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I am by no means an expert, so I ask if any and all answers can be put into simple terms. Regarding the DCQE, there is seen to see both non-interference diffraction patterns (when "which-way information is revealed") and an interference pattern(s) (when "which-way information is erased"), but there is some lack of clarity regarding the observed interference patterns. Here I shall list out my qyestions:

1: Why is there a pi phase shift between the two observed interference patterns (R01 and R02)?

2: Do the observed interference patterns have anything to do with "which-way information"? If not, then what causes them?

3: Is it not significant that the supposed collapse of the wave function, causing the non-interference patterns at R03 (R04), happens after the photon "hit" at D0?

This is a link to the pdf of the experiment which I am referring to: https://drive.google.com/file/d/17ilMXlwwgfJSaNWAJoa0iKV0s7jFdPEn/view?usp=drivesdk

Thank you for any and all information!

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  • $\begingroup$ There is no "which way" information in quantum mechanics. Quanta don't have paths. The "difficulty" with understanding interference patterns of light start with Maxwell's theory. The reason that light can map one surface (e.g. Monet's "Sea Roses") to another surface (your retina) is because it shows absolutely no self-interaction in the volume between those surfaces. This is a general property of linear equations of this type: they have transient solutions and then there is a steady-state solution that only depends on the boundary conditions. Photons replicate that classical behavior. $\endgroup$ Commented May 28, 2023 at 18:48

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The link you posted is not accessible to the general public, but I assume you mean the Kim et.al. experiment from 1998, as described on Wikipedia.

1: Why is there a pi phase shift between the two observed interference patterns (R01 and R02)?

At the beam splitter before D1/D2, the two paths for the idler photon come together. The way the beam splitter works, is if the paths are in phase, the photon is sent to D1, and if they are out of phase, the photon is sent to D2.

The signal photon has the same phase difference between its paths as the idler photon has. Due to that, for idler photons that are in phase (and therefore land at D1), the signal photons make a certain interference pattern on D0. For idler photons that are out of phase, the signal photons make the shifted interference pattern.

2: Do the observed interference patterns have anything to do with "which-way information"? If not, then what causes them?

The interference patterns are due to the interference on the signal side. The two possible paths the signal photon can take come together at D0, and interfere there.

3: Is it not significant that the supposed collapse of the wave function, causing the non-interference patterns at R03 (R04), happens after the photon "hit" at D0?

The probabilities for each signal photon makes a certain interference pattern at D0, in line with the phase difference between its paths. But the signal photons don't all have the same phase difference, and therefore their individual interference patterns are shifted wrt. each other, and add up to the single bulge you see on D0.

The same happens for the subset of signal photons whose entangled partner lands at D3. The individual photons all have different phases, and so their individual interference patterns add up to a single bulge. That is, the bulge is not due to non-interference; the pattern of an individual signal photon on D0 never "collapses" into a bulge pattern.

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