Predictions for laser-cooled Rb clocks

Using information from a recent [Formula Presented]Rb two-color photoassociation experiment, we evaluate the merits of fountain clocks based on [Formula Presented]Rb and [Formula Presented]Rb isotopes as alternatives to [Formula Presented]Cs and find that they offer significant advantages. In the case of [Formula Presented]Rb the collisionally induced fractional frequency shift is 15 times smaller than for [Formula Presented]Cs. This small shift is associated with a small difference in the triplet and singlet scattering lengths for [Formula Presented]Rb. For [Formula Presented]Rb, the shift produced by the two [Formula Presented] clock states may have opposite signs allowing the shift to be eliminated by controlling the relative populations of these states. We also present collision quantities relevant to atomic fountain clocks containing multiply launched groups of atoms, and for evaporative cooling of [Formula Presented]Rb atoms.