Two Big Steps Toward the Quantum Computer(popularmechanics.com) |
Two Big Steps Toward the Quantum Computer(popularmechanics.com) |
I know almost nothing about Quantum Mechanics, but this sounds amazingly ingenious.
So what did they do, some sort of parallel universe transistor where the rubidium atom acts as the gate? If you assemble a processor out of this, will it compute all possible computations at the same time? And, last but not least... how do you make it converge to the computation you actually want?
Quantum computers make my head spin.
The article however paints another picture.
Very interesting listen if you have the time
It feels wrong to suppose that answers come out in an "instant", or a mere one step.
I'm so looking forward to that.
(Dropping some credentials doesn't seem inappropriate here: I worked professionally as a researcher on quantum computing and quantum information for 13 years.)
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
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).
http://curious.astro.cornell.edu/question.php?number=612
http://en.wikipedia.org/wiki/Superluminal_communication
The book, "Why E=mc^2" is an exceptional read. If I recall correctly, it explains why light-speed is a universal limit:
http://www.amazon.co.uk/Why-Does-mc2-Brian-Cox/dp/0306819112
If we could do that at all, we'd almost certainly be able to to it today, anyhow. We have multi-q-bit QM computers, they just aren't of a practical size for computation. But if FTL communication was possible, it would have been done with them already.
to really reap the benefits of entanglement for anything other than microsecond HFT algos, like for human telecommunication, you would need to physically separate these photons by hundreds of thousands to millions of miles. i dont think you can keep them entangled while transporting them that kind of distance.
i could be really really wrong on all of this :)
The main reason for this is decoherence (the pair of entangled photons interact with other photons, or particles along the way) ... and lose their connection because they have to share it with the other particles [1].
People have come up with clever tricks to fight that. It's called distillation of entanglement [2]. You share 1000 pairs of particles, all weakly entangled because they have travelled a long distance and rubbed with the wrong particles along the way. Then each party at each end combines the particles with some measurements and classical communications (phone or internet) and ends up with a single pair, far away, highly entangled.
These systems are high latency as they often are supplemented by classical communication and post processing. They are used by banks (in Switzerland) to do secure point-to-point communication. But they only protect the channel and not the copy of Windows 2000 running on either end.
Perhaps they can be entangled at the factory in the "special room" then kept stable over the next 10 years. That would be enough, no?
They could even be literally the same object. See http://en.wikipedia.org/wiki/One-electron_universe
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.)
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.
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.
