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The coupling of two macroscopic quantum states through a tunnel barrier gives rise to Josephson phenomena such as Rabi oscillations, the a.c. and d.c. effects, or macroscopic self-trapping, depending on whether tunnelling or interactions dominate. Nonlinear Josephson physics was first observed in superfluid helium and atomic condensates^,, but it has remained inaccessible in photonic systems because it requires large photon–photon interactions. Here we report on the observation of nonlinear Josephson oscillations of two coupled polariton condensates confined in a photonic molecule formed by two overlapping micropillars etched in a semiconductor microcavity. At low densities we observe coherent oscillations of particles tunnelling between the two sites. At high densities, interactions quench the transfer of particles, inducing the macroscopic self-trapping of polaritons in one of the micropillars^,. The finite …