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Two Big Steps Toward the Quantum Computer

popularmechanics.com

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Re: Two Big Steps Toward the Quantum Computer

#21

Post by sp332 »

devilshaircut wrote: post #19
sp332 wrote: post #15

Earlier quoted context omitted.

Sure. Last year, they stopped a photon for a solid minute. http://www.extremetech.com/extreme/162289-light-stopped-comp... Note this preserves the quantum state of the photon - it's not just storing the energy and then re-emitting a different photon.

It is my limited understanding that the idea of "a different photon" would be sort of irrelevant here as quantum state is the only thing that differentiates objects such as photons from one another. Outside of that, photons are essentially fungible. I don't really understand the finer points of the experiment however and I could be totally wrong in my understanding.

Oh yeah, I think the current model is that it gets stuck in an electron somehow. But since it preserves the quantum state, it's still usable for quantum crypto and quantum computing.

Re: Two Big Steps Toward the Quantum Computer

#22

Post by knodi123 »

devilshaircut wrote: post #19
sp332 wrote: post #15

Earlier quoted context omitted.

Sure. Last year, they stopped a photon for a solid minute. http://www.extremetech.com/extreme/162289-light-stopped-comp... Note this preserves the quantum state of the photon - it's not just storing the energy and then re-emitting a different photon.

It is my limited understanding that the idea of "a different photon" would be sort of irrelevant here as quantum state is the only thing that differentiates objects such as photons from one another. Outside of that, photons are essentially fungible. I don't really understand the finer points of the experiment however and I could be totally wrong in my understanding.

I can imagine a world where they're more than just fungible-

They could even be literally the same object. See http://en.wikipedia.org/wiki/One-electron_universe

Re: Two Big Steps Toward the Quantum Computer

#24

Post by forgotprevpass »

sp332 wrote: post #14
callesgg wrote: post #13

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.

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.

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)

Re: Two Big Steps Toward the Quantum Computer

#25

Post by alphydan »

Strilanc wrote: post #8
chris_mahan wrote: post #3

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.

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 d…

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).

Re: Two Big Steps Toward the Quantum Computer

#26

Post by alphydan »

leeoniya wrote: post #7
chris_mahan wrote: post #3

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.

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 phy…

>> 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

Re: Two Big Steps Toward the Quantum Computer

#27

Post by hcarvalhoalves »

> 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.

Re: Two Big Steps Toward the Quantum Computer

#29

Post by SoftwareMaven »

m_mueller wrote: post #23

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?

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.

Re: Two Big Steps Toward the Quantum Computer

#30

Post by hackuser »

alphydan wrote: post #25
Strilanc wrote: post #8

Earlier quoted context omitted.

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 d…

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 re…

>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.)

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