Two-Photon Blockade in an Atom-Driven Cavity QED System

The $n$-photon blockade is a dynamical quantum-nonlinear effect in which the absorption of $n$ photons blocks the absorption of the $(n+1)$th photon. This effect can occur in driven systems with an anharmonic ladder of energy eigenstates, e.g. a single atom strongly-coupled to a high finesse optical resonator. While single-photon blockade has been demonstrated in such a system before~[1], we now report on the first observation of two-photon blockade~[2]. As a signature, we show a three-photon antibunching with simultaneous two-photon bunching observed in the light emitted from the cavity. The effect occurs for atom driving, not cavity driving. While the two-level atom can only add photons stepwise one-by-one, the bosonic enhancement for cavity driving increases the transition strengths towards higher manifolds which reduces the inherent nonlinearity of the system. We consider these results as a significant step towards multi-photon quantum nonlinear optics. \newline[1] Birnbaum et al., Nature 436,87 (2016) \newline[2] Hamsen et al., arXiv 1608.01571 (2016)