Quantum Telecloning Demonstrated?

An anonymous reader writes "According to Physorg eavesdropping on a quantum encrypted link can now be done without detection. From the article: 'The scientists have succeeded in making the first remote copies of beams of laser light, by combining quantum cloning with quantum teleportation into a single experimental step. Telecloning is more efficient than any combination of teleportation and local cloning because it relies on a new form of quantum entanglement - multipartite entanglement.' There is also a PDF of a related paper available here for background material."

Comments

I really dig this stuff...

[Mark_Uplanguage]

but I'm starting to get discouraged now that the already hard to grasp concepts of quantum mechanics are being infused with new more complicated forms. In the end I just want to know if we can teleport ourselves cause I'm tired of my f'ing commute.

Re:I really dig this stuff...

[websaber]

I remember the good old days when unbreakable encryption would stay unbreakable for 15 years. Now it's being broken before it is even released.

ahh yes

[la htris]

so now we can listen in on quantum encrypted... wait a second... that doesn't exist yet.

O well, must be the FBI getting an early start.

Re:ahh yes

[kebes]

I think you're confusing quantum computing (which is still mostly theoretical... the largest experimental proof has only involved a few qbits, and for all we know a full-fledged computer will be impractical) and quantum cryptography, which actually has been experimentally demonstrated.

Amazing as it may sound, researchers have used commercially available fiber-optics to send quantum encrypted signals. There are even companies that will sell devices, although right now the tech is not quite ready for prime-time. Still, it has been shown in a laboratory many times, and it's not fanciful to say that it may be deployed within our lifetimes (just depends on when the technology becomes affordable, compared to its benefits).

Also, as others have pointed out, this new result actually doesn't show that quantum crypto is breakable... it only shows that under some circustances the eveasdropper can remain anonymous... but the users of the channel will still know that it has been compromised, and will thus not use the keys that have been generated. That is, quantum crypto is still mathematically unbreakable when properly implemented (assuming that Quantum Mechanics is correct, that is).

Re:ahh yes

[Anonymous Coward]

http://en.wikipedia.org/wiki/Quantum_entanglement

If the composite system is in this state, it is impossible to attribute to either system A or system B a definite pure state. Instead, their states are superposed with one another. In this sense, the systems are "entangled".

Now suppose Alice is an observer for system A, and Bob is an observer for system B. If Alice performs the measurement A, there are two possible outcomes, occurring with equal probability:

      1. Alice measures 0, and the state of the system collapses to |0\rangle_A |1\rangle_B
      2. Alice measures 1, and the state of the system collapses to |1\rangle_A |0\rangle_B.

If the former occurs, any subsequent measurement of B performed by Bob always returns 1. If the latter occurs, Bob's measurement always returns 0. Thus, system B has been altered by Alice performing her measurement on system A., even if the systems A and B are spatially separated. This is the foundation of the EPR paradox.

The outcome of Alice's measurement is random. Alice cannot decide which state to collapse the composite system into, and therefore cannot transmit information to Bob by acting on her system. (There is a possible loophole: if Bob could make multiple duplicate copies of the state he receives, he could obtain information by collecting statistics. This loophole is closed by the no cloning theorem, which forbids the creation of duplicate states.) Causality is thus preserved, as claimed above.

http://en.wikipedia.org/wiki/No_cloning_theorem

The no cloning theorem is a result of quantum mechanics which forbids the creation of identical copies of an arbitrary unknown quantum state. It was stated by Wootters, Zurek, and Dieks in 1982, and has profound implications in quantum computing and related fields.

Note that the state of one system can be identically entangled with the state of another system, such as by using a CNOT gate, but this does not constitute cloning since the systems will always yield the same value upon measurement. The no cloning theorem describes the inability to make separately measurable states.

What?

[compuguy84]

I was just gonna say that...

Seriously though, no matter how much I learn/study/pay tuition, there're always posts that make me realize how little I know about anything.

It's both humbling and inspiring.

Off topic, but someone had to say it... :)

Heisenberg Uncertainty Principal?

[icleprechauns]

What ramifications does this have on the heisenberg uncertainty principal? I may be no expert, but doesn't this mean that you could make a remote copy of a particle, and measure one's momentum and the other's position with great accuracy?

Re:Heisenberg Uncertainty Principal?

[LiquidCoooled]

There is such a place, but the lesson plan is a nightmare.
Just as you think you know where your next lesson is you rush to get there to realise you've already missed it.

Finding out if the lecturers are still alive after opening the classroom door is an entirely different and wholey worrying scenario unto itself.

Re:Heisenberg Uncertainty Principal?

[Concerned Onlooker]

What ramifications does this have on the Heisenberg uncertainty principle?

Obviously no one is quite sure.

Re:Heisenberg Uncertainty Principal?

[da cog]

Actually, if it could be done it wouldn't violate the Uncertainty Principle at all. A particle cannot have both a definite momentum and position, it can only have (roughly speaking) a probability distribution of each. So if you could clone a particle a zillion times, then each time you wouldn't get the same position, but rather if you looked at all of the clones together you'd get a distribution which would be identical to that of the original particle.

Having said that, cloning a particle perfectly is nonetheless forbidden by the No Cloning Theorem. Basically (as I understand it) what this says is that there is an underlying principle of Quantum Mechanics that you can never know what position distribution a particle originally had, since the moment you measure it you focus it at that point and kill the original distribution. Cloning the particle would be a way of "cheating" that would let you get the distribution of the particle without destroying it, so it ends up being forbidden.

Now, even though you cannot perfectly clone a particle, you can imperfectly clone it, which is what these guys have claimed to have done. If you look at the abstract, you will note that they are only claiming a fidelity of 58% +/- 1%. (The theoretical limit is five-sixths (83%) according to this article in New Scientist.)

A non-perfect fidelity, however, isn't so bad. Alice and Bob probably can't get their own optimal fidelity when using Quantum Cryptography anyways; in theory they should expect to see 50% of the bits get through, and then worry if they see it goes down below that -- even, say, to 49%. In practice, their equipment might only be able to get 40% of the bits through, and sometimes even less than that, so they'll tolerate lower rates than 50% since they are figuring that eavesdropping would lower this rate all the way down to 25%, and that is something that they'd surely notice. However, by using the techniques like those discussed in the article you can apparently eavesdrop less than perfectly in a way that, while still lowering the bit transmission, does not make it as bad as 25%. Thus, if Alice and Bob were naive they'd just assume that their equipment was faulty and not that there was an eavesdropper.

So the moral of this story is that from now on Alice and Bob will have to make their apparatus work much more reliably so that they can expect a success rate of say, 45-50% rather than 35-50%, and thus be more likely to notice a slight degradation in the signal due to an eavesdropper.

Don't worry...

[NotQuiteReal]

Don't worry. No matter how hard you try to know anything, you'll still be dead soon, in the cosmic scheme of things.

Have a nice day!

Can I get this in English?

[ip_freely_2000]

Most of the time, I at least read TFA and make a dumb comment. This time, I read TFA and just felt dumb.

Can some explain it and use real-world examples?

As a physics major...

[physicsphairy]

As a physics major who has taken the time to look over the paper (read: barely skimmed--I am a lazy college student afterall), I would just like to offer my sincere opinion of "HUh?"

I hope that will be helpful to other Slashdotters outside the field.

Re:It is not "encryption", it is "modulation"!

[Secret Rabbit]

No mathematical proofs about this security can be given, since we still do not unterstand the physical universe completely!

Perhaps you haven't read:

http://www.amazon.ca/exec/obidos/ASIN/0521635039/q id%3D1140401059/701-1812336-3224355

I am not surprised that once again claims of perfectness by ethically challenged researchers and businesspeople have turned out to be wrong.

Perhaps you are not aware of a phrase that states "within current theory" that is implied everytime a theorist speaks. Or weren't you aware of that?

Or how about all those classical encryption schemes that were thought to be secure for long periods of time, but them turned out to be [near] trivial to break.

New attacks are created all the time. It doesn't mean the the researcher is ethically challenged. It just means that he thought he was right at the time, given the information at hand.

This is cutting edge research. Get a clue. Or at least your head out of your ass.

Re:It is not "encryption", it is "modulation"!

[wwwrench]

Wrong.
All the article claims is that the Evesdropper's location will be undetected. The fact that someone is attempting to eavesdrop will still be detected, and there are several well known proofs of security of this fact.
FTF Press Release
"Quantum cryptographic protocols are so secure that they can not only discover tapping but also where and how much information is leaking out. Now, using telecloning, the identity and location of the eavesdropper can be concealed."
Quantum cryptography is absolutely secure as long as the laws of quantum mechanics are true. And even if the laws of quantum mechanics are false, one can still do secure cryptography from some very weak assumptions (it follows from violating Bell's inequalities and no-signalling) see this

Hmm, how seredipitous...

[kopasa]

It's interesting that we were just talking about this very article (well the actual release, not this article about it) in a analytical mechanics class I'm taking. One of the things that wasn't mentioned in this article was the fact that the beam of light cloned was only done so to about 66% accuracy. I'm sort of kept from going into more details about this by my own fairly limited grasp on the matrix mechanics, but as the clone wasn't perfect, the uncertainty principle was upheld. It is fairly worrisome to see this study spun much out of proportion though. The opening blurb about Captain Kirk only reinforces untrue stereotypes about the potential of quantum teleportation. Alas, if journalists were physicists...

the cloning is only approximate

[wwwrench]

The reason that it doesn't violate the Heisenberg uncertainty principle is that the cloning is only approximate. You have one good photon, and you create two copies, neither of which are like the original. They are only somewhat like the original. This means that the evesdropper will get detected. Telecloning, just means that you clone the photon (approximately), and move it to another location (cloning+teleportation). The article claims that this means the location of the evesdropper will thus be safe, even if her attack is noticed. The article is actually about an experimental realisation of telecloning, not the discovery of telecloning itself.

Re:It is not "encryption", it is "modulation"!

[tbo]

Disclaimer: IAAQIS (I Am A Quantum Information Scientist).

The parent poster (wwwrench) is completely, 100% correct. Is this really Slashdot, or did I type the wrong URL?

Seriously, though, the parent poster is bang-on. To elaborate a bit, quantum cryptography would be more informatively called quantum key distribution (although both names are common in practice). All it does is allow you to distribute a key for a one-time pad in a secure method, given that the laws of quantum mechanics are at least partially correct (one-time pads are information-theoretic secure, provided the key is not compromised or re-used). If somebody tries to eavesdrop, you can detect it, and respond accordingly. That response could be privacy amplification (if the information the eavesdropper gained was only partial), re-trying the protocol, or bombing the eavesdropper to smithereens. That last possibility is why quantum telecloning might be useful.

One other hitch is that quantum key distribution requires a small shared secret in order to authenticate the two parties trying to generate a key. Thus, quantum key distribution is not a complete replacement for public-key cryptography.