Effect of cold collisions on spin coherence and resonance shifts in a magnetically trapped ultracold gas

We have performed precision microwave spectroscopy on ultracold ${}^{87}\mathrm{Rb}$ confined in a magnetic trap, both above and below the Bose-condensation transition. The cold collision frequency shifts for both normal and condensed clouds were measured, which allowed the intrastate and interstate density correlations (characterized by sometimes controversial “factors of 2”) to be determined. Additionally, temporal coherence of the normal cloud was studied, and the importance of mean-field and velocity-changing collisions in preserving coherence is discussed.