Ultracold Collisions Observed in Real Time

In laser-induced collisions between ultracold atoms, the combination of low velocities and long-range interactions results in collision times which can exceed the excited-state lifetime. We use a cooperative effect between two lasers to explicitly observe this time dependence. The first laser, tuned near resonance, excites the atom pair at long range and enhances the collisional flux available for short-range excitation by a second, far-detuned laser. Using pulsed excitation, we find this collisional process to take place on a ${10}^{-6\mathrm{}}\mathrm{s}$ time scale, in reasonable agreement with trajectory simulations.