Localisation d'Anderson d'atomes ultra-froids: nouvelles signatures et étude quantitative du régime critique à 3D

par Musawwadah Mukhtar

Projet de thèse en Physique quantique

Sous la direction de Vincent Josse.

Thèses en préparation à Paris Saclay , dans le cadre de Ondes et Matière , en partenariat avec Laboratoire Charles Fabry (Palaiseau, Essonne) (laboratoire) , Optique atomique (equipe de recherche) et de Institut d'optique théorique et appliquée (établissement de préparation de la thèse) depuis le 01-10-2015 .


  • Résumé

    En résumé, les objectifs de ma thèse sont d'observer la signature d'un effet interférentiel nouveau, le pic CFS («Coherent Forward Scattering»), et d'étudier la transition entre états diffusifs et localisés qui apparaît dans les systèmes tridimensionnels. Pour cela nous aurons besoin d'une source d'atomes ultra-froids puis de les lancer dans le champ de «speckle» optique. Il s'agit ensuite de laisser les atomes évoluer dans le désordre pendant suffisamment longtemps pour observer la signature de CFS. L'originalité de cette étude est d'examiner la signature de la localisation forte dans l'espace des vitesses au lieu de l'espace des positions. Nous essayerons aussi d'observer la transition d'Anderson qui apparaît à trois dimensions en contrôlant et en changeant l'énergie des atomes qui sont dans le potentiel aléatoire. Cette démarche permet d'obtenir plus d'information sur la dynamique de la localisation.

  • Titre traduit

    Anderson localization of ultracold atoms: new signatures and quantitative study of the critical transition in three dimensional system


  • Résumé

    The PhD studies are carried out at Institut d'Optique Graduate School, under supervision of Vincent Josse. The aim is to study experimentally the phenomena so-called Anderson phase transition. The works are mainly experimental, we prepare Bose Einstein Condensation from an atomic cloud of rubidium atoms. The atoms are then subjected to an artificial disordered potential made from speckle laser. The atoms are sufficiently cold to exhibit matter wave behavior. Depending on the energy inside the disordered system, the propagation of matter wave could be diffusive or localized. This project could allow us to understand related phenomena in condensed matter physics such as metal-insulator transition in certain materials. We are currently developing a technique to achieve matter wave with well resolved energy. This has two important objectives. The first one is to perform direct measurement of mobility edge defining the transition from diffusive to localized states. We also had collaboration with Prof. Dominique Delande for simulation works. Secondly, we would like to observe the critical dynamics around the mobility edge. This is done by measuring the diffusion constant and the localization length associated with the diffusive state and the localised state respectively. In the first year of the studies, we had developed the technique using so-called radio frequency spin flip and so-called spin dependent disorder. Currently our technique uses speckle field from a single laser. We had made measurements of the spectral function associated with the matter wave in the disordered potentials. The results show excellent agreement with the numerical works provided by our collaborators, Dominique Delande and Michael Pasek. Unfortunately, the current set-up would feature high loss of atoms due to spontaneous emission which prohibited us to achieve the two aforementioned goals. Nevertheless, these results show that the technique works in principle and we need to upgrade the set-up using speckle field from two laser sources. Currently, we are writing an article with these results. After a rather smooth progress in the first semester, we faced with many experimental issues in the second semester. The problems came mainly from the laser amplifiers, unidentified electrical noises, and instrument control. However, we made solid progress with the speckle set-up with two laser sources. Therefore, my focus for the third semester would be stabilizing the experiment as much as possible and the replication measurements of the spectral function with the new set-up. Hopefully by April 2017, we already attempt the first objective to make direct measurement of the mobility edge.