Rather than quote the entire thing I'll just give the link to what I wrote out. Mar 19 2005 http://www.geocities.com/tdunc01/PG21_Physics_and_Theory_Journal_-TDuncan.html "I question if it is even possible to collapse the wave-function of a photon." "It is unclear to me which particle has been used in all 2-slit experiments." and "Now the only thing I want to know is, is it possible to reintroduce the wave-function to an electron after you have collapsed/separated it?" because if we can re-establish the wave-function then we have a good idea of what we took away by what we put back into it. Edit: Guess what I'm after is similar to 'quantum eraser', found this http://www.fortunecity.com/emachines/e11/86/qphil.html [Broken] So only when the 2 photons are of opposite polarization does the interference patterns disappear. When you re-polarize the photons they become like again, and bingo the interference pattern is re-established. The wave-like nature or wave-function is linked to polarization and more specifically like polarization. I've always thought of polarization as spin, so when both photons have say spin up we get the wave-function yet if 1 is spin down the wave-function is considered "collapsed". In any case we have to treat the 2 separate photons as a (entangled) single system otherwise there would not be this link. Can we then assume in another more common 2-slit experiment that feyman describes that detection of the Electron also involves a change in Polarization? Yet remembering that in that more simple experiment it is not recognized that more than 1 particle is present eg. its not an entangled system - where a polarization change of 1 would alter the system. Here we have only 1 to begin with so to change Its polarization should be meaningless because wave-function depends on a coherent 'more than 1 system' to achieve 'both paths taken' and interference. But I have to disagree and say that the Electron is a 'more than 1 system' all along otherwise it dont make sense.