[xaetium]

It may be the combination is new; I don't know the exact state of the field, but: This experiment uses a single photon, so they don't have to sample multiple times and make a statistical analysis on that part. If they did, that might open the efficiency loophole. The communication loophole isn't opened, as they are in sufficiently distant labs, with short enough measurement frames, but that's been done before.

As far as I can tell, the disjoint measurement loophole doesn't apply here, either, as it opens when correlations are drawn from multiple samples; here there's one. I'm not sufficiently expert to tell whether the rotational invariane, or other loopholes are closed here. Can anyone shed some light on this?

[stangeek]

That would be most useful indeed. Re-read the paper and still can't pinpoint the main difference vs. previous experiments, and why this is a significant achievement...

Any QM expert around here who could help us?

[lisper]

I wouldn't really call myself an expert so take this with an appropriate quantity of NaCl, but AFAICT yes, what is new here is an experimental violation of the Bell inequalities with a "single particle" rather than an EPR pair.

Note that the reason I put "single particle" in scare quotes is that there really is no difference between a "single particle" and an EPR pair. Both are single (non-separable) quantum systems. The only difference is that the "single particle" is in a state that constrains it to deliver its energy at a single location whereas the "EPR pair" can split its energy between two locations. So a "single particle" is really just a special case of an EPR pair, which is in turn a special case of an EPR N-tuple.

[tjradcliffe]

I'm competent in QM but not a quantum optics expert. In particular homodyne detection is new-ish to me. There is a bit of a description of it here that might be useful: http://relativity.livingreviews.org/Articles/lrr-2012-5/arti... That said, this is my take, which is mostly me trying to wrap my head around the problem, so take it all with a grain of salt.

The idea is that Alice mixes the (weak) signal photon stream in her lab with a (strong) "local oscillator" of the same frequency (that is the "homo" in "homodyne") and uses the interference between them to perform measurements on the signal without doing photon counting on it, which when combined with Bob's measurements on the other part of the signal photon wavefunction can demonstrate non-local effects. It is important, as always in "spooky-actions-at-a-distance" experiments to emphasize that nothing Bob sees can be used to infer what Alice measures or vice versa: there is no possibility of faster-than-light communication, and it is only when the measurements are combined after the fact that the non-locality becomes manifest.

Homodyne measurement seems to be the key thing that makes measurements on single photons possible, and this may be one of those cases where the notion of "collapse" breaks down in favour of "entanglement": the part of the signal wavefunction in Alice's detector doesn't collapse, it just gets entangled with the local oscillator, and because everything is still coherent her results can still be combined with results from the wave function components in Bob's lab. Entanglement with a heat bath emulates collapse; entanglement with a coherent local oscillator does not. [I'm still agnostic on the claim "entanglement solves the measurement problem" because I don't think it properly answers the question "why is there a classical world at all?", but that may be just me.]

There are a number of loopholes in previous experiments that this closes. I'm pretty sure it closes all detection efficiency loopholes, and there is a subtle critique of Aspect's experiments regarding the timing of the two-photon cascade that this makes irrelevant. There is a small (and in my view fairly implausible) literature on timing and photon-pair-identification issues that goes after two-photon experiments, and this work is not subject to any of these criticisms. I'm not sure how Joy Christian's work on Clifford algebras would be applied to this experiment either, although I expect they will have something to say about it.