  We demonstrate a cooperative optical non-linearity caused by dipolar
interactions between Rydberg atoms in an ultra-cold atomic ensemble. By
coupling a probe transition to the Rydberg state we map the strong
dipole-dipole interactions between Rydberg pairs onto the optical field. We
characterize the non-linearity as a function of electric field and density, and
demonstrate the enhancement of the optical non-linearity due to cooperativity.
