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Two Big Steps Toward the Quantum Computer

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Re: Two Big Steps Toward the Quantum Computer

#31

Post by tedsanders »

hcarvalhoalves wrote: post #27

> 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…

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).

Re: Two Big Steps Toward the Quantum Computer

#32

Post by gtremper »

hackuser wrote: post #30
alphydan wrote: post #25

Earlier quoted context omitted.

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…

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....

Re: Two Big Steps Toward the Quantum Computer

#33

Post by alphydan »

hackuser wrote: post #30
alphydan wrote: post #25

Earlier quoted context omitted.

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…

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.

Re: Two Big Steps Toward the Quantum Computer

#35

Post by hackuser »

alphydan wrote: post #33
hackuser wrote: post #30

Earlier quoted context omitted.

>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…

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…

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.

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