The dark side of the optical force (Vol. 44 No. 2)

Lithium and Potassium are the only alkali species possessing stable fermionic isotopes, and as such, they have played a key role in the recent development of quantum simulation of strongly correlated systems using cold atoms. These two species also share an excited hyperfine structure hindering efficient laser cooling below the Doppler limit. In this work, we have implemented a laser-cooling scheme based on dark resonances, which allowed us to achieve record high phase space densities for laser-cooled 40K atoms. This strategy was initially developed in the early 90’s and relies on the existence of a family of so-called dark states in which the atoms do not interact with light and do not scatter photons. These states alleviate some of the detrimental effects of traditional schemes, such as spontaneous emission or multiple photon-scattering, which respectively limit the final temperature and density of “bright” optical molasses. This scheme is rather general and can be extended to other atomic species, such as Lithium, as demonstrated by preliminary results obtained in our group.

D. Rio Fernandes, F. Sievers, N. Kretzschmar, S. Wu, C. Salomon and F. Chevy, ‘Sub-Doppler laser cooling of fermionic 40K atoms in three-dimensional gray optical molasses’, EPL, 100, 63001 (2012)