  We present an experiment that probes polariton quantum correlations by
exploiting quantum complementarity. Specifically, we find that polaritons in
two distinct idler-modes interfere if and only if they share the same
signal-mode so that "which-way" information cannot be gathered. The
experimental results prove the existence of polariton pair correlations that
store the "which-way" information. This interpretation is confirmed by a
theoretical analysis of the measured interference visibility in terms of
quantum Langevin equations.
