
Chinese Physicists Smash Distance Record For Teleportation - llambda
http://www.technologyreview.com/blog/arxiv/27843/
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codehotter
For anybody reading this and having visions of teleporting to spaceships, this
is not _that_ kind of teleportation.

Imagine two indistinguishable envelopes. Put a red piece of paper in one of
them and a green piece of paper in the other. Shuffle them so you don't know
which is which. Have a friend take one of the envelopes a hundred kilometers
away. Then open the envelope you still have with you. It contains the green
piece of paper. WHOA INSTANT TELEPORTATION OF INFORMATION! You know that the
other envelope has the red paper instantly, even though it is a hundred
kilometers away! Information transfer faster than the speed of light. Amazing.

Now, instead of using envelopes, imagine Alice generates a pair of entangled
photons. Alice send one half to Bob and measures the other half. Since the
photons are entangled, Alice knows what Bob must be measuring, just like with
the envelopes. So what makes quantum teleportation special? What can we do
with quantum teleportation that we cannot do with envelope teleportation?

One interesting thing about the quantum world is that we cannot measure
something without disturbing it. You can use that property to make an ultra-
secure cryptographic system.

Alice generates an entangled photon pair so that the photons are either
'pointing left' or 'pointing right' or 'pointing up' or 'pointing down'. Now,
there is no way to measure directly what direction the photon is pointing in.
You can choose among two tests. Test A gives you the correct answer if the
photon is pointing left or right, but gives you a random result if it's
pointing up or down. Test B gives you the correct result if it's pointing up
or down but a random result if it's pointing right or left.

Doing Test A or Test B completely disturbs the photon, giving it a random
state.

If Alice chooses a direction to measure randomly, and Bob chooses a direction
to measure randomly, Bob's measurements will agree with Alice 75% of the time.
Half the time, they'll choose the same method and their measures will agree,
and half the time they will choose a different method and both get a random
result, but through pure chance, they will still agree half the time.

If Eve tries to do a 'Man in the middle' attack, intercepting Alice's photons
and copying the result to send to Bob, Eve will send with only 75% accuracy
and Bob's measurements will be only 62.5% accurate. Thus, Eve can be detected.
Watch this video for more details:
<http://www.youtube.com/watch?v=UVzRbU6y7Ks>

This result in the link is interesting because the researchers have been able
to transport an 'entangled' photon over more than 100 kilometers, maintaining
its state. It is quite hard to do since entangled photons are fragile.

However, the photons are not being teleported, just transmitted, and sticking
to the speed limit like good space citizens. It is not that Alice can make a
change to her half of the entangled pair and have that be reflected in Bob's,
it's just that Alice can MEASURE her half and know what Bob must be measuring.

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unreal37
Now I get it. Wikipedia to the rescue:

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

An entangled qubit is created and half of it is stored at location A, and half
of it is stored at location B. That's the insecure part, but once A and B have
two halves of the qubit, the rest of the communication is guaranteed secure.

It's amazing that you can make a change to the half at A, and its reflected in
the half at B, because they are quantum entangled. Now B has no idea how to
interpret the change since it seems random, but A sends a kind of "public key
to unlock the data" over the air to B, and B can use that to measure his half
and extract the data.

The data is "teleported", but the public key used to read the data is open.
The public key itself tells you nothing about the data. It's brilliant.

Mind = blown.

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melvinmt
I don't hear too often that anything exciting is being developed in China so I
think it's great for them. I expect more of this in the future.

I just don't think having the most secure quantum cryptographic communication
system is the type of technology they would gladly want to share with the rest
of world. Sooner or later, the secret service (do they have one?) or military
will be involved. Will we see another Cold War-ish information technology
race?

~~~
mladenkovacevic
>> I just don't think having the most secure quantum cryptographic
communication system is the type of technology they would gladly want to share
with the rest of world. Sooner or later, the secret service (do they have
one?) or military will be involved. Will we see another Cold War-ish
information technology race?

You make it sound as if this selfishness and militaristic lean is specifically
Chinese in nature and that other freedom-lovin', rainbow-kissin' nations
engage in free and open exchange of knowledge with no military aspirations
whatsoever. "Oh if only those poor misguided Chinese shared like the rest of
us do we wouldn't have another cold-war." I mean I understand why you might be
prejudiced to think that way because I presumably live in the same NATO bubble
as you do, but this kind of "our values vs. their values" thinking is the
exact Hollywood-like narrative that starts wars in the first place (and by
that I'm not accusing only westerners of this, I'm pretty sure it happens in
China too).

~~~
melvinmt
Not at all. I'm quite aware of the US having similar export restrictions of
technology (esp. cryptography) in place for the same reasons China will not
share this military-grade technology with others. Hence my Cold War mention.

~~~
mladenkovacevic
For what it's worth I do agree with you that there will be another Cold War
and it may in fact take place on a cyber-battlefield. I would argue though
that the Cold War never ended (maybe somebody just hit a pause button).

But while technology might be used for purposes of conflict, I think that the
digital age also has the potential to break the barriers of distrust and
xenophobia between people of different nations. It's sort of happening on a
small scale already. You are able to have a conversation with a person in
China (when their internet is not being blocked) and realize that they are not
a DVD-copying, cat-eating, environment-hating communist that your society
might suggest they are. Governments often use the lack of knowledge about
different nations to justify going to war or blocking trade. Maybe with the
new age of global digital awareness we will not have such an easy time
cultivating distrust and hate. Or maybe it's gotten to the point where we all
distrust and hate each other on an individual level.. in which case we're all
fucked

------
bcl
This isn't teleporting as far as I can tell. They're still using a
transmission medium (fiber, laser). There is no instant appearance of the
information at the target, photons are still constrained by the speed of
light.

~~~
evincarofautumn
Somehow I wouldn’t expect teleportation to be faster than light, even if the
information doesn’t actually travel through the intervening space.

~~~
RuggeroAltair
The information does not travel faster than light.

------
themgt
I don't quite understand high-level how this works for secure communication.
Alice tosses an entangled photon to Bob and he gets the information out by
reading the spin?

How do you stop Carol from just grabbing the entangled photon herself and
reading the information? How can Alice know that Bob received the entangled
photon? Even Carol as a MITM seems possible if she just sent Bob new entangled
photons

~~~
nullspace
The same way you cannot prevent someone from reading the amplitude of the EM
wave you send through generic wired or wireless media?

The trick has always been in encrypting the aggregate information. But, I am
sure that quantum cryptography will be a completely different ball game
compared to what we have now.

~~~
ajross
No, this is wrong; pejoculant has it right above: the act of measuring the
spin state destroys the superposition. It's not physically possible for three
observers to see the entanglement at once. There's no cryptography required.

------
pastaking
I'm missing something. If it's teleportation, why does glass or air currents
interfere with transmission? It shouldn't matter what's in the way, right?

~~~
ajross
It's not "teleportation" in the Star Trek sense. They need to get the
entangled photons to both places so observers at both sites can see them. That
means that nothing can interact with the photons on their path (or rather that
any interaction needs to be precisely defined such that it doesn't break the
superposition). So you can't just send them through the air.

~~~
pastaking
Understood. So this means we can create awesome walkie talkies.

~~~
cpeterso
A quantum tin-cans-and-string telephone might be a better analogy since the
photon beam can be obstructed, unlike a walkie talkie's radio signal

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joshontheweb
It would be awesome to have the web run using this. When you connect to a
service they issue you a photon that you then use for transmission of data,
relieving the pipes of the burden. I Wonder what the bandwidth is on one of
these connections.

~~~
unreal37
From the article, 1000 bytes took 4 hours to send. That's 0.5 bps.

~~~
BHSPitMonkey
Who wants to set up the first quantum BBS?

------
csomar
I would love if there is a simple diagram or layout that explains how
teleportation of information works. Does anyone knows a good resource, video
or lecture for that?

~~~
rabbitonrails
At Station 1, we prepare a batch of particles that are correlated in a quantum
way. Now we have to get some of the particles to Station 2 without destroying
their correlations. Someone can intercept the particles on their way to
Station 2, but once Station 1 knows the particles are at Station 2, the system
is secure.

The operator at Station 1 performs a measurement on a particle there that
changes the spin of that particle, as well as a measurement on a desired
quantum bit that we want to transmit to Station 2. This process produces 2
classical bits of information. This process destroys the correlation between
the particle pair, and affects the particle at Station 2 but in a randomized
way that is useless without the operator at Station 2 possessing the two
classical bits (thus it is not faster-than-light communication).

Once the laser takes the two classical bits to Station 2, at the speed of
light, the operator at Station 2 can use them to decide what measurement to
perform on the paired particle. Once the correct measurement is performed, the
quantum bit that we measured at Station 1 appears.

The security benefit is that by completing one risky journey from Station 1 to
Station 2, we can allow many future quick, secure journeys.

The propagation happens at the speed of light because the classical bits are
required to understand what has been teleported. Performing the measurements
is much slower than the speed of light. The correlation can only be stored for
very short periods of time, currently.

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CoughlinJ
I thought this was what they referred to as quantum tunneling. I'm probably
wrong about that though...

~~~
rabbitonrails
Quantum tunneling refers to the observation that because we can never know
position and momentum exactly, there is always a nonzero probability that an
particle directed towards a barrier will breach the barrier. E.g. we cannot
accurately measure the position and resistance of the barrier so can't say for
certain what will happen. This results in sometimes observing subatomic
particles on the other side of thin barriers.

And that is how a scanning-tunneling microscope works.

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gren
"Teleportation turns out to be extremely useful." <= lol! Who doubts it?

