  Two-photon intensity interferometry is shown to provide an accurate
measurement of lifetime of quark-gluon plasma created in ultra-relativistic
heavy ion collisions via the difference of outward and sidewardcorrelation
radii. Under the assumption of a longitudinal, boost invariant expansion of the
plasma, we obtain analytical expressions for the correlations from the
quark-gluon plasma phase. A $3+1$ dimensional expansion of the plasma along
with a first order phase transition to hadrons is next considered, and, leads
to a source with two characteristic lifetimes, one for the quark-gluon plasma
phase, and the other for the longer lived mixed phase. This may even help us to
{\em experimentally} determine the order of the phase transition.
