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Re: [Phys-L] Quantum teleportation and information



On 07/20/2017 10:02 PM, antti.j.savinainen via Phys-l wrote:

http://www.nature.com/news/quantum-teleportation-is-even-weirder-than-you-think-1.22321

I was unaware that the nature of information sent in 'teleportation'
is still unresolved to some extent. OTOH, most of the references the
author cites are 10-20 years old. Perhaps some progress on the issue
has been made?

I'm not sure I agree with the premise of the question, but let
me say a few things that might help. If not, please clarify
the question.

1) The universe we live in is quantum mechanical. All of physics,
including classical physics, obeys the laws of quantum mechanics,
with no exceptions known or seriously suspected.

2) Quantum mechanics will tell you when this-or-that classical
approximation is usable. The converse does not hold; classical
mechanics will usually not tell you its own limits.

3) QM is not paradoxical. If you just grit your teeth and turn
the crank, the equations are right. They agree with experiment.
However, the results you get from the equations and experiments
are sometimes contrary to classical intuition. Sometimes this
is summarized by saying QM is weird.

One way to get into trouble is to try to imagine some sort of
mechanistic explanation for how the QM result comes about. In
general, given a black box, you have the option of using it
"as is" or trying to peek inside to discover the mechanism.
With QM there is no mechanism that anybody has been able to
find, so we have to treat it as a black box. QM is what it
is and does what it does.

4) The connection to information is very counterintuitive. This
issue has been known for a long time. You have to work very
hard to create a situation where it matters, but quantum
teleportation is one of those situations. Aspect-style EPR
experiments are essentially the same situation.

The idea is that you create a pair of particles in an entangled
state. You separate the particles by some large distance. At
some pre-arranged time, you measure one and then the other in
rapid succession, using technicians colocated with each particle.
After the dust has settled, you collect reports from the technicians,
and verify that the laws of QM are upheld. The measurements are
correlated, in agreement with the QM laws.

You get into trouble if you try to imagine a mechanism that
could "cause" the correlations. The mechanism would have to
operate faster than the speed of light.

On the other hand, a correlation is not information. In
particular, there is no way that technician A could mess with
his particle in such a way that it would send any information
to technician B.

Let's be clear: If you roll an ordinary die, the top number
and the bottom number are correlated: The top number is random,
and the bottom number is random, but they always add up to 7.
We have a mechanistic explanation for this, based on the
mechanical properties of the material used to make the die.
Information travels through the die at the speed of sound
to maintain the correlation all during the randomization
process. If you change the top face by force, the other
side will find out about it after a slight delay.

With QM the same sort of correlation occurs, but there is
no mechanism, no propagation, no delay due to the speed of
sound (or light), and no way to force the issue. Einstein
was horrified by this, but it is what the equations say,
and the experiments all agree.

5) No part of this story has changed in the last 20 years or
more. The equations were right 20 years ago, and they are
still right. There has been a steady accumulation of
experimental data, but it all says the same thing: The
QM equations are right.

Suppose you were a butcher, a baker, or a candlestick-maker,
and you just bought a 4GB memory card for your computer.
You are probably content to treat it as a black box, without
worrying about how it works internally, so long as it gives
the right answers when you plug it in.

Physicists, in contrast, want to know how the black box works
internally. They discover it contains 32 billion smaller
black boxes, whereupon they want to know how /they/ work
internally. Alas, if you try that with Aspect-style experiments
or teleportation experiments, you're going to be disappointed
and frustrated.

I'm not saying that no mechanistic explanation will ever be
found, but I can guarantee that the mechanism, if any, will
not be simple. A lot of ideas have been tried without
success. A very wide class of hidden-variable theories is
ruled out by the Bell inequalities. More generally, it
appears we need a mechanism that can propagate quantum-
mechanical correlations but cannot be used to propagate
information. That seems like a daunting task.

Keep in mind that the mechanism is not needed for doing
QM theory or experiments. QM is what it is and does what
it does. A mechanistic explanation would soothe the
feelings of physicists, but it is not really required.