Control of Ultracold Collisions with Nonlinear Frequency Chirps

We report on measurements of ultracold excited-state collisions between Rb atoms induced by frequency-chirped laser light. The chirps, either positive or negative, sweep over 1 GHz in 100 ns and are centered at a variable detuning below the atomic resonance. If the laser light is resonant with an attractive potential of an atom-pair at some point during the chirp, the pair is excited, potentially resulting in loss from the trap. In previous work with linear chirps,$^{1}$ we observed that at certain center detunings, the negative chirp yielded a lower collisional loss rate than the positive chirp. We attribute this suppression to the fact that the negative chirp follows the excited atom-pair trajectory and can therefore de-excite the atom-pair during the collision. In the present work, we incorporate nonlinear frequency chirps. For the negative chirp, we find a significant dependence on the details of the nonlinearity. This indicates the importance of matching the chirp to the atom-pair trajectories. This work is supported by DOE. \\* \\*$^{1}$ M.J. Wright \emph{et al.}, Phys. Rev. A \textbf{75} 051401(R) (2007)