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I'm in humanities, not physics, so please bear with me. I am trying to understand this experiment and have a few unanswered questions. I have read the other posts on this site that discuss the experiment, but I'm still fuzzy on a couple of things.

As I understand, there are two setups for this experiment, one involving splitting a photon into two after it passes through the double slits. One photon will reach a screen and create an interference pattern or not. The other has a 50% chance of indicating which-path information or not. This 50% chance, however, is not decided until after the first photon reaches the screen.

In the second setup, the photon leaves a trace that could indicate which-path information but the trace is not observable to the experimenter. The experimenter then chooses whether to keep or erase this information.

My major question pertains to the second setup. Must the experimenter decide whether to erase the which-path trace before the photon hits the screen? (It is still delayed choice because the decision has made after the photon has left the chamber where the trace was made.) What happens if no choice is made? Does the existence of the trace (unobserved but not yet erased) mean no interference pattern will show up?

In the first set up, I have a similar question. I understand the photon that may indicate which-path information will go randomly to one of four detectors, two that can distinguish which path information and two that cannot. The interference pattern can only be observed after the data from these detectors has been collected and correlated, as only those that reached the which-path detectors will form an interference pattern. What I don't understand is what happens if photons are sent through the apparatus one at a time. What shows up on the screen in this case? The screen should be observable before the second photon's fate has been determined.

Any insight would be greatly appreciated.

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First point : Once a type of information "which path" is likely to be acquired (even if it is not materially acquired), there is no interference pattern. The important thing is not that a (which-path) measurement had be done, but that a (which-path) measurement could be done (this is how quantum mechanics works, even if it seems strange). –  Trimok Jan 17 '12 at 19:07
Second point : there is no miracle (you cannot have physical effects back in time), if the experimenter decide to change something in the apparatus measurement, i.e. the possibility of whether a (which-path) measurement could be done or not , but after the photon hits the screen, this could not change the pattern of interference (or no interference) after collecting all the results about one type of experience (interference or no interference) –  Trimok Jan 17 '12 at 19:16

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