The quantum-optical Josephson interferometer

The photon-blockade effect, where nonlinearities at the single-photon level alter the quantum statistics of light emitted from a cavity(1), has been observed in cavity quantum electrodynamics experiments with atomic(2,3) and solid-state systems(4-8). Motivated by the success of single-cavity quantum electrodynamics experiments, the focus has recently shifted to the exploration of the rich physics promised by strongly correlated quantum-optical systems in multicavity and extended photonic media(9-14). Even though most cavity quantum electrodynamics structures are inherently dissipative, most of the early work on strongly correlated photonic systems has assumed cavity structures where losses are essentially negligible. Here we investigate a dissipative quantum-optical system that consists of two coherently driven linear optical cavities connected through a central cavity with a single-photon nonlinearity (an optical analogue of the Josephson interferometer). The interplay of tunnelling and interactions is analysed in the steady state of the system, when a dynamical equilibrium between driving and losses is established. Strong photonic correlations can be identified through the suppression of Josephson-like oscillations of the light emitted from the central cavity as the nonlinearity is increased. In the limit of a single nonlinear cavity coupled to two linear waveguides, we show that photon-correlation measurements would provide a unique probe of the crossover to the strongly correlated regime.