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Unread 23 Feb 2006, 10:12   #1
Flavius
 
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Quantum computer solves problem, without running

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Quantum computer solves problem, without running
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.

Quantum computers have the potential for solving certain types of problems much faster than classical computers. Speed and efficiency are gained because quantum bits can be placed in superpositions of one and zero, as opposed to classical bits, which are either one or zero. Moreover, the logic behind the coherent nature of quantum information processing often deviates from intuitive reasoning, leading to some surprising effects.

"It seems absolutely bizarre that counterfactual computation – using information that is counter to what must have actually happened – could find an answer without running the entire quantum computer," said Kwiat, a John Bardeen Professor of Electrical and Computer Engineering and Physics at Illinois. "But the nature of quantum interrogation makes this amazing feat possible."

Sometimes called interaction-free measurement, quantum interrogation is a technique that makes use of wave-particle duality (in this case, of photons) to search a region of space without actually entering that region of space.

Utilizing two coupled optical interferometers, nested within a third, Kwiat's team succeeded in counterfactually searching a four-element database using Grover's quantum search algorithm. "By placing our photon in a quantum superposition of running and not running the search algorithm, we obtained information about the answer even when the photon did not run the search algorithm," said graduate student Onur Hosten, lead author of the Nature paper. "We also showed theoretically how to obtain the answer without ever running the algorithm, by using a 'chained Zeno' effect."

Through clever use of beam splitters and both constructive and destructive interference, the researchers can put each photon in a superposition of taking two paths. Although a photon can occupy multiple places simultaneously, it can only make an actual appearance at one location. Its presence defines its path, and that can, in a very strange way, negate the need for the search algorithm to run.

"In a sense, it is the possibility that the algorithm could run which prevents the algorithm from running," Kwiat said. "That is at the heart of quantum interrogation schemes, and to my mind, quantum mechanics doesn't get any more mysterious than this."

While the researchers' optical quantum computer cannot be scaled up, using these kinds of interrogation techniques may make it possible to reduce errors in quantum computing, Kwiat said. "Anything you can do to reduce the errors will make it more likely that eventually you'll get a large-scale quantum computer."
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like .. wtf ?
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Unread 23 Feb 2006, 10:19   #2
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Re: Quantum computer solves problem, without running

It's not impressive if it can't also deduce the existance of income tax and rice pudding.
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Unread 23 Feb 2006, 10:20   #3
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Re: Quantum computer solves problem, without running

I wanna play Unreal Tournament 2007 on one if these bad boys!!!
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Unread 23 Feb 2006, 11:13   #4
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Re: Quantum computer solves problem, without running

This kind of thing makes me laugh. Manically.
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Unread 23 Feb 2006, 11:32   #5
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Re: Quantum computer solves problem, without running

So what they're saying is they've pumped loads and loads of money into quantum computer research just so it can do nothing even when it does something

Its very confusing
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Unread 23 Feb 2006, 12:12   #6
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Re: Quantum computer solves problem, without running

does it do something when it does nothing though?
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Unread 23 Feb 2006, 13:14   #7
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Re: Quantum computer solves problem, without running

I only wish they had used this computer to write a report about it in an articulate and decent fashion
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Unread 23 Feb 2006, 13:28   #8
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Re: Quantum computer solves problem, without running

I followed bits of it...but other bits i thought 'wtfx'
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Unread 23 Feb 2006, 13:38   #9
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Re: Quantum computer solves problem, without running

Quote:
Originally Posted by Baron Morte
I only wish they had used this computer to write a report about it in an articulate and decent fashion
They didn't write that.

What they did write was:

Quote:
Originally Posted by The letter to Nature
The logic underlying the coherent nature of quantum information processing often deviates from intuitive reasoning, leading to surprising effects. Counterfactual computation constitutes a striking example: the potential outcome of a quantum computation can be inferred, even if the computer is not run1. Relying on similar arguments to interaction-free measurements2 (or quantum interrogation3), counterfactual computation is accomplished by putting the computer in a superposition of 'running' and 'not running' states, and then interfering the two histories. Conditional on the as-yet-unknown outcome of the computation, it is sometimes possible to counterfactually infer information about the solution. Here we demonstrate counterfactual computation, implementing Grover's search algorithm with an all-optical approach4. It was believed that the overall probability of such counterfactual inference is intrinsically limited1, 5, so that it could not perform better on average than random guesses. However, using a novel 'chained' version of the quantum Zeno effect6, we show how to boost the counterfactual inference probability to unity, thereby beating the random guessing limit. Our methods are general and apply to any physical system, as illustrated by a discussion of trapped-ion systems. Finally, we briefly show that, in certain circumstances, counterfactual computation can eliminate errors induced by decoherence.
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Unread 23 Feb 2006, 14:12   #10
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Re: Quantum computer solves problem, without running

Christ, scientific reporting in the media is a joke. Does anyone have a citation of the actual paper? I cant understand from that if its 'just' saying theyve managed to implement Grover's algorithm, or if theyve done something (theoretically) new.

edit: This seems to give more details about the general idea.

Last edited by Nodrog; 23 Feb 2006 at 14:22.
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Unread 23 Feb 2006, 14:13   #11
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Re: Quantum computer solves problem, without running

Quote:
Originally Posted by MrL_JaKiri
What they did write was:
I noticed that a little while ago while looking through a dictionary.

It was under 'What'.
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Unread 23 Feb 2006, 14:14   #12
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Re: Quantum computer solves problem, without running

Now, if only I could utilise this principle to find out what, say, a 19th century german philosopher was doing even when his mobile phone was turned off, he wasn't on MSN, and his forum inbox was full.

But alas! It can't be scaled up!
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Unread 23 Feb 2006, 14:28   #13
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Re: Quantum computer solves problem, without running

I like your thinking.
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Unread 23 Feb 2006, 16:53   #14
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Re: Quantum computer solves problem, without running

Quote:
Originally Posted by Giles
Now, if only I could utilise this principle to find out what, say, a 19th century german philosopher was doing even when his mobile phone was turned off, he wasn't on MSN, and his forum inbox was full.

But alas! It can't be scaled up!
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Unread 23 Feb 2006, 17:06   #15
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Re: Quantum computer solves problem, without running

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Originally Posted by MrL_JaKiri
So it turns out GD can solve problems beyond the reach of quantum computation. You should write a paper.
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