Sub-Poissonian atom-number fluctuations using light-assisted collisions

We investigate experimentally the number statistics of a mesoscopic ensemble of cold atoms in a microscopic dipole trap loaded from a magneto-optical trap and find that the atom-number fluctuations are reduced with respect to a Poisson distribution due to light-assisted two-body collisions. For numbers of atoms N greater than or similar to 2, we measure a reduction factor (Fano factor) of 0.72 +/- 0.07, which differs from 1 by more than four standard deviations. We analyze this fact by a general stochastic model describing the competition between the loading of the trap from a reservoir of cold atoms and multiatom losses, leading to a master equation. Applied to our experimental regime, this model indicates an asymptotic value of 3/4 for the Fano factor at large N and in the steady state. We thus show that we have reached the ultimate level of reduction in number fluctuations in our system.