  The macroscopic coherence of quantum fluids enables for the observation of
interference effects in their wavefunctions and leads to practical applications
based on Josephson tunnelling such as superconductor quantum interference
devices. The Josephson effect manifests in both fermionic and bosonic systems
and it has been extensively studied in superfluid helium and atomic
Bose-Einstein condensates. Exciton-polaritons can facilitate the transition to
integrated semiconductor platforms, but realizing weak links in ring geometries
has proven to be challenging. Here we realize a Josephson junction in a
polariton ring condensate and, leveraging the optical control of the barrier,
we impose a net circulation around the ring. Our experiments reveal two
distinct regimes of the polariton weak link: a hydrodynamic regime with a
linear relation between the phase jump at the barrier and the average current
around the ring, and a Josephson regime characterised by a sinusoidal behaviour
for which the mass transport through the barrier drops to zero. Our theory in
terms of the 'tilted washboard' potential explains the appearance of these
regimes from energy considerations using experimental values. These findings
show that weak links in ring condensates can be investigated using all-optical
configurations and hold potential for exploration even at room temperature.
