We present a new method based on a transfer of population by adiabatic passage that allows one 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 fountain accuracy at the ${10}^{-16}$ level realistic. With improvements, the adiabatic passage would allow measurements of density-dependent phase shifts at the ${10}^{-3}$ level in high precision experiments.

• Received 9 August 2002

DOI:https://doi.org/10.1103/PhysRevLett.89.233004