Consider this report: http://www.physics-astronomy.com/2015/01/quantum-teleportation-of-subatomic.html "Then, the researchers shot a third particle of light at the photon traveling down the cable. When the two collided, they obliterated each other. Though both photons vanished, the quantum information from the collision appeared in the crystal that held the second entangled photon." There have been several reports of this result, all claiming photon-photon annihilation. Is that possible? I am not asking if it is rare, I am asking if it is at all possible. The two photons that annihilated each other were not in this case even entangled. If it is possible (however rare) for a neutrino and an antineutrino to annihilate giving a photon pair, then the time reversal would be photon-photon annihilation... but I'd guess you would have to get the phase just right. In which case you would not need the MEV energy of an electron-positron pair, but only the rest-mass energy of twice the neutrino, meaning that very-long-wavelength photons would work. Is this result some kind of artifact of being in an optical fiber? The researchers do not seem to have any concern for conservation of energy; they do not mention what products of the annihilation there might be, the photons simply vanish, in their view.