We report on the observation of spontaneous coherent oscillations in a
microcavity polariton bosonic Josephson junction. The condensation of exciton
polaritons takes place under incoherent excitation in a disordered environment,
where double potential wells tend to appear in the disordered landscape.
Coherent oscillations set on at an excitation power well above the condensation
threshold. The time resolved population and phase dynamics reveal the analogy
with the AC Josephson effect. We have introduced a theoretical two-mode model
to describe the observed effects, which allows us to explain how the different
realizations of the pulsed experiment have a similar phase relation