I understand that one can measure a single photon being absorbed using a photomultiplier tube or CCD. Can one measure a single photon being emitted by monitoring the current through an LED or the recoil of an emitting ion? Is it therefore possible to detect the same photon both being emitted and later being absorbed? After the photon's emission has been detected, but before its absorption has been registered, is it real or virtual? I assume a QED calculation would proceed on the assumption that the photon is virtual but if its emission has been measured then surely it should be treated as real? Is there a paradox here?