Work is underway on a new atomic fountain using Rubidium atoms. 

Since a goal of the fountain devices is to provide a stable reference rather than to define the second, we are not constrained to use Cesium. The cold-collision shift, which ends up being the largest contributor to frequency fluctuations in Cesium Fountains, is much smaller in Rubidium.

Since the Rb frequency is smaller than that of Cs, the wavelength is longer, and the entrance and exit holes to the microwave cavity can be larger without having leakage that would perturb the atoms in the drift region. This means the atom cloud can be physically larger, containing more atoms, and yields a larger fluorescence signal.

Rb also has a smaller nuclear spin, so there are fewer magnetic states. A larger fraction of the trapped atoms will start out in the m = 0 "clock state" that is less sensitive to magnetic fields.


These factors outweigh the disadvantage of having a smaller transition frequency.