One simple question regarding delayed choice quantum eraser

Question

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My question is, in the delayed eraser setup, if I completely remove all the D1/D2/D3/D4 parts and only record D0, what will D0 be like?

Shouldn't it be an interfered pattern?

When I add in D1/D2/D3/D4 and all related apparatus for the entangled idler photons, then D0 will change into no interference can be seen.

Is my understanding right?

If it's right, then after D0 is detected, I quickly remove all the apparatus for D1-D4, before the entangled photon touches them, what will the D0 pattern be like?

Thanks.

Answer 1

$D_0$ will only ever display a blurred band that shows no interference to the naked (or other) eye. It doesn't matter what you do with $D_1$-$D_4$.

As has been explained in multiple other questions and answers on this topic, the ONLY way you can recover an interference pattern (note, I said "recover" not "see") is if you correlate locations of photon hits on $D_0$ with hits on your detectors that do not reveal which-path information.

If those detectors are $D_1$ and $D_2$, what you'll find is that the peaks of the interference fringes for $D_1$ align with the troughs of $D_2$, so that without teasing them apart with your stopwatch after the fact, even if you got rid of your which-path detectors entirely, you will not see any interference.