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I am familiar with the standard Quantum Eraser experiment. I was thinking of the following.

Quantum Eraser My hunch is that the screen will show an interference pattern since the which way information for the second slit (X & Y) is not available.

If that is the case, we have forced the Wave function to collapse in the first slit (A & B) and then un-collapsed it for the second slit!

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  • $\begingroup$ Let's say the photons are released one at a a time, as the observer sees them they collapse and do not exist anymore, so nothing travels to the final slits and screen. If many photons go thru only the unobserved get to the screen and a diffraction pattern is seen. $\endgroup$ – PhysicsDave Jul 10 '19 at 1:22
  • $\begingroup$ Observing the photon can be done without destroying them. It's only when the photon hits a screen that it's "destroyed"."so nothing travels to the final slits" could you please elaborate on this? Why would the photons not travel beyond the first slit? $\endgroup$ – Momshad Alvee Jul 10 '19 at 7:17
  • $\begingroup$ In the classic double slit experiment, the photons are observed before they pass through the slit and afterwards they definitely go through the slits and hit the screen. That's how we see the particle pattern. So again, I don't understand why you're saying only the unobserved photons will go through. If that was the case there would be no way to see the particle pattern on a screen. $\endgroup$ – Momshad Alvee Jul 10 '19 at 7:27
  • $\begingroup$ We know photons go thru because typically they are many from the source and we can even see a few reflected from the front, and some on the screen, all those we see are destroyed. A photon is like an agreement between 2 atoms to exchange energy, one atom in the sun or bulb filament and the other eventually in your eye or a detector. $\endgroup$ – PhysicsDave Jul 11 '19 at 1:07
  • $\begingroup$ There are many interesting single photon experiments for the double slit which show that "interference" is not why we see the pattern, (because the photons build up these pattern over time). Photons prefer to travel integer multiples of their wavelength, the dark spots are spots where few photons go. $\endgroup$ – PhysicsDave Jul 11 '19 at 1:09

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