Coherent merging of counterpropagating exciton-polariton superfluids

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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 synchronize and the topological excitations (vortex or soliton) disappear to form a single quantum superfluid.