We present here a new method based on a transfer of population by adiabatic
passage that allows to prepare cold atomic samples with a well defined ratio of
atomic density and atom number. This method is used to perform a measurement of
the cold collision frequency shift in a laser cooled cesium clock at the
percent level, which makes the evaluation of the cesium fountains accuracy at
the 10−16 level realistic. With an improved set-up, the adiabatic passage
would allow measurements of atom number-dependent phase shifts at the 10−3
level in high precision experiments.Comment: 4 pages, 3 figures, 2 table