A similar question has been asked here, but there was no answer: Why Delayed choice quantum eraser doesn't produce two lines
I learned that in the original double-slit experiment, an interference pattern is observed after firing many photons on the detector.
I also learned that in Delayed-choice quantum eraser experiment, when you look at the detector, what you initially see is just noise. Only after separating out the photons according to which detector detected their entangled pair, you start to see interference patterns on D1 and D2 detectors.
What I'm wondering is what causes the interference pattern seen on the main detector in the original double-slit experiment to turn into noise in the quantum eraser experiment. I am suspecting it's because the beam splitter BSC but I don't understand why.
Let's consider a simpler experiment than quantum eraser, removing the two initial splitters BSa and BSb. The photons travel from both paths and which-way information is deleted when they pass through beam splitter BSC.
What do we see in the detector D0? Interference pattern or noise? If noise, why? In the original experiment we saw interference pattern, what caused the difference in this case? Since we removed the information, I would expect the result to be equivalent to the one in the original experiment.
In other words, I don't see a difference between this experiment and original experiment, other than the fact that we have entangled photons here which doesn't provide any information thanks to BSC. Why is the outcome different?