Nature 502, 71–75 (2013)

In principle, photons do not interact with each other. However, if coherent interactions between individual photons became a reality, it would potentially open the door to applications such as all-optical switching and deterministic photonic quantum logic. Now, Ofer Firstenberg and co-workers from the USA have created a quantum nonlinear medium within which individual photons travel as massive particles with strong mutual attraction. This causes the propagation of photon pairs to be dominated by a two-photon bound state. The researchers realized mutual interaction between photons through dispersive coupling of light to strongly interacting rubidium atoms under the condition of electromagnetically induced transparency in systems with highly excited Rydberg states. When two Rydberg atoms were located within a distance of the Rydberg blockade radius (about 18 μm in this work), the doubly excited Rydberg state was largely detuned from the electromagnetically induced transparency resonance because of the repulsive van der Waals interaction. By tuning the frequency and narrowing the beam waist to 4.5 μm of the laser beam used to excite a cold rubidium gas, the researchers created a narrow region in which photon–photon interaction occurs. The experimentally measured concurrence of 0.09 at a delay time of zero indicates deterministic entanglement of previously independent photons during propagation through the quantum nonlinear medium.