Spin polarization in a freely evolving sample of cold atoms

We have implemented and optimized a technique of spin polarization by optical pumping in a caesium atomic fountain, gaining a nearly fivefold increase in the useful cold atom signal in detection. This allows an improvement of the fountain clock stability without compromising its accuracy. We present a detailed study of optical pumping in a freely evolving cloud of cold caesium atoms: we have investigated theoretically and experimentally the dynamics of the pumping process and the associated heating due to random photon scattering. The heating limits the potential gain in the fountain signal due to an additional cloud expansion. A high degree of spin polarization was achieved with accumulation of up to 97 % of the population in a single magnetic (m (F) = 0) sublevel of the ground state. Factors affecting the achievable spin polarizations, such as the purity of the pumping light polarization and the shadowing effect in the cloud, were considered. This technique may also be used in atom interferometers and for other alkali metal systems.