Title:
Anomalous momentum diffusion of strongly interacting bosons in optical lattices

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

Abstract:
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).