Delayed Choice Quantum Eraser - Experimental Details

[albroun]

What exactly happens during the time gap between the detectors being activated, and the eraser subsequently wiping out the information after the results screen has received the photons? For example, do we "observe" the interference pattern being absent and then (gradually or immediately) re-appearing during this time gap, or does the "observation" of what is happening on the results screen during the time gap prevent the eraser from being able to restore the interference pattern?
Any illumination in layperson's non-mathematical form would be appreciated.

 

[.Scott]

That's not what happens during the Delayed Choice Quantum Eraser Experiment.
There are two or three real events: Photon emission, signal photon detection (possible); idler photon detection.
So the "gap" (or time period) you are talking about is between the signal photon detection (or, if it is not detected, when it would have been expected) and the idler photon detection.

If we interpret the detection or lack of detection of the signal photon as a spot - bright or dark; then that spot will be tallied against one of four plots depending on where the idler photon is detected.

What is interesting in this experiment is that the half silvered mirrors that process the idler photon perform their "decisions" after the signal photon has either constructively of destructively interfered with itself.

You could do this experiment in another interesting way - though I don't know if this alternative has ever been tried:
Instead of leaving the detection of the idler photon to chance, write a preprogrammed detection sequence and try to conceal this information from the erasure experiment. Then run the experiment with mirrors that are setup at the last femptosecond based on your secret sequence. Will the QM experiment be able to create the interference patterns even though the signal photon has no good reason to know whether the which way information will be collected? The answer is yes, but the results from the signal photon are not enough to tell you what the secret sequence was. So the secret is not revealed until it is actually used in the experiment.

You might suspect that detecting the signal photon before the idler measurement was decided could allow you to use the signal photon detection to force the idler measurement to be "wrong". You could try to take the detection of the signal photon as a trigger to measure the idler the "wrong" way. The detection of the signal photon implies that either which-way information was collected from the idler photon, or that you were not at a null in the Ma or Mb detector - whichever detected it. So you would set the first two half-silvered mirrors (BSa and BSb) to not reflect (thus avoiding which-way detection) but you still don't have enough information to decide whether BSc should be transparent or reflective. So there is no way to make this attempt.

The point is that the system does not act as though the photons are following definite paths and that trying to see it that way forces you to conclude that some detections are effects that precede their causes.

 

[Nugatory]

There's is a pretty decent explanation of the experimental setup here: https://en.wikipedia.org/wiki/Delayed_choice_quantum_eraser#The_experiment_of_Kim_et_al._.282000.29

There is no "results screen" upon which a pattern is observed. Instead, there is a long list of detector readings, all of the form "The moving detector D0 was at this position when it detected a photon at the same time that detector D1|D2|D3|D4 detected a paired photon". The interference pattern only becomes apparent when we process this list after the fact and see that the probability of D0 detecting a photon at given position is different according to whether one of D3/D4 was triggered (we have which-slit information) or one of D1/D2 was triggered (we have no which-slit information).