  A measurement for studying the mass dependence of the dilepton interferometry
in relativistic heavy-ion collision experiments as a tool to characterize the
quark-gluon phase is proposed. In calculations involving dileptons, we show
that the mass dependence of radii extracted from the virtual photon (dilepton)
interferometry provide access to the development of collective flow with time.
It is argued that the non-monotonic variation of HBT radii with invariant mass
of the lepton pairs signals the formation of quark gluon plasma in heavy ion
collisions. Our proposal of experimentally measuring the ratio, $R_{\mathrm
out}/R_{\mathrm side}$ for dileptons can be used to estimate the average life
times of the partonic as well as the hadronic phases.
