Two Big Steps Toward the Quantum Computer

by wherkewitz • 12 years ago

87 32 12 years ago

IpxqwidxG• 12 years ago

Continuum of Singularity, so to speak.

chris_mahan2• 12 years ago

When two network quantum chips can communicate instantly (not at speed of light--instantly) over infinite distances, using entangled atoms, then we can ditch the tecos.

I'm so looking forward to that.

thangalin• 12 years ago

Chances are FTL communication is not possible.

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

leoc• 12 years ago

Mind you, this would imply that the telcos probably can't send data FTL either, so it's all good. :)

jerf• 12 years ago

Stating whether FTL communication is possible at all may require hedging. Stating that it can't occur over quantum superposition does not, as I understand it. The math is clear; there's no information traveling. It does not matter how sophisticated our quantum computers get, there will not be any FTL communication coming out of them.

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.

leeoniya• 12 years ago

how would you entangle the photons in the first place? from what i understand they have to be in close proximity or even emitted from the same source. once entangled, they need to be maintained in this state very very carefully. i think a few hours is the record currently.

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 :)

chris_mahan2• 12 years ago

Yeah, not saying this was possible now. But in 10, 20 years?

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?

frozenport• 12 years ago

>to really reap the benefits of entanglement for anything other than microsecond HFT algos

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.

alphydan3• 12 years ago

> i dont think you can keep them entangled while transporting them that kind of distance.

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.

[1] http://www.quantiki.org/wiki/Monogamy_of_entanglement

[2] https://en.wikipedia.org/wiki/Entanglement_distillation

Strilanc• 12 years ago

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

alphydan3• 12 years ago

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).

michael_nielsen• 12 years ago

Faster-than-light communication using entanglement is not possible, according to the standard rules of quantum mechanics.

(Dropping some credentials doesn't seem inappropriate here: I worked professionally as a researcher on quantum computing and quantum information for 13 years.)

bashinator• 12 years ago

If at any point in the future of human history FTL travel or communications gets invented, we would already know about it.

wlievens• 12 years ago

As far as I understand, faster-than-light communication violates causality in some reference frames but not in all. FTL communication without arbitrary tims travel might be possiblem

callesgg• 12 years ago

I was under the Belief that we currently have no way of adding the qbits together. We can set a qbit and read it but we can't currently get the particles in the quantum state to interact.

The article however paints another picture.

sp3323• 12 years ago

Some groups have even been running Shor's algorithm on multi-qubit computers. https://en.wikipedia.org/wiki/Quantum_computer#Developments It requires the qubits to be entangled to do the computation, and in the later ones (not IBM's 2001 work) the researchers did observe entanglement.

forgotprevpass• 12 years ago

Technically, you can run Shor's algorithm on non quantum computers, with an exponential speed up (but fully within capabilities of modern computing vs. small quantities of qubits)

rqebmm• 12 years ago

This is all I can think of when I hear about this stuff: http://upload.wikimedia.org/wikipedia/commons/4/4e/Eniac.jpg

m_mueller• 12 years ago

IANAQP, but doesn't this article describe measuring the state of a quant? Shouldn't measuring put a quant into a defined state, i.e. destroying its superposition?

SoftwareMaven• 12 years ago

Measuring certainly would, but I don't think that's what it's describing. Instead, I think it is describing connecting the qbits together and allowing them all to share the same super-position through the entangled photon.

hcarvalhoalves• 12 years ago

But here's the tricky part. The scientists can put the rubidium atom in superposition, so that it is simultaneously in that energetic state and not in the energetic state. It's on and off. Because of this, the photon both does and does not enter the mirror, mingle, and gain its polarization change. And the photon, by virtue of having both changed and not changed, carries that superposition information and can bring it to a different atom-based qubit.

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.

tedsanders• 12 years ago

Yeah, one interpretation of what a quantum computer does is that it performs all computations in parallel. However... you run into problems when you try to read the output of this computer. In order to read the output you must measure a quantum state, and when this happens you only get to see one answer at random, not all of them. And that's not really useful. But if you can design an algorithm so that all of the parallel computations add together to form one answer (i.e., get all the wrong answer's probabilities to cancel out), you can get an exponential speed up. The key takeway is that quantum computers are not faster for general problems - quantum computers are only faster for problems where a special algorithm exists (like Shor's algorithm for factoring).

cordite• 12 years ago

Am I missing something? Isn't a quantum computer just a non deterministic computation mechanism?

It feels wrong to suppose that answers come out in an "instant", or a mere one step.

bluejellybean• 12 years ago

Here is a Google tech talk with Eric Ladizinsky that got posted a few days ago. https://www.youtube.com/watch?v=eIEy1KHk0rk

Very interesting listen if you have the time

Two Big Steps Toward the Quantum Computer

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