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In Kim's delayed choice experiment as described in Wikipedia, you get interference patterns or not depending on whether you look at detectors D3 and D4 (which give you path information) or look at detectors D1 and D2 (which do not give you path information) and therefore gives you no interference patterns, or interference patterns, respectively.

My question is what does it mean to "look at" the results for either pair of detectors? Is not the coincidence detector operating in both cases? ..or for all beams? How do you control what you "look at"?

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In Kim's delayed choice experiment as described in Wikipedia, you get interference patterns or not depending on whether you look at detectors D3 and D4 (which give you path information) or look at detectors D1 and D2 (which do not give you path information) and therefore gives you no interference patterns, or interference patterns, respectively.

That's an imprecise and therefore incorrect description of the experiment by Y.-H. Kim et al.; in particular, there seems no mentioning of "(looking at) detectors D1 and D2" or "(looking at) detectors D3 and D4" in the present Wikipedia article ("Delayed choice quantum eraser").

The setup is rather such that coincidences are recorded between

  • detector D0 and

  • exclusively either detector D1, or detector D2, or detector D3, or detector D4.

Correspondingly, the separately recorded rates (obtained as detector D0 is gradually scanned along the $\vec x$ direction) are called "R01", "R02", "R03", and "R4".

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  • $\begingroup$ Aha. So you record the coincidences as you describe in real time as D0 oscillates. If you had four separate coincidence detectors and each measuring against one of D0-D4 simultaneously would they all result in no interference detected, or two of each? $\endgroup$ Oct 23, 2015 at 0:04
  • $\begingroup$ @John Fistere: "So you record the coincidences as you describe in real time as D0 oscillates." -- To say that "D0 oscillates" is a gross mis-characterization. Instead, D0 records at each "x position"; long enough for differences in rates to show up with some significance. We could think of D0 as one large camera sensor. "If you had four separate coincidence detectors" -- That's what we/Kim et al. have: Coinc. counters "D0D1", "D0D2", "D0D3" and "D0D4"; and they record what's shown in Fig.s 3 and 4. Obviously: always only one of these finds a coicidence, given one single laser photon. $\endgroup$
    – user12262
    Oct 23, 2015 at 4:48
  • $\begingroup$ @user12261: Thanks. Is there a reason your setup is such that you can record coincidences only exclusively with one of D1-D4? You also say that D0 records "long enough". Could it not be a permanent record? It must be clear that there are elements of your experiment I don't understand. I really appreciate your clarification. $\endgroup$ Oct 23, 2015 at 18:25
  • $\begingroup$ @John Fistere: "Thanks." -- Sure. "Is there a reason [Kim et al.'s] setup is such that you can record coincidences only exclusively with one of D1-D4?" -- The main idea is to analyze one laser photon "at a time". Asking for coincidences between D1 and/or D2, D3, D4 necessarily involves several photons. (I'm sure this could be set up and analyzed, too; but I'm not sure that's in any way considered "interesting".) "You also say that D0 records "long enough"." -- A large count ($n \ge 150$) of "D1+D2+D3+D4" being recorded with D0 "at any one spot $x$"; with recorded coincidences $\le n$. $\endgroup$
    – user12262
    Oct 27, 2015 at 19:09
  • $\begingroup$ I realize now that "simultaneous" recording of coincidences of D1-D4 makes no particular sense because the coincidences occur at different times anyway, and the result would be the same for each recorder, running concurrently or not. $\endgroup$ Oct 28, 2015 at 6:28

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