
'Quantum' bounds not so quantum after all - dnetesn
http://phys.org/news/2016-07-quantum-bounds.html
======
lisper
This article nearly pegs my bogometer. It reads more like numerology than
science. Yes, it is true that quantum bounds "arise naturally" from quantum
theory, but what matters is not _that_ they arise but _how_ they arise, and on
this the article is completely mute. The mere fact that you can coax the same
numbers out of a classical experiment is completely meaningless. What matters
is _how_ you coax those numbers out, and in particular, whether the manner in
which you coaxed those numbers out involved a quantum effect or simply
manipulating unconstrained degrees of freedom to get the result you want.
There may be something interesting going on here, but there is no way to tell
from reading this article. If you really could get a quantum bound out of a
classical system in a non-bogus way, that would be _big_ news, the biggest
breakthrough in physics in decades. I'll give you long odds against.

~~~
gone35
Indeed. From the discussion section of their paper:

"This shows that quantum correlations can be universally recreated with
classical systems _at the expense of some extra resources_ " (Emphasis added)

I might be wrong, but judging from Fig. 1 in the paper, it seems these "extra
resources" involved are exponential, which is exactly what we would expect
since we already know arbitrary quantum circuits can be simulated classically
with exponential slowdown! But I guess the setup is interesting in of itself
--from an experimentalist point of view at least.

~~~
lisper
That is _exactly_ the sentence that I was about to point out!

------
xyience
Skimmed the PDF, it seems like the point of this paper is just to show that
yes, classical systems can (with memory, e.g. the reference
[https://arxiv.org/abs/1007.3650](https://arxiv.org/abs/1007.3650)) simulate
quantum ones, and thus finding "characteristic quantum numbers" shouldn't make
one immediately suspect something quantum is going on. In the discussion it's
like the ultimate nitpick: "The characteristic trait of QT rely on the fact
that the quantum bounds are achieved _without_ employing extra resources such
as memory. Therefore, the principles needed to fully derive QT (in the spirit
of Refs. [33–37]) should account for that."

There are some people who find the concept of quantum physics philosophically
displeasing and try however they can to ignore all the experimental evidence
and say we're really in a classical universe. This isn't a case of one of
those.

------
amluto
I quit after the abstract. No one ever said something was quantum because the
number 2 * sqrt(2) appeared, kind of like no one really thinks that something
is a circle just because you can coax pi out of it.

Now, if they actually violated Bell's inequality in a non-quantum experiment
in which Bell's inequality were actually applicable, that would be huge news
(and grounds to try to confirm the results). But finding a _number_ that
violates it out of context isn't news at all.

tl:dr 2.78 3.14. Hence this post is quantum and circular.

~~~
sp332
I take your point about this paper, but if pi shows up, I would absolutely
suspect a circle to be involved.

~~~
ccvannorman
There are many ways to arrive at Pi which themselves are quite divorced from
"a circle", since you could arrive at the conclusion in a one dimensional
Universe.

It's true that if you were to _graph_ these equations you would see circles,
but that is besides the point - a one dimensional being wouldn't be able to
understand your graph, but could still read you the digits of pi.

[http://www.geom.uiuc.edu/~huberty/math5337/groupe/expresspi....](http://www.geom.uiuc.edu/~huberty/math5337/groupe/expresspi.html)

~~~
sp332
The first one there is derived from a definition of a circle: x^2 + y^2 = c.
Most of the rest use an arctangent. Only the last one is not obviously related
to a circle.

We are in 3 dimensions but we use higher dimensional math all the time, so I
don't know why a 1D creature would be so constrained.

------
semi-extrinsic
Actual paper:
[http://dx.doi.org/10.1103/PhysRevLett.116.250404](http://dx.doi.org/10.1103/PhysRevLett.116.250404)

Preprint: [http://arxiv.org/abs/1511.08144](http://arxiv.org/abs/1511.08144)

Since this is published in PRL, it's been through some fairly stringent peer
review. So I'll be mildly sceptical until someone wiser weighs in.

Paging Dr. Aaronson, Dr. Aaronson to the blogosphere.

~~~
jessriedel
PRL does a fine job of checking scientific soundness, but their assessment of
importance/notability is unimpressive, like most processes involving a handful
of humans. (I've published there, and it was far from my most notable result.)
Lots of boring stuff passes the filter.

------
sriku
I've once heard a statement that polarization can only have a quantum
explanation. The effect is that when you have two polarizers at right angles,
no light gets through, but when you insert _another_ polarizer at, say, 45
degrees to the others, some light gets through all the polarizers. The thing
is, classical EM wave theory predicts the exact same result.

~~~
ivan_ah
You raise an interesting point, and you're right that quantum treatment is not
necessary in the regime where many light particles pass through the circuit,
in which case we can talk about light as a continuous thing, that can be
infinitely subdivided. A quantum treatment of polarization is only required in
the single-photon regime.

Specifically consider an experiment where a light beam passes through three
polarizing filters: first a horizontally-polarizing lens H, then a diagonally
polarizing lens D, and finally a vertically polarizing lens V.

    
    
         photons ------>  H  ---->   D  -->   V  ->  
     

The result of this "triple filtering" for a beam of light (consisting of
gazillions of photons, modelled as a continuous wave) can be explained as the
wave amplitude being "projected" along the polarization axis of each lens. At
each polarization step the projection angle is 45 degrees, so the wave
intensity is reduced by cos(45)=1/sqrt(2) which is equivalent to a reduction
in optical power of 1/2\. The power of the beam that passes through all three
lenses is 1/4 of the power leaving the first lens. No need for quantum, since
we model the beam of light as a continuous quantity that can be infinitely
subdivided.

If we repeat the same experiment sending one photon at a time however, we
can't say "the photon divides" since by definition a photon is the smallest
possible quantum of light. There is not such thing as 1/sqrt(2) of a photon.
Basically, when a H-polarized photon reaches a D-polarizing lens, a classical
physicists is forced to pick whether the photon goes through, or is
reflected—the option "partially goes through" is not allowed. It's only in
this regime that the "photon is a wave" explanation fails.

The quantum explanation uses a probabilistic approach and explains the
probability of a H-polarized photon going through the D-polarizing lens is 1/2
and subsequently the (now D-polarized photon) going through the third
V-polarizing lens is also 1/2 so the overall probability of passing through
all three lenses is 1/4.

~~~
sriku
When considering a single photon the classical explanation certainly fails.
However, this is different from the claim that an effect perceived at macro
levels cannot be explained by classical theory and only by quantum mechanics.

The paper's result sounds to be along similar lines - claiming that a certain
number "could only have come from quantum theory" while classical physics
perfectly well yields the same number.

By virtue of being a super-theory of classical mechanics, all phenomena _are_
quantum phenomena with quantum explanations, so that's not what is being
talked about here.

