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Quantum computer solves problem without running

dogjones

Graduate Poster
dogjones
Joined
Oct 3, 2005
Messages
1,303
Found this via boing boing.

http://www.eurekalert.org/pub_releases/2006-02/uoia-qcs022106.php

By combining quantum computation and quantum interrogation, scientists at the University of Illinois at Urbana-Champaign have found an exotic way of determining an answer to an algorithm – without ever running the algorithm.
Using an optical-based quantum computer, a research team led by physicist Paul Kwiat has presented the first demonstration of "counterfactual computation," inferring information about an answer, even though the computer did not run. The researchers report their work in the Feb. 23 issue of Nature.

(snip)
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This is extremely interesting, even though I am pretty much clueless as to how quantum mechanics works.

This will doubtless be cited to back up various ESP woo though.
 
You're not being skeptical enough.
This is just a joke.

P.S.: This is my first post. I'm so proud of me.
 
Well, I have to admit that I haven't a clue what that article said. I'd really appreciate it if someone here could translate a bit of that into layman's terms.

Meg
 
Doesn't that Zeno effect mean we can stop radioactive elements decaying by continually observing them?

It all sounds very nice. I don't actually believe any of it, and probably won't until a quantum computer is sitting on my desktop.
 
Wow... that's some weird... well, you know. Quantum just never ceases to amaze me.

P.S. The potential for abuse of this stuff is huge. :(

Edit: bad grammar
 
meg said:
Well, I have to admit that I haven't a clue what that article said. I'd really appreciate it if someone here could translate a bit of that into layman's terms.
Click to expand...
I didn't understand the article either. I don't think it was very well written.

I found a much better explanation on the web site of Paul Kwiat himself, the physicist mentioned in the article. It was written some time ago, I think, but it seems to deal with roughly the same topic.
 
Thanks 69dodge. I think I actually did understand a small part of that one. My head hurts now, though. :confused:

Meg <--heading off to go read some Winnie the Pooh or something
 
Underemployed said:
Doesn't that Zeno effect mean we can stop radioactive elements decaying by continually observing them?
Click to expand...

If my understanding is correct, yes. HOWEVER, you would have to "observe" it by bombarding it with the radiation you are trying to prevent it from emitting.

Basically, you hit it with a form of radiation that would bring it from the lower state to the excited state. If it doesn't absorb the radiation, you've "observed" it in the excited state, and it's probability of decaying vs. time is now the same as if it had just been excited to that state. Therefore, if you keep hitting it with this radiation and keep the time intervals between hits small enough that the pobability is really low that it would decay, you could hold the system in the excited state. But, good luck aiming and producing the radiation necessary to prevent radioactive decay. And better luck shielding the set-up. But most importantly, what's the point of preventing the radioactive decay of an element if you're doing it by generating gobs and gobs of the radiation!
 
I'm pretty skeptical about the last paragraph.

I find this a bit funny. Phillip Ball actually called me to get a comment very shortly before they had to go to press, and since I hadn't seen the paper and hadn't read about the idea for 5 or so years I really couldnt help - I suggested he get a hold of Josza, whose idea the whole thing was. Now I presumed Ball found me because of a citation in the article to this paper http://prola.aps.org/abstract/PRL/v85/i14/p2925_1 of mine, which is pretty much completely ignored but which Paul Kwiat (the guy who did the experiment) encouraged me to publish in the first place. But instead its a citation to a completely mad paper of mine http://www.arxiv.org/abs/quant-ph/0206066 which I only wrote up because of the potential for jokes (of which there are a few, though several to do with personalities within the community). I never even bothered submitting it to a journal...
 
It's a step in the right direction, considering our current understanding of why and when atoms decay is limited to plotting half-life graphs.

In a high-radiation environment like a nuclear reactor, wouldn't the constant bombardment from all sides mean the Zeno effect is seen on a small scale - ie we see less radioactive decay than expected, because some atoms are going to be 'observed' more than normal? (although the effect would probably be so minute you'd be hard-pressed to find it)

Likewise wouldn't this mean a star would be producing slightly less radiation than expected - Did I read somewhere that this was the case with the Sun?...Maybe not, I can't find that one anywhere.

Anyway, I'm sure all the experiments are spiffy. So I'll modify my statement to read: I'll believe it when my desktop quantum computer plays my games before I've installed them.
 
PixyMisa said:
Doesn't look like it. See this post on Nature's web site. The Quantum Zeno Effect is real, by the way.
Click to expand...

Still...
I'm pretty sure I read an article about this topic a few years ago which was supposed to be a hoax. Of course, I might be wrong. I guess I'd need a quantum computer to find that article now.;)
 
gipp said:
Still...
I'm pretty sure I read an article about this topic a few years ago which was supposed to be a hoax. Of course, I might be wrong. I guess I'd need a quantum computer to find that article now.;)
Click to expand...

Nah, its not a hoax, though there have been plenty of hoaxes that sound a lot less crazy!
 
It'll be interesting to see if the Zeno's paradox trick will enable quantum computing to be reliable. At any rate, this is an important first step.
 
Underemployed said:
In a high-radiation environment like a nuclear reactor, wouldn't the constant bombardment from all sides mean the Zeno effect is seen on a small scale - ie we see less radioactive decay than expected, because some atoms are going to be 'observed' more than normal? (although the effect would probably be so minute you'd be hard-pressed to find it)
Click to expand...

To be honest, Although I know quite a bit about optical physics and a moderate amount about atomic phisics, I know almost nothing about nuclear physics. So, I have no intuition about the stability of the states, absorption properties, intermediate states, etc. Furthermore, my understanding is based on systems involving photon interactions, so although I would be hesitantly comfortable extrapolating to gamma radiation, I have little feel for the quantum states involved with proton/electron/neutron-nucleus interactions.

That said, I suspect that the nuclear case isn't as clear cut as the simple systems I picture when conceptualizing the Zeno effect. I suspect (but like I said I have little intuition about this) that the pretinent decay times in the types of chain reactions used in a nuclear reactor are sufficiently small compared to the rate of bombardment (combined with the small target a nucleus presents to this bombardment) that this effect would be extremely small.
 
TonyL said:
If my understanding is correct, yes. HOWEVER, you would have to "observe" it by bombarding it with the radiation you are trying to prevent it from emitting.

Basically, you hit it with a form of radiation that would bring it from the lower state to the excited state. If it doesn't absorb the radiation, you've "observed" it in the excited state, and it's probability of decaying vs. time is now the same as if it had just been excited to that state. Therefore, if you keep hitting it with this radiation and keep the time intervals between hits small enough that the pobability is really low that it would decay, you could hold the system in the excited state. But, good luck aiming and producing the radiation necessary to prevent radioactive decay. And better luck shielding the set-up. But most importantly, what's the point of preventing the radioactive decay of an element if you're doing it by generating gobs and gobs of the radiation!
Click to expand...

I dont think this "bombardment" is necessary. I'm pretty sure in the original formulation it was simply imagined that there was a sphere of scintillaiton counters aranged around the decaying nucleus. But I too am no expert on nuclear physics.

The same thing can happen with a photon trapped in a leaky cavity. Shove a detector outside the leaky walls and turn it on. If no photon is detected, turn it off and let some of the photonic wavefunction leak out of the cavity. Turn the detector back on. With high likelihood you simply collapse the photon back to being inside the cavity (and you start all over again), but with some probability you do end up detecting it. In the limit of very rapid switching you freeze the photon in the cavity..

That example is not perfect because of the implicit modification to the electromagnetic boundary conditions (which are defining the leaky cavity modes) if one simply leaves the detector on. I'm not sure this is a problem, but it may be. The much better example is using photon polarization as in the original "Interaction Free Measurement" experiments where this doesn't occur.
 
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