Quantum computer solves problem, without running

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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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In addition to Kwiat and Hosten, co-authors of the Nature paper are graduate students Julio Barreiro, Nicholas Peters and Matthew Rakher (now at the University of California at Santa Barbara). The work was funded by the Disruptive Technologies Office and the National Science Foundation.

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Source: http://www.eurekalert.org/pub_releas...-qcs022106.php

Discuss. :blink:

Everything I ever believed, ever, was wrong.

I use the same technique and algorithm in deciding whether or not I should go to work...every day... and it works.. I never have to go to work...yet my work gets done.

Intresting....

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Originally Posted by Barbarossa

Discuss. :blink:

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we've already had the discussion,
without having to discuss it....
:blink:

I read about a computer like that in some sci-fi novel a while back, can't remember the title now, but it was the sequel to Coalescent. By Stephen Baxter, I think.


EDit: the title of the book is Exultant.
EDit: Dunno' if the mechanics of it are comparable, that one explicitly used time travel, several short jumps, to go through all possible solutions and then sent the information back, cancelling out iterations in earlier moments as it went. But the result sounds similar.

strange how these sort of topics seem to crop up, as April approaches..

I thought it was customary to wait until it was actually April before doing an April-joke :huh:

I thought he meant the end of the tax year.

Because as we all know, only accountants and super-quantum-computers can formulate UK tax returns :smilie4:

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Originally Posted by manker

I thought he meant the end of the tax year.

Because as we all know, only accountants and super-quantum-computers can formulate UK tax returns :smilie4:

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tackth doethn't have to be tackthing:smilie4: