Accumulation of a clock state population by optical pumping in a caesium fountain

We have implemented a technique of optical pumping (OP) to increase the clock state populations in a caesium fountain frequency standard. Our initial experiments have demonstrated accumulation of 75% of the atoms in the m_F = 0 sublevel (clock state), which can be compared to a typical 11% population of this sublevel with no additional OP applied. As the (OP) process requires scattering of some 20 photons per atom, the atomic cloud is heated by nearly 1 μK and the final number of atoms reaching the detector is reduced by up to a factor of two, depending on the molasses temperature. Using the OP technique in the NPL-CsF2 fountain we have observed a four-fold net increase of the clock state population.