Delayed quantum eraser, turning off sensor to bring back interference

Question

I've been learning about the delayed-choice-quantum-eraser experiment. To sum up; I imagine a double slit which has behind it a crystal which splits photons into entangled pairs. The crystal sends a photon to the right, from either slit, toward a single sensor (s1) which simply measures that a photon was received and in a pattern, and then the crystal also sends an entangled photon to the left, presumably from the same slit to an apparatus that allows the entangled photon to be detected (s2, s3) based on which slit it passed through 50% of the time, or the entangled photon is reflected into a path which combines both slits into a single censor (or set of sensors, s4 and s5 which cannot determine which slit the photon passed through).

I was watching a laymens video which made the claim that the crystal, in a sense, is making a measurement when the photon passes through it and the entangled pair are created, therefore an interference pattern cannot be made at sensor 1. This was new to me, hearing this interpretation. Part of my confusion is that I am under the impression that sensors 4 and 5 are showing interference, and this seems to contradict the claim that the crystal is collapsing the wave form. My question is; what if sensors 2 and 3 are turned off, or completely removed from the experiment/apparatus? Does sensor 1 still have no interference? I'm assuming sensor 4 and 5 still have interference.

Answer 1

The whole experiment relies on a coincidence detection scheme .... so if you turn off 2,3 you will never know what's happening on 1 when 2 photons hit 2,3. The video? There are many interpretations .... the more fanciful the more it is likely to gain interest, beware.

It can easily be assumed that the wave collapses in the crystal .... but I think its incorrect. In all these "interference" type experiments it is better to think of a 2 step process: 1) the excited electron in the source is already interacting with the EM field ("virtually"), long before the photon is emitted, 2) at photon emission time the path is pretty much predetermined ... just don't move any mirrors mid-flight as that becomes a whole new experiment/wave function.

Thus each photons wave function encompasses the whole apparatus, first "virtually" and then you can maybe consider a 2nd wave function of the actual photon path .... The Feynman path integral is a good way to calculate the probabilities of the virtual one .... it will show you where the photons are likely to hit. Turns out photons like to travel lengths that are multiples of its wavelength .... kind of like a music note on a guitar string.

The experiment also claimed to offer faster than light communications ..... this is has been refuted as well.