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An answer/comment on a sister site cited a paper that seems to have been published in a legitimate journal and seems to be claiming the possibility of communicating information via entanglement, which we should all know is not possible:

Intuition from our everyday lives gives rise to the belief that information exchanged between remote parties is carried by physical particles. Surprisingly, in a recent theoretical study [Salih H, Li ZH, Al-Amri M, Zubairy MS (2013) Phys Rev Lett 110:170502], quantum mechanics was found to allow for communication, even without the actual transmission of physical particles. From the viewpoint of communication, this mystery stems from a (nonintuitive) fundamental concept in quantum mechanics-wave-particle duality. All particles can be described fully by wave functions. To determine whether light appears in a channel, one refers to the amplitude of its wave function. However, in counterfactual communication, information is carried by the phase part of the wave function. Using a single-photon source, we experimentally demonstrate the counterfactual communication and successfully transfer a monochrome bitmap from one location to another by using a nested version of the quantum Zeno effect.

I haven't read the paper and don't really have time to devote to it, but since it seems to be an extraordinary claim, I'm outsourcing this to physics.SE for debunking (at least of the interpretation of the result; the paper is probably legitimate). Can any one summarize what the actual result is and how it's consistent with QM?

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  • $\begingroup$ What aspect of this do you believe is inconsistent with QM and why? $\endgroup$ – WillO Sep 3 '19 at 23:33
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    $\begingroup$ @WillO: A claim that you can transmit information via entanglement (which does not seem to be what the paper is claiming, but how it was being interpreted when cited) is inconsistent with QM. $\endgroup$ – R.. GitHub STOP HELPING ICE Sep 3 '19 at 23:34
  • $\begingroup$ FWIW, here's a Scientific American article about that paper. It includes a fair amount of basic quantum background info, and IMHO doesn't do a great job of explaining this counterfactual communication experiment, but I guess it's worth reading. Also see en.wikipedia.org/wiki/Elitzur%E2%80%93Vaidman_bomb_tester $\endgroup$ – PM 2Ring Sep 4 '19 at 8:25
  • $\begingroup$ But yes, you're correct, quantum interference or entanglement cannot be used to transmit information FTL or backwards in time. And as patstew says, the experiment by Cao et al doesn't attempt to do that. $\endgroup$ – PM 2Ring Sep 4 '19 at 8:31
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The actual paper is Cao et al., "Direct counterfactual communication via quantum Zeno effect," PNAS 114, 4920--4924 (2017).

I don't think they're claiming to be communicating via entanglement in either the faster-than-light or I-can-control-which-way-my-particle-collapses senses, which would be impossible. It relies on a photon being able to traverse the gap, even though subsequent detection proves that it didn't.

It's like having a single photon double slit experiment where Bob can only look at the screen and Alice can open and close one of the slits. Alice can send a message to Bob, because Bob can observe whether or not there's a interference pattern. But no photon or other particle goes from Alice to Bob. Spooky.

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