I Mandel quantum eraser: No signal/idler-correlations needed?

[greypilgrim]

Hi.
I found slightly different depictions of the Mandel quantum eraser experiment (1991). They are all from German sources, but I think they are pretty clear nonetheless.
The first is from a German schoolbook:

The second is from here:

They claim that interference appears when the origins of the idler photons are made indistinguishable (second picture in the first setup, first picture in the second setup) and disappears otherwise. However it seems to me that this choice (inserting or pulling out the idler beam splitter in the first setup or the red block in the second could be arbitrarily delayed by making the optical paths of the idler photons longer, even longer than the paths of the signal photons, causing all the weird things like retrocausality or FTL communication.

I know other setups (e.g. Kim et al. (2000)) don't exhibit this since they need to measure correlations between signal and idler photons, which make everything consistent with causality and SRT. Also, in Kim's setup it's not really a choice whether or not the idler photon are indistinguishable, it just happens in 50% of all cases.

Are the depictions above just terrible oversimplifications, or do they really not need to measure correlations? Unfortunately I couldn't find Mandel's original publication.

 

[zonde]

Are the depictions above just terrible oversimplifications

Yes. Not only coincidence counting is missing but some interference sensitive filtering has to be present before detection of idler photon.

 

[greypilgrim]

interference sensitive filtering

What do you mean by that? Is it a theoretical necessity or something to reduce experimental noise?

 

[zonde]

What do you mean by that? Is it a theoretical necessity or something to reduce experimental noise?

Actually I was thinking that the setup is like in Walborn experiment: http://laser.physics.sunysb.edu/~amarch/eraser/Walborn.pdf
Then this Cthugha's comment applies: https://www.physicsforums.com/threads/quantum-eraser-experiment.300985/#post-2125687 (this is what I meant with "interference sensitive filtering")

I would say that the first experiment (first two pictures) is quite similar to Walborn experiment.
The second experiment however is different. In first picture of second experiment the first idler overlaps in downconverter crystal with the second idler. So for the second downconversion the phases are correlated for idlers and so they are correlated for signal photons as well and you can observe interference without coincidence counter. But you can't propose any FTL communication as second downconversion happens only after you have blocked/not blocked first idler.