  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 to form a single superfluid.
