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I just rewatched Sidney Coleman’s famous lecture “Quantum Mechanics, In Your Face”. I say “rewatched” because I had the privilege to attend his repeat of this lecture at UCSD in ’94 or so. But unfortunately, I didn’t have the presence of mind as a lad to ask him:

Has anyone ever actually run the “Diehard” experiment he describes in this lecture? See

                  Sidney Coleman, Quantum Mechanics in Your Face [1994]

I’ve timestamped it to the section about the Diehard experiment, but I recommend treating yourself to the whole lecture.

I have to humbly agree with Pr Coleman that this thought experiment is “pedagogically superior to the original Bell argument", in that it’s much simpler and demonstrates a more clearly intractable conflict between QM and classical physics. But Pr Coleman's main point is that if you run the experiment and get the results the QM predicts, it’s classical physics that can only be rescued with “spooky action at a distance”!

However the small snag is that in order to run this experiment you must reliably produce entangled particle trios in this state:

$$\psi=\frac{1}{\sqrt{2}}\left(\left|\uparrow\uparrow\uparrow\right\rangle -\left|\downarrow\downarrow\downarrow\right\rangle \right) $$

…where the arrows describe the z-components of spins of three particles to be aimed at three widely-separated detectors. That's a tall order.

So my question is: How would one produce entangled particle trios in that state? Is that even possible? If so, has this experiment ever actually been done?

OP EDIT: Thinking about this more, I don't understand how this experiment rescues QM from action-at-a-distance at Pr Coleman claims. The "measurement" being done is actually three measurements happening vast distances apart, and because their are only certain sets of results they can get, they are very much affecting each others results, at a distance.

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  • $\begingroup$ ui.adsabs.harvard.edu/abs/2000Natur.403..515P/abstract $\endgroup$
    – Thomas
    Commented Oct 6, 2019 at 21:15
  • $\begingroup$ Do you mind summarizing the thought experiment? $\endgroup$ Commented Oct 6, 2019 at 22:56
  • $\begingroup$ @NorbertSchuch I really tried, but it got rather long. Pr Coleman explains it well in the video I linked. $\endgroup$ Commented Oct 6, 2019 at 23:55
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    $\begingroup$ Not related to your question, but I can't help mentioning that using Pr for Prof. is unconventional, at least in English, and I would urge you not to do it as it is quite distracting. $\endgroup$
    – Virgo
    Commented Oct 7, 2019 at 0:33
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    $\begingroup$ The experiment Coleman describes is the GHZ/GHSZ/GHZM experiment. (He calls it GHZM at 31:52 in the video.) As far as I know "diehard experiment" is not a standard name for it and Coleman just used it to make the lecture more entertaining. $\endgroup$
    – benrg
    Commented Oct 7, 2019 at 22:45

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It has been done: D. Bouwmeester, J.-W. Pan, M. Daniell, H. Weinfurter and A. Zeilinger, Observation of three-photon Greenberger–Horne–Zeilinger entanglement, Phys. Rev. Lett. 82 (7), 1345–1349 (1999).

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  • $\begingroup$ I believe that's a different experiment that makes similar claims, but it's not the experiment Coleman described. Thank you for the link though, it's fascinating. $\endgroup$ Commented Oct 6, 2019 at 21:46
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    $\begingroup$ It only differs from the experiment that Coleman describes in non-essential ways (for example, Coleman uses spin-$\frac{1}{2}$ particles, while Bouwmeester et al use photons, and it's not clear whether the 3 detectors were spacelike separated). $\endgroup$ Commented Oct 7, 2019 at 11:59
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    $\begingroup$ @PeterShor The spacelike separation is a MAJOR difference. Please see the follow-up question I added to the end of the post. I think he might not have saved QM from action-at-distance as well as he thought. $\endgroup$ Commented Oct 7, 2019 at 22:23
  • $\begingroup$ Where does Coleman actually say that it rescues QM from action-at-a-distance? I don't believe it does, and I don't believe that Coleman would have said that. $\endgroup$ Commented Oct 8, 2019 at 13:23
  • $\begingroup$ @PeterShor He doesn't use the word rescue, but after describing the experiment he does claim that the classical explanation would require action-at-a-distance, and so (he claims) we are left with a choice between QM and aaad, not QM requiring aaad. That's the part I don't quite buy, because there are only certain sets of results the three detectors can get, so some sort of superluminal coordination is still necessary. $\endgroup$ Commented Oct 9, 2019 at 15:11

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