Coherent merging of counter-propagating exciton-polariton superfluids

We report the formation of a macroscopic coherent state emerging from colliding polariton fluids. Four lasers with random relative phases, arranged in a square, pump resonantly a planar microcavity, creating four coherent polariton fluids propagating toward each other. When the density (interactions) increases, the four fluids synchronise and the topological excitations (vortex or soliton) disappear, forming a single superfluid. We confirm the observations with a numerical time-evolution of the fluid wavefunction and explain this behavior with a reduced analytical approach based on the local fluid properties.