  We investigate the evolution of quantum correlations over the lifetime of a
multi-photon state. Measurements reveal time-dependent oscillations of the
entanglement fidelity for photon pairs created by a single semiconductor
quantum dot. The oscillations are attributed to the phase acquired in the
intermediate, non-degenerate, exciton-photon state and are consistent with
simulations. We conclude that emission of photon pairs by a typical quantum dot
with finite polarisation splitting is in fact entangled in a time-evolving
state, and not classically correlated as previously regarded.
