Talk - Fabrice Gerbier: Anomalous momentum diffusion of strongly interacting bosons in optical lattices

06.09.2018 | Mette Alstrup Lie

Dato ons 12 sep
Tid 11:00 12:00
Sted 1520-737

Anomalous momentum diffusion of strongly interacting bosons in optical lattices


Fabrice Gerbier, Laboratoire Kastler Brossel, Institut de Physique du Collège de France, Frankrig

Dissipative quantum systems are subject to decoherence, the disappearance of interference phenomena due to irreversible loss of information. This plays a role in many modern areas of research, from research on the foundations of quantum mechanics to quantum information processing (and quantum technologies in general), where decoherence is a threat that must be countered. While decoherence has been extensively studied for simple systems, such as a two-level atom or a harmonic oscillator, much less is known for many-body systems of interacting particles. In this talk, I will report on an experimental study of how spatial coherence in a superfluid gas of bosonic atoms in an optical lattice is lost when the atoms undergo spontaneous emission. For independent atoms excited by a near-resonant laser, repeated laser photon absorption-spontaneous emission cycles destroy spatial coherences on distances larger than the wavelength of the optical transition, or equivalently leads to diffusion in momentum space with a momentum width scaling as $t^{1/2}$. This momentum diffusion process is well-known in quantum optics and limits the temperature achievable in laser cooling. For strongly interacting bosons, we observed that the momentum diffusion is anomalously slow : After a short time, the decay of spatial coherences slows down, and momentum space dynamics becomes sub-diffusive with a momentum width scaling as $t^{1/4}$. We explain this behavior in terms of a model proposed by Poletti et al. [1], where the long-times dynamics is understood in terms of a diffusion in Fock space. Dissipation leads to the formation of "long-lived" clusters of atoms with higher occupancy than the average site. These clusters decay slowly (through high-order processes) due to their energy mismatch with more typical configurations. In classical statistical mechanics, the transport dynamics of systems with a distribution of lifetimes featuring a slow tail typically shows sub-diffusion [2]. Using three-body losses as a probe of on site statistics, we provide a direct evidence of this anomalous diffusion in Fock space which underlies the anomalous momentum diffusion.


[1] D. Poletti \textit{et al.}, Phys. Rev. Lett. \textbf{109}, 045302 (2012); D. Poletti \textit{et al.}, , Phys. Rev. Lett. \textbf{111} , 195301 (2013).

[2] J.-P. Bouchaud, A. Georges, Physics reports \textbf{195} (4-5), 127-29 (1990).

Institut for Fysik og Astronomi, Offentligheden / Pressen, CQOM seminar