[Strilanc]

Quantum mechanics doesn't allow instantaneous communication.

QM does allow some additional forms of spontaneous coordination, like in quantum pseudo telepathy games [1], but there is no communication. There is no back-and-forth decision making, just after-the-fact correspondence.

Unfortunately, the distinction between classical communication and quantum coordination is kind of hard to explain. It has to do with the difference between stochastic and unitary matrices [2].

1: http://en.wikipedia.org/wiki/Quantum_pseudo-telepathy 2: http://en.wikipedia.org/wiki/Unitary_matrix

[alphydan]

I'll bite the bullet and try to explain it in layman's terms (with not attempt at rigor):

When you measure particle A, something happens to particle B. Unfortunately, particle B always has 2 potential outcomes (let's say, with 50% chance of being RED and 50% chance of being BLUE). So when you measure B, you find "B is red", or "B is blue".

Now when you measure particle A, imagine you change the probabilities for B remotely to 90%/10%.

But when you're the guy at B, and you machine say "RED!" ... did that just happen because A changed the likelyhood, or did it happen because, well, there was a 50/50 chance of it happening?

It is more subtle than that, but that's the gist of why you can't send information. (yes when you repeat the experiment and A and B compare their results, the probabilities have changed, but for a single measurement you never know if you got it by chance or if you got it because A did something).

[hackuser]

>When you measure particle A, something happens to particle B.

Couldn't that be the signal? For example, one person could tell the other: "When B resolves, press the button!" It wouldn't matter if B resolved to red or blue.

Is it that we cannot detect whether B is in a superposition state without observing it and therefore resolving its state to one 'position' or the other?

(I hope I'm not the only one on HN with an incomplete understanding of current quantum theory.)

[alphydan]

What do you mean "When B resolves, press the button"? "When you measure A, something happens to B" ... that's the problem, we can't know what happend. Only after the fact, and after repeating the experiment and sharing the results between the two parties.

You're Alice, and you want to send Bob the message "1001010". Let's start with the first "1". You measure A and see "red", and thus alter the probabilities of B to "90/10" ... and you think to yourself: Awesome, I just sent a "90/10" probability to Bob, and that means "1". If I had gotten "blue", Bob would be receiving a "50/50" probability.

Now, you're Bob at Alpha Centauri, and a particle arrives. Then what? No matter if you get "red" or "blue", you'll never know if it happened as 50/50 (the inherent randomness of any quantum measurement), or because of the "90/10" probability. So when you have to write down was it a "1" or a "0" ... you can't know.

At that specific moment, in your lab, when particle B arrives .... the result doesn't tell you anything.

Once you meet again, or send an email (you can compare your stats and find out that, statistically A affected B ... but if you need email (classical communication) to find out, then it's definitely not faster than light)

There are more subtleties about the uncertainty principle, orthogonal basis, etc ... but you would need a more formal language to express it.

[hackuser]

I understand what you are saying but that's not what I meant to ask. The signal I'm proposing is not red/blue, it is superposition/resolved. Maybe this is more clear:

1) On Earth, Alice creates two entangled particles, both in superpositions, and gives one to Bob. She tells Bob: 'If your particle ever loses its superposition and resolves, whether to red or to blue, press the button!'

2) Bob goes to Alpha Centuri, Alice remains on Earth.

3) Alice wants Bob to press the button. She does something to resolve her particle.

4) Instantly, Bob's particle loses its superposition and also resolves. Bob gets the message and presses the button, in much less time than 4 years.

Why wouldn't that work? I suspect because you can't determine whether or not the particle is in a superposition, but my understanding is limited. Maybe the basis of my question is wrong.

[gtremper]

You have to know the "phase" at which to measure the quibit, which you would need to communicate through other means. Here's some more info from a class a took a couple years ago. http://www-inst.eecs.berkeley.edu/~cs191/sp12/notes/chap1&2....