Delayed choice and information erasure

[StevieTNZ]

Sorry if this sounds stupid, but what exactly are idler photons? I'm reading this book that mentions the University of Rochester experiment by Mander that contains signal photons and idler photons. So we have a laser and a beam splitter, each path contains either the signal photons or the idler photons?

So when you say the subset of signal photons and their corresponding idler photons, what do you mean? Is that some of the signal photons that went on one path at the first beam splitter and some of the idler photons that went on the other path at the beam splitter?

Not too sure. Sorry if it sounds silly. Hopefully I'll realize what is being said once I'm given some clarification!

But either way, the original interference pattern is NOT recovered, but two new interference patterns are created which cancel each other out?

 

[JesseM]

Sorry if this sounds stupid, but what exactly are idler photons?

"signal" vs. "idler" are just two labels to distinguish members of an entangled pair of photons, they don't imply any intrinsic difference between the two photons, the "idlers" are just the ones that go in the direction of the D1/D2/D3/D4 detectors in the diagram while the "signal" photons are the ones that go in the direction of the D0 detector in the diagram.

So when you say the subset of signal photons and their corresponding idler photons, what do you mean? Is that some of the signal photons that went on one path at the first beam splitter and some of the idler photons that went on the other path at the beam splitter?

Each photon is created as part of an entangled pair, so each signal photon has an entangled idler "twin". So if you have 1000 signal photons recorded at D0, you can label each one according to where their idler "twin" was detected, at D1 or D2 or D3 or D4. Suppose there are 250 signal photons whose idler twins went to D1, if you graph only that subset of signal photons (i.e. not including the positions of any of the other 750 signal photons on your graph) then you will see an interference pattern. Likewise if there are 250 signal photons whose idler twins went to D2, if you graph that subset you also see an interference pattern, but displaced from the first one so that the peaks of the second interference pattern line up with the valleys of the first and vice versa. Because of this displacement, if you graph the 500 signal photons whose idler twins went to either D1 or D2, you see a non-interference pattern. And since the D3/D4 subsets also are non-interference patterns, if you graph the total pattern of all 1000 signal photons, you get a non-interference pattern.

But either way, the original interference pattern is NOT recovered, but two new interference patterns are created which cancel each other out?

By "original" do you mean what would be seen in a regular double-slit experiment with non-entangled photons? If so, assuming what you mean here matches what I said above (that the total pattern of all 1000 signal photons shows no interference, but if you graph the 250 whose idlers went to D1 or the 250 whose idlers went to D2, you do get two interference patterns which add up to a non-interference pattern when you combine their data) then yes.