
Quantum physics at a distance - icey
http://medienportal.univie.ac.at/presse/aktuelle-pressemeldungen/detailansicht/artikel/quantum-physics-at-a-distance/
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Xcelerate
Quantum teleportation is pretty cool. But it's one of those things that a lot
of people get confused about.

First, what is a quantum state? A "quantum state" is all of the information
you need to _completely_ determine a system. If you know a quantum state, then
you know everything that there is to know about what it describes. And if we
take all of the particles that constitute your body to be "the system", then
we can teleport it to a new location. Of course, something as large as a
person will probably never happen -- right now we're just doing one particle
systems (although a wavefunction can describe as many particles as you want).

The news article says that "matter isn't transported, just the state". This is
kind of misleading. There's a property in physics called indistinguishability.
Electrons are an example of something that has this property. What this means
is that you _cannot_ tell two electrons apart. They are identical in every way
(coarsely put, electrons are just an excitation of the same "electron field").
This indistinguishablity leads to some awesome physics (see
[http://en.wikipedia.org/wiki/Bose%E2%80%93Einstein_statistic...](http://en.wikipedia.org/wiki/Bose%E2%80%93Einstein_statistics)).

So it's meaningless to say that the original matter has or hasn't transported
because there's no such thing as the "original matter". If the state of the
"new matter" is the same as the old once was, then for all intents and
purposes you have actually teleported that object.

But if you have teleported the state, haven't you just cloned the object? Aha!
Nature's too tricky to let you pull that off. There's something called the no-
cloning theorem which proves that it is impossible to duplicate a quantum
state -- you can only move it around.

And to clarify one other misconception, quantum teleportation does NOT allow
faster than light information transfer. You still require a classical
information channel to complete the teleportation (so it's not instantaneous
teleportation in the sci-fi sense).

~~~
exch
This subject has always caused me a great deal of headaches while trying to
wrap my mind around it.

If I understand you correctly then, the term 'teleportation' is entirely
misleading in this context.

I am also wondering about another issue: Heisenberg's uncertainty principle.
How can you accurately gauge the state of a particle, in order to 'transport'
it elsewhere, when Heisenberg seems to be convinced this is impossible to do?

Thirdly: 'teleportation' as you explained it, seems to me to be more of a game
of "Let's see what's over there, then observe this spot here and wait until
some particle randomly displays characteristics similar enough to consider it
equal. We can then pretend we did it with science!". Is this the case, or do
they actually 'do' something on the other end to create a new particle with a
specific state?

Disclaimer: I am very much an arm-chair physicist with as much understanding
of this subject matter as the average doormat.

~~~
galadriel
Heisenberg tells you that you cannot measure locations and speed, but
Physicist have just accepted it and moved on. What you can measure, and what
quantifies a state, are probabilities of things happening. That is all the
information that nature gives you, and if you can recreate a set of
probabilities from point A at point B, you have teleported the object. The
laws of nature ensure that object at point A is destroyed, so you cannot clone
states.

The way how you interpret is now philosophy. As far as nature goes, there is
no way to distinguish one electron from other, or to differentiate a
'teleported' object from its 'original' object.

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walrus
Article ($32):
[http://www.nature.com/nature/journal/vaop/ncurrent/full/natu...](http://www.nature.com/nature/journal/vaop/ncurrent/full/nature11472.html)

Preprint: <http://arxiv.org/abs/1205.3909>

~~~
sillysaurus
Why is it $32? I have about $1k left in the bank. I'm deeply fascinated by
quantum mechanics (I spend most days researching optical quantum mechanical
phenomena), but I can't afford this.

~~~
alecdibble
Because publishers want your money. This is actually a HUGE problem in
academia that has yet to be disrupted.

    
    
        "we believe the publisher adds relatively little value to the publishing process... We are simply observing that if the process really were as complex, costly and value-added as the publishers protest that it is, 40% margins wouldn't be available."[0]
    
    

[0]: <http://en.wikipedia.org/wiki/Academic_publishing>

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juiceandjuice
The future of telecommunications, brought to you by Bell's lab instead of Bell
Labs.

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lmirosevic
Nice to know there's some exciting stuff happening in my city for once.

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Roboprog
Weird. So they can send an FTL signal with quantum coupling/entanglement (or
whatever). I wonder how "simultaneity" is set by the universe: average light
speed between 2 points to determine average time frame???

If they can make it scale, it would be interesting to see how this affects
space exploration vehicles: "get me a subspace channel to starfleet!", er,
"open a quantum channel to deep space network!". This should make it possible
to have almost realtime monitoring and control of a remote probe with a
quantum relay link covering most of the interplanetary distance.

~~~
pranjalv123
It's not FTL. Quantum teleportation is like this:

Alice shows Carol and Bob a red ball and a blue ball, and two empty boxes.
Without showing Carol and Bob which ball goes in which box, Alice puts a ball
in each box and gives one box to Bob and the other to Carol. Carol is an
astronaut going to Mars, and they've agreed to open their boxes when she gets
there.

Fast forward 6 months later, Carol is on Mars, and she opens her box and sees
a red ball. Immediately, without any speed-of-light delay from Earth, Carol
knows what Bob will see in his box when he opens it. Note how there's no
"useful" information transferred faster than the speed of light - Carol and
Bob can't use this to make any useful decisions.

The quantum version is weirder because the balls are in a superposition of red
and blue until one of the two opens the box - but there's still no useful
information transferred faster than light.

~~~
drostie
What you have described is an EPR experiment, not a quantum teleportation
experiment. Also, we don't have any way of persisting a quantum state for 6
months. And, the above discussion is a little misleading because there exists
a set of experiments which show "coherence effects" -- basically, that the
universe is not obeying the classical probabilities. In this case neither Bob
nor Carol see coherence effects on their own qubits (the blue/red state is
really a 0-vs-1 distinction, hence quantum bits), but there are coherence
effects to be seen in correlations between their measurements -- so if they
ever communicate about some experiments they ran, they can do experiments
which are inconsistent with the idea that Nature has "made up its mind"
whether the ball is red or blue, the same as in double-slit diffraction where
Nature has not "made up its mind" whether a photon has gone through the one
slit or the other. That's the gist of the EPR experiment.

Let's get into what makes this a _teleportation_ experiment though. Bob and
Carol, we assume, still share these EPR pairs of well-entangled objects. What
can you do with them? Let me just use "normal" to describe a channel which
communicates normal 0's and 1's -- like the Internet does, or radio
communications to Mars or what have you.

It turns out, you can communicate one of these clearly-abnormal states from
Bob to Carol, using only normal bits. Bob wants to send another quantum state
S to Carol but can only send normal bits. He entangles S with his side of the
entangled pair, measures the two to get some normal bits, then sends these to
Carol. Carol can use the normal bits to modify her own side of the EPR pair
into the state S. All of the weird quantum nature of S seems to have "hopped
over" sometime between the entanglement and the measurement: the classical
information sent just seems too puny, in the sense that we know you can't use
a normal channel to communicate a quantum state without such an entangled
pair.[1]

Still, the two normal bits that Bob sends seem to be important, in the sense
that they tell Carol the right way to "look" at the teleported quantum system,
which is not trivial. In that sense there is a much firmer speed-of-light
barrier established: the information about the "right way to look at the
teleported qubit" is classical, and cannot go faster than light, even though
the weird quantum nature of the qubit seemingly did.

[1] That's not entirely true. If Bob makes the state himself then he can tell
Carol how to make a copy. Instead you have to imagine that Alice gives quantum
states to Bob which Bob doesn't know: Bob can still send these to Carol.

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juiceandjuice
One very practical application of this would be for communications to Mars.
You could be constantly transmitting a "deferred" signal, only to alter the
quantum states when you are ready. We still have a long way to go for ~20
minute quantum state storage times though.

~~~
lwat
You can't use this for FTL communication, sorry.

<http://en.wikipedia.org/wiki/No-communication_theorem>

