An experimental study of the spin-exchange frequency shift in a laser-cooled cesium fountain frequency standard

Abstract

The spin-exchange frequency shift due to cold collisions between ground-state atoms appears to be the systematic effect which could limit the accuracy of a laser-cooled cesium fountain operating as a frequency standard. We have investigated this shift for atoms distributed among all the Zeeman substates of either the F = 4 or F = 3 hyperfine level of the 6S1/2 ground state of cesium and for the case where most of the atoms are prepared in F = 3, MF = 0 sublevel. The measured relative frequency shifts are respectively − 1.0·10−21, − 0.6·10−21 and − 2.0·10−21 (at/cm3)−1. These results agree with the measurements performed by Gibble and Chu at higher atom densities with atoms prepared in F = 3, MF = 0 sublevel. However, the precision of our frequency measurements is much better and is only limited at 2·10−15 by the H-maser used as a reference oscillator. Finally, we show that the relative frequency shift due to cold collisions can be reduced to 10−15, while preserving a short-term frequency stability of about 10−13τ−1/2.