

German scientists link two labs with ‘universal quantum network’ - po
http://www.theregister.co.uk/2012/04/11/max_planck_quantum_network/

======
lbo
The article may be misleading to those not familiar with quantum
teleportation. It doesn't have anything to do with instant messages across the
galaxy.

"That means it would be possible to build a network of 'quantum repeaters'
that use quantum teleportation, rather than photons, to transmit information
between different places."

You cannot convey information over distance faster than light using quantum
teleportation. You simply ensure that both entangled atoms' wave functions
will collapse in a way that's correlated.

Wikipedia has a nice explanation: "Assume that Alice and Bob share an
entangled qubit ab. That is, Alice has one half, a, and Bob has the other
half, b. Let c denote the qubit Alice wishes to transmit to Bob. Alice applies
a unitary operation on the qubits ac and measures the result to obtain two
classical bits. In this process, the two qubits are destroyed. Bob's qubit, b,
now contains information about c; however, the information is somewhat
randomized. More specifically, Bob's qubit b is in one of four states
uniformly chosen at random and Bob cannot obtain any information about c from
his qubit. Alice provides her two measured classical bits, which indicate
which of the four states Bob possesses. Bob applies a unitary transformation
which depends on the classical bits he obtains from Alice, transforming his
qubit into an identical re-creation of the qubit c."

What's cool about this experiment is that it shows you can transmit a qubit's
state over distance in a quantum way, which could be useful for a network of
quantum computers, which themselves are only useful for a small subset of
problems--most involving cryptography. The quantum-repeater application may
also be useful in reducing packet-loss-induced error, though it seems to be an
incredibly impractical method of doing so for most applications.

~~~
aik
Thanks for this explanation. One question: Once the atoms are entangled, I
understand that both take on the same superposition. At this point, in any
possible way, although we don't know the states, do they actually communicate
instantaneously, ie. not at the speed of light?

Put another way -- for the longest time I have believed that through
entanglement atoms transmit information instantly (not at the speed of light).
Is this in any way true?

~~~
lbo
Nothing that you would consider usable information, referred to as 'classical
information', can be transmitted via quantum entanglement. There is 'quantum
information' in the state of the entangled particles that can be transferred
instantaneously over infinite distance, however attempting the observe this
information by any means will destroy it (ie cause the wave function to
collapse).

If it was possible to transmit classical information faster than light, this
would violate causality as we understand it (you could receive a message that
you had been shot before you actually experienced being shot, thus allowing
you to prevent yourself from being shot). It's a bit difficult to understand
why this is true without a solid understanding of relativity, but you can read
up on it a bit here
[http://en.wikipedia.org/wiki/Special_relativity#Causality_an...](http://en.wikipedia.org/wiki/Special_relativity#Causality_and_prohibition_of_motion_faster_than_light).

This is all heavy stuff and very non-intuitive, but if you're curious about it
and eager to learn I'd highly recommend this tome: [http://www.amazon.com/The-
Feynman-Lectures-Physics-boxed/dp/...](http://www.amazon.com/The-Feynman-
Lectures-Physics-boxed/dp/0465023827/ref=sr_1_1?ie=UTF8&qid=1334214079&sr=8-1)

~~~
redwood
Ditto the Feynman lectures recommendation. Those were incredible icing on the
cake of my undergraduate physics course-load, adding significant intuitive
meaning and understanding to a subject with much potential but that's taught
in a very try manner most of the time.

------
cgs1019
Source article:
[http://www.scientificamerican.com/article.cfm?id=universal-q...](http://www.scientificamerican.com/article.cfm?id=universal-
quantum-network)

Source article's source article (Nature mag.; pay wall):
[http://www.nature.com/nature/journal/v484/n7393/full/nature1...](http://www.nature.com/nature/journal/v484/n7393/full/nature11023.html)

EDIT: arxiv pre-print: <http://arxiv.org/pdf/1202.5955v1.pdf>

------
kator
If we figured this out certainly someone/thing way out on the other side of
our galaxy or another galaxy has figured it out.. Could it be that we would
have to figure out how to DHCP across galaxies and manage to finally share
information with a distant intelligence? If so I hope the first thing they
send is a cartoon that looks like garfield and we'll finally know we're not
alone in the universe. :)

~~~
hetman
Unfortunately the initial entangling still requires physical contact (provided
by proxy of the photon on the optical fibre in this case).

Further more, once entangled, it is still not possible to transmit information
(in the way we think of it every day) faster than the speed of light. This
would break causality as we understand it.

What quantum teleportation allows is the instantaneous transmission of quantum
information between the entangled particles. I.e. the superposition of all
possible states can be made the same at both ends. However it is still not
possible to know which of these states actually represent the classical
information we were interested in. Hence classical information can't go as
fast as quantum information.

This is as best I understand it though it would be nice have get an expert to
further expand on this.

~~~
leot
AFAIK, the reason why entanglement doesn't permit instantaneous information
transfer is that, by definition, the quantum state is unknown before you
observe it. While, by observing it you instantly know both sides of the
entangled pair, this information itself requires conventional communication to
the other party in order to be useful, thereby again restricting communication
to the speed of light.

------
perlgeek
Here's something that has always bothered me about this topic:

> That, Ritter says, could extend the application of the network even further:
> once two atoms are entangled, the quantum state of one depends on the
> quantum state of the other.

And what exactly is the benefit from that? It sounds as if now you could
manipulate one of the atoms, and the other would follow suite, allowing you to
transmit information.

But my understanding of quantum physics is that manipulating one atom
immediately breaks up the entanglement, so no information transmitted. And
this is not just a technical limitation, but a conceptual one.

Am I wrong? And if no, how would a pair of entangled atoms help us in building
a communication network?

~~~
redwood
You're spot on, this is why I find "quantum computing" a bit less exciting
than most (just like the "hydrogen economy" say). Both sound great to those
who scratch the surface but both are really misnomers. Quantum networks
presumably would have a security advantage, but not a speed advantage. Thus
there is really not much to get excited about.

Basically entanglement is always dealt with in a misleading way. It's not that
one's state influences the other. It's that both are equivalently altered by a
third entity limited to the speed of light (as far as we know so far anyway).

------
grannyg00se
It is explained that there is a photon traveling from one atom (node) to
another which encodes information (as quantum state of the atom). Ok.

But then there is a glossing over of what I think is the more impressive part:

"The group also was able to entangle the atoms, which allowed them to share
information with each other no matter how far the distance between them. "

So they go from wired-up 60m distance photon information transmission to
infinite distance quantum entanglement information transmission. In one
sentence. Without any details!

Over what distance did they test this entanglement? Was it as reliable as the
wired-up version? Were they able to repeat it under different conditions? Is
there a reason they were limited to only two nodes?

~~~
gwillen
Quantum entanglement works over any distance, but can't be used to transmit
information. They most likely did not try further separating the atoms; the
fact that entanglement is an unbounded-distance effect is just a statement of
the physics involved.

------
andrewfelix
More articles like this please! I find these articles and the informed
comments that flow out of them fascinating.

More than can be said for some of the questionable articles headlining HN
lately.

------
brainless
This is surely genuinely spooky. To quote Arthur C. Clarke:

"Any sufficiently advanced technology is indistinguishable from magic."

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mooom
I wonder if a SETI experiment can be established by sending information by
quantum codification, do we know how to receive or scan for this kind of
signals?

------
kolev
Too bad you can't use this technology to implement an ISP-free Internet. Or...
is it possible to have more than two particles entangled?

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ktizo
So, how long now before wintermute merges with neuromancer?

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cyanbane
The beginnings of the Illusive man.

