The phase distribution in a Bose-Einstein condensate can realize various
topological states which can be classified according to distinct winding
numbers. While states with different winding numbers are topologically
protected in the linear Schr\"odinger equation, when nonlinearities are
introduced, violations of the topological protection can occur, leading to
unwinding. Exciton-polariton condensates constitute a weakly nonlinear open
dissipative system that is well suited to studying such physics. Here we show
that exciton-polariton condensates display a spontaneous phase unwinding from a
Ï€- to zero-state. While such an effect was previously observed in a
one-dimensional polariton-condensate array and explained as occurring due to
single-particle mode competition, we offer a new explanation in terms of
collective phase unwinding of metastable states. We clarify that the collective
transition is caused by the combined effect of nonlinearity and topological
defects in the condensates. Reanalyzing the experimental data, we find an
evidence of the collective phase unwinding.Comment: 8+4 pages, 4+1 figure