
Quantum entanglement of a single particle has been observed by researchers - jonbaer
http://www.cnet.com/au/news/researchers-demonstrate-quantum-entanglement-prove-einstein-wrong/
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beloch
That's a pretty brutal press writeup.

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.

~~~
stangeek
Hi Beloch

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?

Many thanks

~~~
xaetium
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'.

~~~
stangeek
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?

~~~
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.

------
timnic
Preprint: [http://arxiv.org/abs/1412.7790](http://arxiv.org/abs/1412.7790)

------
lisper
Previously on HN:

[https://news.ycombinator.com/item?id=9283263](https://news.ycombinator.com/item?id=9283263)

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ck2
[https://news.ycombinator.com/item?id=9288941](https://news.ycombinator.com/item?id=9288941)

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phkahler
AFAICT no one has ever been able to distinguish a particle whose wave function
has collapsed from one that hasn't.

That would of course allow faster than light communication by modulating the
"collapsedness" of a stream of entangled particles.

~~~
nsm
Could you explain how this would be faster than light? Wouldn't the particles
be traveling at the speed of light?

~~~
phkahler
>> 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.

~~~
maxerickson
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).

Are you talking about something more than that?

------
Devid2014
This is just another example of Sen­sa­ti­o­na­lis­mus.

