  We have used quantum control to suppress the impact of random atom positions
on coherent population transfer within atom pairs, enabling the observation of
dipole-dipole driven Rabi oscillations in a Rydberg gas with hundreds of atoms.
The method exploits the reduced coupling-strength sensitivity of the
off-resonant Rabi frequency, and coherently amplifies the achievable population
transfer in analogy to quasi-phase-matching in non-linear optics. Simulations
reproduce the experimental results and demonstrate the potential benefits of
the technique to other many-body quantum control applications.
