Achieving optical nonlinear interactions at the level of single photons has been the goal of extensive efforts in the last couple of decades. I will present our recent demonstration of deterministic, all-optical photon-photon interactions based on a single atom. This scheme performs deterministic quantum state-transfer between a photon and a single quantum emitter (a 87Rb atom, in our case). Based on passive, interference-based nonlinearity, this scheme requires no control fields, and can function as a quantum memory and even a universal quantum gate. It therefore provides a building block for scalable quantum networks based on completely passive photonic devices interconnected and activated solely by single photons.

Refs:
[1] S. Rosenblum, A. S.  Parkins, and B.Dayan, "Photon routing in cavity QED: Beyond the fundamental limit of photon blockade", Phys. Rev. A. 84, 033854 (2011)
[2] I. Shomroni, S. Rosenblum, Y. Lovsky, O. Bechler, G. Guendelman, and B. Dayan, 
"All-optical routing of single photons by a one-atom switch controlled by a single photon", 
Science 345, 903 (2014)