, with the waist remaining stationary for an appropriate choice of parameters. We analyze our observations by using numerical simulations based on a simple 2-level atom model.Comment: 3 pages, 4 figures, submitted to Optics Letter

The optical very large array and its moon-based version

An Optical Very Large Array (OVLA) is currently in early prototyping stages for ground-based sites, such as Mauna Kea and perhaps the VLT site in Chile. Its concept is also suited for a moon-based interferometer. With a ring of bi-dimensionally mobile telescopes, there is maximal flexibility in the aperture pattern, and no need for delay lines. A circular configuration of many free-flying telescopes, TRIO, is also considered for space interferometers. Finally, the principle of gaseous mirrors may become applicable for moon-based optical arrays. Fifteen years after the first coherent linkage of two optical telescopes, the design of an ambitious imaging array, the OVLA, is now well advanced. Two 1.5 m telescopes have been built and now provide astronomical results. Elements of the OVLA are under construction. Although primarily conceived for ground-based sites, the OVLA structure appears to meet the essential requirements for operation on the Moon

Conservation of tropical root and tuber crops agrobiodiversity : on farm true seeds production and use as a mean for geographic distribution of allelic diversity

The tropical root and tuber crops (cassava, sweet potato, taro and yam) are cultivated throughout the tropics where they play a major role for food security. In Vanuatu, anarchipelago of 81 islands situated in the South West Pacific, traditional food gardens are at the basis of self-sufficiency with tuber and root crops as the main source of carbohydratesfor smallholders. Emerging environmental changes such as climatic ones, the introduction of new plant diseases and/or changing diets are now endangering the local agrobiodiversity and the country's food security. As several molecular genetic studies have shown, the local tuber and root crops diversity of Vanuatu is narrow and in the course of genetic erosion, its resilience to changes is thus quite limited. To enlarge this genetic pool, introduction of exotic varieties has been conducted through in vitro genotypes but excessive distribution costs and the limited success of the operation, mainly due to the high fragmentation of the country, pointed up the need for other means of allelic dispersion. True botanical seeds production and use seems to be a promising way since it is easier to distribute and because it enables efficient protection of local allelic pool by crossing it with introduced one. However, the adoption of this innovative practice by traditional smallholders raises numerous problems since they are practicing exclusively clonal propagation and are not aware of tuber and root crops sexual reproduction. Our project aims at evaluating the potential for on-farm true seeds production and use through participatory methods. This study will focus on mains environmental, economical, social and cultural constraints to the development of this practice in on-farm conditions. Since the sexual reproduction of these plants is poorly documented, this study will also assess the efficiency of this method through the characterization of reproductive biology and estimation of the percentage of new cultivars created during a cycle. We will finally evaluate the possible practical improvements by building on farmer's own experiments and analysis of their difficulties. As most of the work on tropical tuber and root crops true seed of has been limited to scientific research stations, it is of interest to develop this technique for on farm activities. (Résumé de l'auteur

Self-sustained oscillations in a Large Magneto-Optical Trap

We have observed self-sustained radial oscillations in a large magneto-optical trap (MOT), containing up to $10^{10}$ Rb$^{85}$ atoms. This instability is due to the competition between the confining force of the MOT and the repulsive interaction associated with multiple scattering of light inside the cold atomic cloud. A simple analytical model allows us to formulate a criterion for the instability threshold, in fair agreement with our observations. This criterion shows that large numbers of trapped atoms $N>10^9$ are required to observe this unstable behavior.Comment: submitted to Phys. Rev. Lett. 4 pages, 3 figure

Does social organisation shape crop diversity? A case study among Tharaka farmers in Kenya

Identifying the factors that influence crop diversity patterns in-situ is a major challenge for its conservation. Despite the role of farmers in the construction and management of crop diversity, social factors have been widely neglected in this approach. Can different social groups living in the same environment have different folktaxonomy underlying crop diversity ? In the Tharaka community on the Eastern side of Mount Kenya, farmers living in the same neighboring group (ntora) usually cooperate for agricultural tasks. In contrast, cooperation between groups is uncommon and reflects the scarcity of their social relations which limits potentially the exchange of knowledge. Sorghum (Sorghum bicolor (L.) Moench) folktaxonomies of 95 tharaka farmers belonging to 11 ntora were compared to describe knowledge exchange patterns through social network. Inter-class multivariate analysis was applied to compare crop species and sorghum landraces inventory between ntora. The comparison or crop species inventory between ntora reveals differences. Free lists of sorghum landraces cultivated by tharaka farmers were recorded, enabling us to compare the sorghum folktaxonomy between ntora. The names used by farmers to identify sorghum landraces were significantly different between ntora and reflected differences of folktaxonomy. The influence of social network on crop diversity patterns in- situ is hence discussed. (Résumé d'auteur

Approximated center-of-mass motion for systems of interacting particles with space- and velocity-dependent friction and anharmonic potential

We study the center-of-mass motion in systems of trapped interacting particles with space- and velocity-dependent friction and anharmonic traps. Our approach, based on a dynamical ansatz assuming a fixed density profile, allows us to obtain information at once for a wide range of binary interactions and interaction strengths, at linear and nonlinear levels. Our findings are first tested on different simple models by comparison with direct numerical simulations. Then, we apply the method to characterize the motion of the center of mass of a magneto-optical trap and its dependence on the number of trapped atoms. Our predictions are compared with experiments performed on a large Rb85 magneto-optical trap.Comment: 9 pages, 8 figure

Thermal breakdown of coherent backscattering: a case study of quantum duality

We investigate coherent backscattering of light by two harmonically trapped atoms in the light of quantitative quantum duality. Including recoil and Doppler shift close to an optical resonance, we calculate the interference visibility as well as the amount of which-path information, both for zero and finite temperature.Comment: published version with minor changes and an added figur

The social diffusion pathways of sorghum varieties and associated knowledge in the Mount Kenya region

Small-scale farming systems present a major interest for the conservation of crop diversity in situ. In these systems, farmers act on crop diversity through their management practices, mainly seed selection and exchange. Understanding the diffusion pathways of crop seeds and associated indigenous traditional knowledge (ITK) within and among farmers' communities would hence be usefull for designing conservation strategies for agrobiodiversity. In smallholders' communities, the relationship among farmers are largely determined by social rules and customs, partly inherited from the pre-colonial period. It is notably the case in the Mount Kenya region, which still shelters an impressive cultural diversity with more than nine ethnolinguistic groups coexisiting on its Eastern slope. This study focused on three of these ethnolinguistic groups: the Chuka and Tharaka groups, which had a long standing alliance but have no alliance with a third group, the Mbeere, despite their spatial adjacency. Does this social organization determine the diffusion pathways of the sorghum varieties and that of the ITK associated? This study attempts to adress this question. The sorghum seed exchange network was characterized using Social Network Analysis. The dependance of seed exchanges regarding the social organization of farmers was tested based on the interviews of 218 farmers in the three ethnolinguistic groups. We also measured the consistency of 96 farmers belonging to the three ethnic groups in naming sorghum varieties, in order to test whether both the material (seed) and the cultural (sorghum variety names) diffusion pathways corresponded. This study showed that the social organization of farmers in the Mount Kenya region still shapes today not only the seed exchange network, but also the ITK diffusion pathways. This study hence illustrates the relevance of considering the social organization of farmers' communities for the conservation of agrobiodiversity in situ. (Résumé d'auteur