
Quantum teleportation over 143 kilometres using active feed-forward - antimora
http://www.nature.com/nature/journal/vaop/ncurrent/full/nature11472.html
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carbocation
I confess I haven't read the entire article, but from the abstract I was
wondering, why 143 kilometers (they answer this: distance to satellites), and
why those two islands (except to maybe show off that they were teleporting in
the lap of luxury)? I'm guessing it's because there is no physical link
between the two, so it will reduce skeptics' abilities to say "this is a hoax,
there is a cable connecting the two stations."

~~~
ricardobeat
But quantum teleporting does require a link between the two points. From the
Wikipedia article:

    
    
        The prerequisites for quantum teleportation are a qubit
        that is to be teleported, a conventional communication
        channel capable of transmitting two classical bits (...)
    

There is no teleporting in the classical meaning of the word, just
communication between two locations to make a remote qubit match the (unknown)
state of the local one. That state is 'teleported'.

~~~
javert
_There is no teleporting in the classical meaning of the word_

So isn't it dishonest to call it teleportation? Even if all the scientists
working in this area are calling it that?

~~~
chc
No? How on earth is it dishonest to use the technical term for the thing? It
doesn't seem any more dishonest than calling the constituent parts of matter
"atoms" even though they can actually be split.

~~~
javert
The discovery that atoms are not atomic (could be split) came after the term
was coined.

On the other hand, in this case, a bunch of people decided to call something
"teleportation" when it isn't teleportation.

~~~
chc
AFAIK, it _is_ teleportation -- something is going from one place to another
without having to move through the space between them -- but it just isn't
what you think of when you think of "teleportation".

~~~
rodly
If it is moving from A to B via some physical continuous medium, then it is
not teleporting.

~~~
tgb
(Note: my full qualifications are an on-going Coursera course on quantum
computing that covers quantum teleportation lightly. FWIW, I recommend the
course.)

In vague terms, the process of quantum teleportation is the following:

* You have a qubit that you want to transmit to a friend and that you don't know anything about.

* create an entangled pair of qubits (such as photons) and separate them. Keep one yourself and give another to your friend.

* Now you have two qubits; one entangled with your friends and one that you want to transmit.

* Send your two bits through a quantum circuit (the main part being a 'controlled NOT gate').

* Your bit is now entangled with the bit that is entangled with your friends bit.

* Measure each of your bits, hence 'collapsing' them. Each measurement gives you a classical bit.

* Transmit your two classical bits to your friend over any classical connection.

* Your friend then performs an operation on his or her qubit depending on what you send him or her.

* Your friend's qubit is now in the state that your original qubit was in. Your original qubit is now in a different state.

Call it teleporation if you want; it's not that unlike some sci-fi
teleportation constructs of 'beaming' things places.

Why bother with this?

* It lets you send qubits to distant places that you have only classical channels to (and previously close enough contact share two entangled qubits).

* It allows more error-resistant computations: if your qubit is difficult to construct and you wish to perform a difficult operation on it, you can actually have your friend perform the difficult operation on his or her bit - _prior_ to teleportation - and, if it succeeds, you can then proceed with the teleportation. If it doesn't succeed, your friend may try again on another entangled qubit without messing up your difficult-to-prepare qubit. After a success and teleportation, you get the same result (up to a commutator which is usually relatively small and risk-free) as performing the difficult operation on the difficult-to-prepare qubit, but with less risk of messing it up.

~~~
javert
Thanks, this is a pretty helpful summary.

Can you clairify the second reasons to "bother" with it? I mean, if it's
difficult for me to perform the operation, it's presumably equally difficult
for my "friend" to do it (and subsequently "teleport" the result to me), so
why not just do it myself?

~~~
stordoff
In addition to the transportation benefit, it can be used for computation. The
example given in the Coursera course was to suppose that we have a qubit
(quantum bit) Y on which we wish to perform some unreliable computation. If it
fails, then Y's state is lost. Further suppose that Y is costly (e.g. time
consuming) to re-create.

Instead, we can create a pair of entangled qubits (which is easy to do - just
a single quantum gate), as one would when teleporting a qubit. We then apply
the computation to one of these qubits, and try again with a new pair of
qubits if it fails.

Once we have successfully applied the computation, we can teleport our
"expensive" qubit Y using this entangled pair. The result is that we now have
Y with the computation applied, but there is no risk of losing Y's state.

The details are rather more tricky (only certain computations can be used,
some require a correction after teleportation etc.), but this basis can (and
has) be built on to develop, for example, fault tolerant quantum computing.

------
jvdh
The full article is also available on Arxiv.org:
<http://arxiv.org/pdf/1205.3909v1.pdf>

------
jstalin
Could someone translate the abstract into English?

~~~
antimora
Here is another article that tries putting in English.

[http://www.zmescience.com/science/physics/physicists-
quantum...](http://www.zmescience.com/science/physics/physicists-quantum-
photons-08092012/)

~~~
alecco
The author fails to understand the basics of Quantum Teleportation. Think of
it this way: Alice has 2 boxes with the same _unknown_ bit, and sends one of
those boxes to Bob. When Alice opens her box she can see the value and Bob can
see it too. Something like that.

Very good for cryptography (think untamperable shared secret keys) and for
some cases of parallel computing.

Superluminal (FTL) communication would break Special Theory of Relativity. If
anybody achieves that it would be an instant Nobel and front page on every
newspaper on the planet. Even a mistake like the FTL neutrinos fiasco earlier
this year.

Tech journalists tend to skim the abstracts and hype it to sell more
clicks/ads.

~~~
newhouseb
I'm trying to think of a physical analogy to better understand this: Image you
(Alice) have a randomly shuffled (say, by god) deck of cards. You cut the deck
in half and distribute one half to Bob and keep the other half (such that by
knowing one half, you would know the other). Until either of you look at your
respective half-deck, the distribution of cards is in a superposition of all
possible distributions (a la Schrodinger's cat). Where the analogy breaks down
(thereby illustration what's special about Quantum Teleportation), is that as
soon as Alice flips her cards, Bob's cards flip as well (and visa versa). Thus
by the time the cards got to Bob, Bob would be able to detect that someone
else could have read the cards and thus the information may have been
compromised. (This is kind of how I remember quantum cryptography to work,
actually).

Does this analogy make sense? My physics background is pretty weak...

~~~
Saavedro
This is really just an illustration of quantum cryptography, rather than
quantum teleportation. Quantum cryptography is a method of exploiting quantum
effects to create a channel that can be used to transmit classical information
in a manner that cannot be eavesdropped.

Quantum teleportation is kind of the opposite. It uses a classical information
channel in order to transmit -quantum information-, which can't actually be
represented clasically. It is actually moving the superposition itself from
one place to another. The term "teleportation" is used because the "no
cloning" rule (quantum information cannot be "copied") means that the
information is no longer in the original location.

This is mostly useful in quantum computers, because in order to actually
_process_ information it's pretty important to be able to get it from one
piece of the machine to another, and until the recent teleportation
breakthroughs building quantum computers that could handle more than a couple
"qubits" was pretty much not doable.

------
newhouseb
Is it incorrect to say that, with quantum teleportation - while faster than
light travel of _matter_ is impossible, faster than light travel of
_information_ is possible provided that you have previously shipped the
endpoints of the link?

~~~
alecco
<http://en.wikipedia.org/wiki/Quantum_teleportation>

First paragraph.

~~~
newhouseb
Thanks, the key sentence here is

> However, it does not _immediately_ transmit classical information.

As I understand it, once you distribute an EPR pair, you can transmit one
qubit of quantum information which thereby destroys the pair. Provided you can
encode your classical information into quantum information, the amortized rate
of classical information communication cannot be faster than light because you
need to first distribute the EPR pairs (which you cannot do at FTL), but the
_instantaneous_ rate of classical information communication could be faster
than light.

The biggest assumption here is that you can encode/decode classical
information to/from quantum information at a rate (if at all) faster than it
would take for matter to travel over the potentially infinite distance between
the EPR pair. Some googling reveals that this is possible via means such as
time-bin encoding. (Edit: time-bin is only for photons, which I don't know if
you can entangle, but it still appears that you might be able to encode
classical information in electron spin)

What am I missing?

Edit - alecco's other answer in this thread make it much more clear what is
going on. Thanks again.

~~~
Variance
It sounds like you already figured this out, but you can't encode classical
information into the system in such a way that would allow for FTL transfer of
information. Depending on what you are specifically thinking of as "encoding",
there will be an equivalent reason why it won't work; often because you
collapse the wave function of your local quantum pair-element when you
transfer energy into it, which is a form of observation.

------
walrus
Some previous discussion: <https://news.ycombinator.com/item?id=4485347>

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elorant
I find it extremely intriguing that although we do not fully understand the
fundamentals of the quantum world we are able to exploit it.

------
kalmsy
"... whose location is unknown ..."

Wait. Is this the dawn of truly anonymous communication, with no means of
tracing the origin or target by wiretap?

------
moondowner
Beam me up, Scotty! (I couldn't resist...)

