Eliminating cold-collision frequency shifts

A technique to cancel the frequency shift due to cold collisions of laser-cooled atoms is proposed. Results for future clocks based on ${}_{}{}^{135}{}_{}{}^{}\mathrm{Cs}$ and ${}_{}{}^{137}{}_{}{}^{}\mathrm{Cs}$ are presented. We find a ${}_{}{}^{137}{}_{}{}^{}\mathrm{Cs}$ fountain clock can be operated with no cold-collision shift by adjusting the population ratios in the fountain, and it may also be possible for ${}_{}{}^{135}{}_{}{}^{}\mathrm{Cs}$. This technique can also be applied to other phase-sensitive measurements using cold atoms, including atom interferometers.