Strongly correlated photons on a chip

Optical nonlinearities at the single-photon level are key ingredients for future photonic quantum technologies(1). Prime candidates for the realization of the strong photon-photon interactions necessary for implementing quantum information processing tasks(2), as well as for studying strongly correlated photons(3-6) in an integrated photonic device setting, are quantum dots embedded in photonic-crystal nanocavities. Here, we report strong quantum correlations between photons on picosecond timescales. We observe (i) photon anti-bunching upon resonant excitation of the lowest-energy polariton state, proving that the first cavity photon blocks the subsequent injection events, and (ii) photon bunching when the laser field is in two-photon resonance with the polariton eigenstates of the second Jaynes-Cummings manifold(7,8), demonstrating that two photons at this colour are more likely to be injected into the cavity jointly than they would otherwise. Together, these results demonstrate unprecedented strong single-photon nonlinearities, paving the way for the realization of a quantum optical Josephson interferometer(9) or a single-photon transistor(10).