  We demonstrate the ability to coherently control ultracold atomic Rb
collisions using frequency-chirped light on the nanosecond time scale. For
certain center frequencies of the chirp, the rate of inelastic trap-loss
collisions induced by negatively chirped light is dramatically suppressed
compared to the case of a positive chirp. We attribute this to a fundamental
asymmetry in the system: an excited wavepacket always moves inward on the
attractive molecular potential. For a positive chirp, the resonance condition
moves outward in time, while for a negative chirp, it moves inward, in the same
direction as the excited wavepacket; this allows multiple interactions between
the wavepacket and the light, enabling the wavepacket to be returned coherently
to the ground state. Classical and quantum calculations support this
interpretation.
