# Has this experiment really demonstrated wave-function collapse?

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!

• Apr 9 '15 at 21:44
• @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. Apr 9 '15 at 22:57
• @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. Apr 10 '15 at 2:26
• 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). Apr 10 '15 at 2:27
• 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 ;) Apr 11 '15 at 3:23

• 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. Apr 10 '15 at 16:19