Chinese Physicists Smash Distance Record For Teleportation

http://www.technologyreview.com/blog/arxiv/27843/

by llambda • 14 years ago

78 38 14 years ago

themgt• 14 years ago

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• 14 years ago

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.

ajross2• 14 years ago

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.

talmand2• 14 years ago

If you are talking about intercepting one of the entangled particles during tranmission, then you are right, that's not much more secure than any other transmission. But once the entangled pairs are at their respective secured locations then I imagine it would be rather difficult to get to the transmitted data. But I'm not an expert on these matters.

pejoculant• 14 years ago

Carol's measurements would destroy the entanglement in a way that Alice and Bob would be able to detect. However QKD is still vulnerable to a MITM (as opposed to mere interception). You would still need some sort of a mechanism so that Alice knows she's talking to Bob and vice-versa.

melvinmt2• 14 years ago

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?

IsaacL• 14 years ago

China has two of the world's fastest supercomputers and an active space programme, so they're not doing too badly. Slightly more outlandish, they're also the world leaders in cloud seeding, as they wanted to make sure that the weather for the Beijing Olympics was under control.

partisan• 14 years ago

They wanted to reduce smog.

comportboy• 14 years ago

> I don't hear too often that anything exciting is being developed in China

What world do you live in?

draggnar• 14 years ago

The same world as me apparently. I never hear of Chinese innovations. Where are some good places to learn more about things developed in China?

mladenkovacevic2• 14 years ago

>> 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).

melvinmt2• 14 years ago

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.

pastaking2• 14 years ago

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?

talmand2• 14 years ago

That confused me as well. They seem to be talking about the relocation of one of the entangled pairs to its intended position. But it doesn't seem like what I would call teleportation, since it seems to be moving one of the entangled pairs along a path. They do describe one thought of teleportation in the beginning that fits my thought; copy object data, destroy object, send object data to second position, create object. But what the Chinese did doesn't seem like that unless I too am missing something.

rcfox• 14 years ago

Edit: I came up with a better way to say it.

You aren't teleporting the photons, you're teleporting via the photons. What's in the way matters only for getting the photons to the destination.

ajross2• 14 years ago

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.

pastaking2• 14 years ago

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

SoftwareMaven• 14 years ago

It is the information that is (in a sense) being teleported. The photons are the transporter rooms not the people being beamed aboard (to make a tortured, lossy analogy).

csomar• 14 years ago

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?

powrtoch• 14 years ago

The thing is it really isn't "teleportation" in the typical sense of instantaneous travel. The photons still move at the speed of light, this method just happens to be useful for taking cryptographic advantage of the quantum properties of photons.

It also bothers me that the article specifically says the "information" is teleported, as it seems like the author is going out of his/her way to imply a violation of the No Communication Theorem (http://en.wikipedia.org/wiki/No-communication_theorem), which is not in any way violated by the experiment.

rabbitonrails2• 14 years ago

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.

bcl• 14 years ago

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• 14 years ago

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

RuggeroAltair• 14 years ago

The information does not travel faster than light.

norswap• 14 years ago

Agreed, also the phrase "Teleportation turns out to be extremely useful. Because teleported information does not travel through the intervening space, it cannot be secretly accessed by an eavesdropper. " confused the heck out of me.

unreal373• 14 years ago

The information isn't being transmitted. Only the "public key" to read the information is transmitted from A to B. The information magically appears at B because of quantum entanglement. It's brilliant!

CoughlinJ• 14 years ago

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

rabbitonrails2• 14 years ago

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.

joshontheweb2• 14 years ago

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.

unreal373• 14 years ago

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

joshontheweb2• 14 years ago

lol, guess thats not happening any time soon.

BHSPitMonkey• 14 years ago

Who wants to set up the first quantum BBS?

unreal373• 14 years ago

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.

codehotter• 14 years ago

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.

gren• 14 years ago

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

Chinese Physicists Smash Distance Record For Teleportation

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