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This YouTube video demonstrates a doable-at-home quantum eraser experiment.

To summarize in brief, a laser is shined through a double-slit, causing an interference pattern on a white poster 3 meters away. When a horizontal polarizing filter is added on the left slit and a vertical polarizing filter is added on the right slit, the interference pattern disappears. When a 45-degree polarizing filter is re-added after the slit, the interference pattern reappears.

This is said to prove that knowledge of which slit the photon goes through eliminates the interference pattern. Because with the polarized light we can tell which slit it went through, there is no interference pattern. Because with the 45-degree filter added we no longer can, the interference pattern re-emerges.

However, it’s instead possible that it’s not knowledge of the slit but rather that horizontally and vertically polarized light doesn’t cause an interference pattern with itself, while light polarized in one direction does, that explains the interference pattern or lack thereof.

What if instead we put the 45-degree filter 3 meters away and the white poster 6 meters away? There are 4 different setups:

  • NN: no filters at slits, no filter at 3 meters
  • YN: yes filters at slits, no filter at 3 meters
  • NY: no filters at slits, yes filter at 3 meters
  • YY: yes filters at slits, yes filter at 3 meters

What is the expected pattern for each setup that we will see at the white poster 6 meters away? Is this accurate:

  • NN will give us a 6-meter interference pattern as we have no knowledge of the photon path.

  • NY will give us the same pattern (?) as we have no information about the path. I’m not sure if the 45-degree filter 3 meters in will affect anything.

  • YN: here we expect no interference pattern since we know through which slit the photon went through by the polarization at 6 meters.

  • What is YY predicted to yield?

    • Option 1: identical result to NN as we have no info of the photon path at the 6-meter detector.

    • Option 2: Something like the pattern we’d see if the slits were 3 meters away from the white board , as the path is known for the first 3 meters and therefore the interference pattern starts only once the light is combined back at the 45-degree filter.

    • Option 3: something else?

And if it’s Option 2, why is it the case if we only observe the result at the 6-meter detector?

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For YY the answer is the same as "When a 45-degree polarizing filter is re-added after the slit, the interference pattern reappears."

Your comment" However, it’s instead possible that it’s not knowledge of the slit but rather that horizontally and vertically polarized light doesn’t cause an interference pattern with itself, while light polarized in one direction does, that explains the interference pattern or lack thereof." might be based on classical or the historical concept of "interference" where 2 photons meet at the screen and cancel ..... but this is a fundamental violation of the principle of conservation of energy. This old concept is still taught in school today because of its convenience and testability.

Dirac and Feynman knew that each photon on its own finds its own probable path .... photons do not interact. Read about the Feynman path integral as well.

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  • $\begingroup$ Understood - and the probable path doesn't change based on the fact that instead of 6 meters of unpolarized light after the slit, it's 3 meters of part-horizontally and part-vertically polarized light, followed by 3 meters of 45-degree polarized light? What I mean is, do we expect the same exact interference pattern, or some interference pattern although one that is different? $\endgroup$
    – Claudiu
    Dec 7, 2021 at 9:10
  • $\begingroup$ Exact same pattern, although in the link above, we can get the inverse pattern by adding an additional polarizer in the path! The important point is that an excited atom/electron is radiating virtual photons (my interpretation and a few others) and if this atom sees 2 paths/slits at the same time the "interference" pattern arises, if the polarizers only enable one path or the other ... no pattern. A probable path is taken by the real photon. $\endgroup$
    – PhysicsDave
    Dec 8, 2021 at 21:17

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