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Accueil > Événements > Séminaires 2016

Ultracold matter waves in random potentials, from diffusion to weak and strong localization

Séminaire de Christian Miniatura
le jeudi 6 octobre 2016 à 11h

The concepts of random walks and Brownian motion were developed at the beginning of the 20th century after groundbreaking founding works on the kinetic theory of gases. They could successfully explain numerous statistical phenomena ranging from the transport of classical particles (Boltzmann’s kinetic equation, Drude’s electronic transport) in physics to the migration of mosquitoes in a forest in biology.

The propagation of waves in complex media did not escape this description, as exemplified by Lord Rayleigh’s pioneering works in acoustics (Rayleigh’s law) or those of Chandrasekhar and Milne in astrophysics (radiative transfer theory). Within this framework, a wave packet propagating in a disordered medium is multiply scattered in all directions by the heterogeneities of the medium. The scattered partial waves acquire random phases and, on average, the interference between these partial waves are smoothed out although the initial wave packet can be perfectly coherent. In turn, the memory of the initial propagation direction is rapidly lost and the wave packet spreads diffusively in space, just like the swarm of mosquitoes in a forest.

This powerful and fruitful description of propagation in a random medium was nevertheless questioned in the late 50s when Anderson showed that disorder could bring wave transport to a halt, an interference phenomenon known as strong localization. In fact, we now know that phase coherence is not completely scrambled even far from the localization regime as exemplified by weak localization corrections to the diffusion constant, universal conductance fluctuations and by the celebrated coherent backscattering (CBS) phenomenon. The latter manifests itself as an interference peak in the momentum distribution of the wave packet centered in the direction opposite to the initial propagation direction. Recently a new interference effect was discovered, the coherent forward scattering peak, a marker of Anderson localization.

I will present and discuss these results and developements in the framework of ultracold atoms in as much simple terms as possible.