Talking about this experiment https://en.wikipedia.org/wiki/Delayed_choice_quantum_eraser#The_experiment_of_Kim_et_al._.282000.29
I am assuming that anyone able to answer my question is already somewhat familiar with the experiment and the wiki page explains it better than I can. But here is the gist of it according to my understanding: you send photons through a double slit then split them into two entangled photons. One goes to D0 and the other goes to a setup of half mirrors and detectors where there is a 50/50 change of erasing or keeping information relating to the path or "which slit" the photon passed through.
While the collective pattern of all photons hitting D0 is just a blurred line you can pick out subsets R1 R2 R3 and R4 using the coincidence counter. That is R1 fx shows only photons hitting D0 which also had a "sister" particle hitting D1. Now R1 and R2 shows an interference pattern, phase shifted in relation to each other such that they cancel out in the collective image of D0.
Now here is what I don't understand: why is there a difference between D1 and D2? On the graphs R1 has a peak in the middle while R2 has a valley, why not reversed? What makes D1 and D2 different? What decides which one gets peaks at certain points and the other gets the opposite?
Also does D1 and D2 always have a peak or valley in the middle? or are they simply always phase shifted pi in relation to each other but not in relation to the middle? Either way, what I really want to know is why/how does R1 and R2 turn out phase shifted?