A pump beam excites a non-linear crystal which produces two entangled photons with perpendicular polarization, namely in the state $|HV>+|VH>$. Are there examples where one of the photons was emitted slightly earlier than the other? Or in other words, is there a measurable distribution in the relative time of detection at equidistant detectors from the source?
I would think that according to quantum mechanics the emission of the two photons can never occur exactly at the same instant. Is this correct?