My question is: why did the following experiment claim that it had demonstrated the wave-function collapse?

Experimental proof of nonlocal wavefunction collapse for a single particle using homodyne measurements. M Fuwa et al. Nature Communications 6 6665 (2015), arXiv:1412.7790.

I would have no problem if they had claimed that, the experiment demonstrated the "non-local" (or: precisely quantum) steering effect. In my humble opinion, there is no logic to justify that "quantum steering effect is equivalent to the wave-function collapse". Here the wave-function collapse is defined in the strict Von-Neumann's postulate form.

I am afraid that, this type of quantum steering experiment would cause the similar misunderstanding to that caused by the Bell's type of experiments. Just a reminder that, the Bell's definition of "non-locality" has no direct relation with the Von-Neumann's wave-function collapse postulate, either. All the "relations" that people had considered/debated were based on many extra assumptions and interpretations.

Did I miss something that are really profound and important here? Many thanks!

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    $\begingroup$ Related: physics.stackexchange.com/q/172794 $\endgroup$ – Martin Apr 9 '15 at 21:44
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    $\begingroup$ @Martin: I'd hardly use acceptance into a journal as a guarantee that a paper's claims should be taken at face value. I know you aren't saying otherwise, just offering my two cents. $\endgroup$ – DanielSank Apr 9 '15 at 22:57
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    $\begingroup$ @Martin that is a questionable criteria under any circumstance, but note that the journal here is Nature Communications, not one of the flagship Nature journals, which is online-only and intended for papers that are not of wide interest. And indeed, this looks like a result that could be interesting for the quantum optics community, with an overselling title. $\endgroup$ – Rococo Apr 10 '15 at 2:26
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    $\begingroup$ That said, I suspect it was meant to be read with the emphasis on "for a single particle using homodyne measurements" (the novel part), not "Experimental proof of nonlocal wavefunction collapse" (which is either trivial or impossible depending on your perspective). $\endgroup$ – Rococo Apr 10 '15 at 2:27
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    $\begingroup$ Like I said, it depends on what you mean by "collapse." An experiment that, for example, somehow invalidated Everettian interpretations of quantum mechanics would be momentous. This is not that experiment ;) $\endgroup$ – Rococo Apr 11 '15 at 3:23

The paper doesn't explain how their predictions would differ from those of non-collapse theories. Since the paper doesn't even discuss what would be predicted without collapse, it is difficult to see how it could rule out quantum theory without collapse. Quantum theory without collapse explains all of the predictions commonly attributed to quantum theory with collapse:


Variations on quantum theory that include collapse, such as the GRW theory, may or may not reproduce the predictions made in the paper, but as this is not discussed it is difficult to tell whether the results are even consistent with such a theory. As such, the title of the paper does not accurately describe its contents.


Yes the wave function "collapses" after Alice measures her part. I am not sure what you call the Von-Neumann collapse. Here you have a bipartite measurement with only one photon. And I guess the steering thing had never been done before with only one photon. No there's nothing profound.

  • $\begingroup$ The "Von-Neumann collapse", for a single particle, is defined as: given the state before the measurement, described by the pure state $c_1|1\rangle + c_2|2\rangle$, the state after the measurement collapses to $|1\rangle$ (or: $|2\rangle$). The precise definition of quantum state steering is defined as, e.g., seen here journals.aps.org/prl/abstract/10.1103/PhysRevLett.98.140402. They are not the same definitions and no one proved they are equivalent, which I guess cannot be done. Conceptually they might be related, but not the same. $\endgroup$ – G. Xu Apr 10 '15 at 16:19
  • $\begingroup$ As I said in my OP, I had no question at all for calling it the new demonstration of quantum steering effect. $\endgroup$ – G. Xu Apr 10 '15 at 16:24
  • $\begingroup$ In the article you have: a|3>|4>+b|4>|3> -> (c|3>+d|4>)(e|3>+f|4>) It fits your definition of collapse if you take |1>=(c|3>+d|4>)(e|3>+f|4>) and |2> accordingly. So yes you can call it collapse (even if you only measure one part). $\endgroup$ – borilla Apr 10 '15 at 17:36
  • $\begingroup$ No. That's not what they demonstrated. What they demonstrated was The violation of the EPR-steering inequality. The violation of the EPR-steering inequality could be explained (or: interpreted) by the collapse. But you don't need the collapse interpretation, instead you could use the standard "shup-up-and-compute" interpretation to explain everything. This means, the standard QM predicts it all correctly - nothing more and nothing less. $\endgroup$ – G. Xu Apr 10 '15 at 17:52
  • $\begingroup$ As a quote from the paper, "The violation of the EPR-steering inequality by seven s.d.’s is a clear proof that Bob’s quantum state cannot exist independently of Alice, but rather is collapsed by Alice’s measurement." Yes, that proved "Bob’s quantum state cannot exist independently of Alice". No, it did not prove "but rather is collapsed by Alice’s measurement." In another word, it proved the quantum entanglement. $\endgroup$ – G. Xu Apr 10 '15 at 17:55

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