Study of mesoscopic clouds of cold atoms in the interacting regime*

We present studies on cold and dense atomic $^{87}$Rb clouds containing $N\sim 2-100$ interacting atoms. We produced such mesoscopic ensembles by loading a microscopic optical dipole trap from a MOT. Due to $2$-body light-assisted collisions, we have found that in steady state such ensembles exhibit reduced number fluctuations with respect to a Poisson distribution. For $N \geq 2$, we measured a reduction Fano factor $F=0.72\pm0.07$ consistent with the value $F=3/4$ predicted at large $N$ by a general stochastic model [1,2]. To enhance interactions between the atoms, we are following two tracks. Firstly we evaporatively cooled a few hundreds of trapped atoms and obtained $\sim10$ atoms close to quantum degeneracy ($n\lambda_{dB}^3\sim1$) in the microscopic trap. In this regime s-wave interactions dominate ($n=2~10^{14}$~at.cm$^{-3}$). Secondly we sent near resonant light ($\lambda_{p}$) on the small cloud (size $l$). When $l<\lambda_{p}/2\pi$, dipole-dipole interactions should lead to collective behaviour.\\[4pt] [1] A. Fuhrmanek, Y.R.P. Sortais, P. Grangier, A. Browaeys, Phys. Rev. A \textbf{82}, 023623 (2010).\\[0pt] [2] Y.R.P. Sortais, A. Fuhrmanek, R. Bourgain, A. Browaeys, ``Sub-Poissonian atom number fluctuations using light-assisted collisions,'' arXiv:1111.5203 (2011).