1. "For the first time, quantum entanglement of a single particle has been observed by researchers." -- No. This has been done many times previously. This is just the first time it's been done with the efficiency loophole closed. That's good, but not what the the reporter who wrote this thought. He'll probably write the same thing all over again the next time somebody repeats this type of experiment with one of the other loopholes closed.
2. "A single photon (particle of light), for example, can be split into two particles that are still connected " -- No. The photon is path entangled. It's still just one particle/wave.
3. "Using homodyne detectors -- that is, instruments that can measure waves and wave-like properties" -- This is hilariously imprecise.
If you're even remotely interested, do yourself a favor and check out the arXiv preprint posted by timnic. This cnet article is much worse than the usual low standard of journalism when it comes to QM.
I just read the arxiv paper, but with my limited QM understanding it's tough to really grasp the significance of this experiment. Would you be able to explain it in layman's terms (assuming basic knowledge of QM) or is it too tricky to explain?
In these types of quantum reality-probing experiments, any problems of experimental design that affect the validity of the findings are referred to as loopholes, as though there's some awkward legal wrangling going on, because the experiments were conceived originally to determine whether the controversial Bell's inequalities hold. The inequalities were designed to test Bell's theorem which states that any hidden variables (things not yet observed that have a causal influence on experimental outcome) are required to be non-local if they are to hold with the predictions of quantum mechanics. Non-local here means 'spooky action at a distance'.
Showing the inequalities to be violated (incorrect by experiment) was originally controversial because Einstein and Bohr had differing notions of what the quantum mechnical theory implied about reality. They engaged in a lengthy, open discussion about it which was never resolved. Einstein believed in local realism, in which there is no spooky action at a distance and properties like position and momentum exist even when not being measured. Bohr, on the other hand, insisted that there simply wasn't an underlying reality and that only when measurements are made are properties like position and momentum condensed out of the quantum mechanical reality. So, you see, the significance of the experiment is in line with the underlying nature of reality; by closing another loophole, we get closer to what's what.
[The rest here is historical context.]
The familiar refrain, "God does not play dice," is almost always taken out of context - within its original statement, Einstein was also talking about a kind of telepathy required with it - the non-local aspect of quantum mechanics. Einstein said in 1954 'it is not possible to get rid of the statistical character of the present quantum theory by merely adding something to the latter, without changing the fundamental concepts about the whole structure'. He was saying he lost conviction in using a hidden variable theory to replace quantum mechanics.
Bohr's view, like Einstein's later view, is more in line with modern thinking. A team led by Aspect in 1981-82 ruled out either locality or objective reality, by testing the inequalities experimentally. This left possible a non-local reality. In 2006, a group tested Leggett's inequality, and showed it to be violated, which refined experimentally what the nature of reality is, though showed only that realism and a certain type of non-locality are incompatible, without ruling out all possible non-local models. (Nature, April 2007) Aspect remarked that philosophically, the 'conclusion one draws is more a question of taste than logic'.
OK - but what's the difference with previous experiments? Is it that they did it with a single photon?
Or is it because they managed to do it from two remote laboratories?
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?
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...
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.
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.
Yes, the article seems to imply that homodyne detectors are some kind of magical machines that can determine whether a particle is quantum entangled without having it collapse into a single state. Which would go against all QM knowledge that I have (little though it is). A quick check of the Wikipedia articles doesn't confirm that, but it leaves me confused.
Current thinking is that "collapse" is a hack. We've always talked about the wavefunction collapsing but never been able to describe the onotology of collapse. Non-locality seemed for a time to explain why that is so: because collapse happened non-locally, it could happily violate the law of non-contradiction from the viewpoint of a Lorentz-invariant observer.
The alternative to collapse is decoherence due to entanglement with a heat bath. There are still unexplained issues with this, in my view, but that may simply be because I don't fully understand how the ontology of the classical world is constructed by this process, or why once that ontology is constructed the beings that are governed by its laws are only capable of being aware of the foundations via the most indirect kind of inference.
In any case, this experiment shows some of the power of "entanglement" vs "collapse" as an account of measurement. If "collapse" is a short-hand for measurements that involve "entanglement with a heat bath" then it is possible perform a measurement that involves entangling part of a wavefunction with a coherent signal (the local oscillator in a homodyne detector) without doing to it what we would call "collapse". That appears (to me at least) to be what is happening in the present case. No magic involved.
Dunno why this is getting downvotes, as it is simply a statement of fact. If we could distinguish a stream of photons that had "collapsed" onto a particular basis from one where individual photons were in a superposition of states on that basis we would be able to communicate faster than light using EPR pairs. It is generally believed this is impossible.
>> Could you explain how this would be faster than light? Wouldn't the particles be traveling at the speed of light?
As maxerickson says, you emit streams of entangled particles from a central location heading in opposite directions. People equidistant from that location can communicate instantaneously. Alice modulates the wave function collapse by either taking a measurement or not. Say measuring indicates a 1 and non-measurement indicates a 0. Bob over at the other end uses his ability to distinguish a collapsed wave function from a non-collapsed one to get 1's and 0's out the other end. Because the measurement induced wave function collapse is instantaneous this will be faster than light communication. Bob can tell weather Alice is measuring or not, right now.
I stand by my assertion that physicists can not tell the difference. I'll also add that the reason is that there is no difference. But by all means continue to downvote without a counterexample.
So if I understand correctly, this experiment is demonstrating that prior to Bob's measurement the wave function is spread out between the labs. I don't think that is the same thing as being able to detect a non-collapsed wave function (the design just assumes it exists, and the outcome implies that it is a useful description).
The idea is that you set up a stream of entangled particles and arrange it so that, for example, one stream is available at Earth at the same moment that the entangled particles are available on Mars.
Then if you somehow gain the ability to twiddle the state of one end and measure the state of the other, you can communicate instantaneously.
But the first step to doing this is to develop a brand new physics where it is actually possible. Results that confirm/refine the current understanding won't ever enable it.
1. "For the first time, quantum entanglement of a single particle has been observed by researchers." -- No. This has been done many times previously. This is just the first time it's been done with the efficiency loophole closed. That's good, but not what the the reporter who wrote this thought. He'll probably write the same thing all over again the next time somebody repeats this type of experiment with one of the other loopholes closed.
2. "A single photon (particle of light), for example, can be split into two particles that are still connected " -- No. The photon is path entangled. It's still just one particle/wave.
3. "Using homodyne detectors -- that is, instruments that can measure waves and wave-like properties" -- This is hilariously imprecise.
If you're even remotely interested, do yourself a favor and check out the arXiv preprint posted by timnic. This cnet article is much worse than the usual low standard of journalism when it comes to QM.