Question about Delayed Choice Quantum Eraser

[LikenTs]

As almost everybody knows, DCQE experiment only shows interference after the data analysis, never in the first instance, regardless of future events for idler photons.

https://en.wikipedia.org/wiki/Delayed_choice_quantum_eraser

So, my question is, why you cannot see interference with this simple and simetric modified experiment, where there is not which way information?

 

[MichPod]

What is that triangle with an on/off switch? And what is the switch?

 

[LikenTs]

What is that triangle with an on/off switch?

Nothing, the picture is not mine. I think the original autor wanted to use this experiment for communication, that is not allowed.
As far as my question goes you can ignore that switch.

 

[MichPod]

where there is not which way information?

This information still does exist (it is not "erased"). The red and blue rays coming at D4 detector have different momentum coming at different angles. Then this information is absorbed into the detector. We cannot get it, but it still is present first in the detector micro-state and then in the environment.

Upd: and you can see that this is not the case in the original DCQE experiment where two light beams are perfectly aligned by a system of mirrors.

 

[MichPod]

BTW, I am not sure that talking here in terms of "which way" or "information erasure" makes much sense. That is, if I write formulas, I can more or less see how this particular configuration may fail to produce interference pattern, but I am not comfortable to discuss this system in terms of "information", "which way" etc (may be somebody else is? ) and my explanations in these terms may be wrong.

 

[DrChinese]

As almost everybody knows, DCQE experiment only shows interference after the data analysis, never in the first instance, regardless of future events for idler photons.

So, my question is, why you cannot see interference with this simple and simetric modified experiment, where there is not which way information?

Is your question: "Why is there no interference pattern on the top side, without considering coincidence detection?"

 

[zonde]

So, my question is, why you cannot see interference with this simple and simetric modified experiment, where there is not which way information?

I suppose you ask about interference in coincidences judging by first sentence in your post.
Then let me ask, why do you think you cannot see interference? Because I think you can.