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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!

Emilio Pisanty
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G. Xu
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  • They did it to get a nature publication. Might be right, might not be. – Martin Apr 09 '15 at 21:43
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    Related: http://physics.stackexchange.com/q/172794/ – Martin Apr 09 '15 at 21:44
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    @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. – DanielSank Apr 09 '15 at 22:57
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    @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. – Rococo Apr 10 '15 at 02:26
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    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). – Rococo Apr 10 '15 at 02:27
  • @Rococo In my humble opinion, the solid proof of impossibility of the wave-function collapse, or experimentally verifying it, shouldn't be a trivial result. I guess, you might mean that, most time when people published the result on the "wave-function collapse", the actual content was trivial in the sense that their results could be explained by it, while the collapse explanation is somewhat unnecessary. – G. Xu Apr 10 '15 at 16:40
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    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 ;) – Rococo Apr 11 '15 at 03:23
  • the article is on arxiv too –  Jun 13 '15 at 01:09

2 Answers2

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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:

http://arxiv.org/abs/1212.3245.

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.

alanf
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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.

rob
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ceillac
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  • 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 http://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. – G. Xu Apr 10 '15 at 16:19
  • As I said in my OP, I had no question at all for calling it the new demonstration of quantum steering effect. – G. Xu Apr 10 '15 at 16:24
  • 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).     – ceillac Apr 10 '15 at 17:36
  • 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. – G. Xu Apr 10 '15 at 17:52
  • 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. – G. Xu Apr 10 '15 at 17:55
  • Ok i think i understand what your disagreement is about now. Of course standard QM predicts correctly their measurements! The purpose of this paper is not about demonstrating one interpretation or another one. They use the word "collapse" just to say that the result of Alice measurement projects Bob's state. They did the steer thing with one photon which is very nice. But there's nothing more fundamental than doing it with two entangled spin for example.
    Also, doing it with one photon has a historical meaning as explained in the abstract. Hence the reference to EPR     – ceillac Apr 11 '15 at 01:02
  • Yes, the paper presented an excellent experiment using a single photon. And yes, it is very relevant to the historical discussions. However, Einstein's very early proposal was against the collapse interpretation. Neither am I sure that Einstein would ever argue against the correctness of any prediction of the quantum theory. This was even stated very clearly in the 1935's EPR paper, which was not reasoning to be against the correctness of the quantum mechanics. – G. Xu Apr 11 '15 at 04:04
  • It's my pleasure to help you understand what I meant. You are more than welcome! – G. Xu Apr 11 '15 at 17:39