Experimental investigation of nonergodic effects in subrecoil laser cooling

We present the first detailed investigation of the line shape of the moment um distribution of atoms cooled far below the recoil limit. This distributi on is deduced from a direct measurement of the atomic spatial correlation f unction of metastable helium atoms cooled by velocity-selective coherent po pulation trapping. The measured line shape is then compared to the predicti on of an analytical model of this cooling based on Levy statistics. A very good agreement is found between experiment and theory and fundamental featu res such as self-similarity and nonergodicity are identified.