Quantum nonlinear optics with Rydberg polaritons

Realizing and engineering optical non-linearity at the level of single
photons is a goal of scientific and technological significance. We obtain
strong interactions between propagating photons by coherently coupling them
to Rydberg atoms in a cold gas. While slowly traversing the medium, the
"Rydberg polaritons” interact via the Van-der-Waals force, owing to their
large electric dipole-moment. We are able to vary the dynamics of the
two-photon wavefunction from dispersive (Schrodinger-like) to dissipative
(diffusion-like) and observe strong bunching, anti-bunching, and a
conditional phase-shifts of 1 radian for two individual photons.