Active Suppression of Dephasing and Observation of Rabi Oscillations in Near Resonant Dipole-Dipole Interactions

We have shown that pulsed electric fields can be employed to actively suppress macroscopic dephasing in dipole-dipole mediated, resonant energy transfer between nearly degenerate atom pairs states in cold random Rydberg ensembles. In particular, we have studied the Stark-tuned (nearly) resonant interaction, 32p_3/2+32p_3/2 → 32s+33s in a Rb magneto-optical trap (MOT). In an electric field of 11.6V/cm, the 32p_3/2+32p_3/2 and 32s+33s states in a pair of non-interacting ⁸⁵Rb atoms are degenerate, and following laser excitation of an isolated 32p_3/2 atom pair, the state of the system will coherently evolve into a coherent superposition of 32p_3/2+32p_3/2 and 32s+33s, exhibiting Rabi oscillations in the probability for finding the atoms in the 32s+33s state. However, in a random ensemble with a large number of nearest neighbor atom pairs, the Rabi frequencies vary significantly over the sample, resulting in macroscopic dephasing of the coherent evolution in each atom pair, such that no Rabi oscillations are observed in the total 32s+33s population. We have implemented a scheme that is a time-domain analog to spatial quasi-phase matching of non-linear optical processes that effectively reduces the variation in Rabi frequencies over the cold atom ensemble. Specifically, we use pulsed electric fields to coherently transport the system across the resonance, from the high to low field side and back, enhancing the off-resonant population transfer and reducing the effective Rabi rate (and its spread). Simulations predict an increase in dephasing times by a factor of 2N where N is the number of back and forth cycles. To date, we have experimentally observed the suppression of dephasing, the reduction of the effective Rabi frequency, and the population transfer enhancement for N=1 and 2. We are exploring the potential of the method for suppressing dephasing associated with atom motion as well.