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A quantum computer at a US University has solved a computational problem without running a program. Scientists at the University of Illinois at Urbana-Champaign gleaned the answer to an algorithm by combining quantum computation and quantum interrogation (a technique that makes use of wave-particle duality to search a region of space without actually entering that region) in an optical-based quantum computer through a process called "counterfactual computation".

"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 Paul 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."

The scientists explain this paradoxical result in the February 23 issue of Nature. The set-up for the experiment is explained in the University's press release (those unfamiliar with exotic nature of quantum physics - as exemplified by the Schrodinger's Cat thought experiment - should look away now) thus:

Utilising 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."

Obscure at this may sound, quantum computers have the potential to outstrip the capabilities of even the most modern of today's supercomputers. Although the University of Illinois' quantum computer cannot be scaled up, using these kinds of interrogation techniques the researchers are pioneering might make it possible to reduce errors in larger systems. ®

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