| Abstract: | Realizing and engineering optical non-linearity at the level of single photons is a goal of scientific and technological significance, pertaining to non-classical light sources, all-optical gates, and correlated many-photon states. Strong interaction between propagating photons can be obtained 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 interaction potential from real to imaginary, changing the dynamics of the two-photon wavefunction from dispersive (Schrodinger-like) to dissipative (diffusion-like). To characterize the final two-photon state, we use time-dependent quantum tomography and delineate the two-photon bound-state. We observe strong bunching or anti-bunching, and large conditional phase-shifts for two individual photons. |
