Quantum entanglement information transfer proven?

[Cane_Toad]

I'm seeing a lot of articles about refining the entanglement process, and how it should be applicable to information transfer, but I haven't seen any experiments where they have actually manipulated the states on one side to be observed on the other. I'm having trouble understanding exactly how far into reality all this has advanced.

Does the explanation of entanglement leap into higher dimensional math, or is there something I can hope to follow. It's taking "spooky" physics to new levels for me, that and teleportation, and supersolids (one quantum state for a whole helium-sicle? does that mean it's one virtual particle?).

 

[Rach3]

We have not experimentally observed information transfer by quantum entanglement, or anything that might suggest it to be possible. In fact, there are strong no-go theorems against it - basically nonlocal or acausal communication would violate Special Relativity. (In short, communication is a causal physical process, hence it is limited by the speed of light.) This does not contradict the usual formalism, in which certain mathematical objects appear to travel FTL, because they are emphatically not observable!

The confusion about "information transfer by entanglement" is probably because there are communications protocols, involving entanglement, but not as an information channel (which we believe it cannot be). In short, if you have an entangled pair of qbits between users A and B, and also a supplementary communication channel between them (classical), then A and B can communicate securely by a mixture of classical communication and operations/measurements on their qbits. Example: they share a pair of entangled spin-1/2 particles, and A performs an operation on his/her qbit, then sends a message to B with instructions which basis to perform a measurement in, to "read" the message. The actual speed of information transfer is limited by the classical channel; however, an eavesdropper with access only to this and not the entangled qbits, cannot possibly decipher it - hence secure communication! (we hope) This is a frequently-discussed topic (related to quantum teleportation) and may explain the common confusion about this being "information transfer" - which it is not, sticking to standard terminology. Usually we talk about "correlations" between the states of the entangled qbits in different places and times.

Isn't QM fun?

 

[Rach3]

...but I haven't seen any experiments where they have actually manipulated the states on one side to be observed on the other.

For the reasons above there aren't any; but as for correlations between entangled states, that's been a hot experimental topic for many decades! They're usually described as "tests of Bell's Theorem" or "violations of Bell inequalities" - the classic one was done by Alaine Aspect in 1982. Usually it involves coincidence counting of photon states in very precise, quantum-optics experiments.
http://en.wikipedia.org/wiki/Bell_inequality#Bell_test_experiments

Does the explanation of entanglement leap into higher dimensional math, or is there something I can hope to follow.

QM formalism is remarkably simple (and elegant) - for things like QComputation all you need is a solid grounding in linear algebra. A broader knowledge in QM requires many other things, basic classical mechanics, statistical mechanics, tons of math.

 

[Cane_Toad]

Thanks. That makes more sense now.

I guess the main thing that they have accomplished is to create a system where a piece of information, ie. a photon's state, can be observed more than once by the "cloning" of the photon.

The way I'm thinking of it, the "cloning" of the photon creates two photons with [some] identical properties, so the fact that when you observe one, then the other "becomes" the same is just a mathmatical model that doesn't have to violate my (rudimentary) intuitive understanding of the universe. Is this correct, or have they actually observed something that leads to proof that the two particles are truly undefined until one is observed?

For cryptographic purposes, it gives new meaning to "this message will self-destruct in ..."

 

[timpeac]

Quantum entanglement

We have not experimentally observed information transfer by quantum entanglement, or anything that might suggest it to be possible. In fact, there are strong no-go theorems against it - basically nonlocal or acausal communication would violate Special Relativity. (In short, communication is a causal physical process, hence it is limited by the speed of light.) This does not contradict the usual formalism, in which certain mathematical objects appear to travel FTL, because they are emphatically not observable!

The confusion about "information transfer by entanglement" is probably because there are communications protocols, involving entanglement, but not as an information channel (which we believe it cannot be). In short, if you have an entangled pair of qbits between users A and B, and also a supplementary communication channel between them (classical), then A and B can communicate securely by a mixture of classical communication and operations/measurements on their qbits. Example: they share a pair of entangled spin-1/2 particles, and A performs an operation on his/her qbit, then sends a message to B with instructions which basis to perform a measurement in, to "read" the message. The actual speed of information transfer is limited by the classical channel; however, an eavesdropper with access only to this and not the entangled qbits, cannot possibly decipher it - hence secure communication! (we hope) This is a frequently-discussed topic (related to quantum teleportation) and may explain the common confusion about this being "information transfer" - which it is not, sticking to standard terminology. Usually we talk about "correlations" between the states of the entangled qbits in different places and times.

Isn't QM fun?

Hi,
I was interested to read this quote

In one seminal experiment in 1982, physicist Alan Aspect at the University of Paris noted that by changing the polarity of one speeding photon (a particle of light) he could induce another photon from the same source speeding in the opposite direction to change its polarity. The interaction happened faster than light, with sufficient distance between the photons that they shouldn't have “known” what was happening to the other. And yet, inexplicably, there was some sort of link.

http://www.signonsandiego.com/news/science/20060622-9999-lz1c22cause.html" .

Have they got that wrong then? It makes it sound like there is meaningful information being transmitted instantaneously since if they could see that the second photon changed its polarity, then that could be viewed as a binary "did change, didn't change" information, couldn't it? Or am I (probably am!) misunderstanding it.
Although doing anything as complicated as teleportation may not (may never be?) in our grasp by quantum entanglement, are we any nearer to being able to send simple (meaningful) information by that means? In other words if I give you a photon entangled with one I keep with me and you zip off to Mars, am I able to use that entanglement to send you a meaningful instantaneous (and therefore without recourse to any classical communication methods) signal? Thanks!:)

 

[Rach3]

Timpeac - you will find physics much easier to understand if you learn it from a physics source, rather than from a newspaper, which is written for the general public and is thus devoid of content. There is so very much written about Aspect's experiment and the bell inequalities, there's really no point in me recommending a particular source for reading. You could start with a google search, a more recent QM textbook, Aspect's original paper, any of the hundreds of papers citing it, or perhaps Asher Peres' book "Quantum Theory", which is a very nice introduction to foundations of QM.

 

[Rach3]

are we any nearer to being able to send simple (meaningful) information by that means?

No, see the above posts.

 

[Simone Severini]

No superluminal signals with entanglement

The no cloning theorem prevents superluminal communication via quantum entanglement.

This is an interesting paper on the subject.

http://arxiv.org/abs/quant-ph/9801014

Simone Severini

http://www-users.york.ac.uk/~ss54

 

[beautiful1]

The no cloning theorem prevents superluminal communication via quantum entanglement.

This is an interesting paper on the subject.

http://arxiv.org/abs/quant-ph/9801014

You may also find this critique of the above paper interesting as well

http://arxiv.org/abs/quant-ph/9803030

 

[nrqed]

Timpeac - you will find physics much easier to understand if you learn it from a physics source, rather than from a newspaper, which is written for the general public and is thus devoid of content. ...

So "written for th egenral public" implies "devoid of content"?

I find this a bit arrogant. And what is the excat definition of "written for the public"? Aren't all undergraduate textbooks in physics "written" for the public? Or is your definition that anything without equations is "written for the public"? So are you saying that nobody should learn anything about physics until they have learned maths for a few years? So when someone for the general public asks me what I do in life and I mention that I teach and do some research in physics and they ask what *is* physics all about, I should reply "well, I can't explain anything to you unless you are at ease with calculus and linear algerbra, etc"? Is that what I should reply?

By the way, I could say that *your* own post was written for the general public and therefore devoid of information

I am sorry...arrogance is one my pet peeves.

 

[Cane_Toad]

So "written for th egenral public" implies "devoid of content"?

...

I am sorry...arrogance is one my pet peeves.

I suspect that it wasn't arrogance, but annoyance at the way popular reporting dumbs things down to the where the point of the thing is lost or actually wrong (as was the case).

Writing on complex subjects for a general/non-expert audience is a hard task. It takes someone who knows the subject well, and can distill the important ideas. What exactly is chosen as "important" is an art. Too often we see entertainment or sensationalism instead of education.

 

[timpeac]

Thanks guys for your replies. I also read Rach's reply and was a bit peeved. I know she's trying to help but you shouldn't assume things. Just to give a bit of background - I am not studying physics. I studied it many years ago and from what I remember when I read that comment in the press it rang false to me - for the reasons you have confirmed - which is why I wanted to ask the question. I did search the internet for references and found nothing that was clear enough for a layman to understand, which is why I asked here. Thanks for the answers though :)

 

[Rach3]

I also read Rach's reply and was a bit peeved. I know she's trying to help

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