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Wednesday, February 22, 2006
 

In a quantum computation sort of way.

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. . .

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 . . .

posted by back40 | 2/22/2006 05:30:00 PM

1 Comments:

A classic interviewing technique is to NOT ask a question, but instead remain completely quiet. Many people are uncomfortable with long silences, so they start speaking, volunteering information unasked. Apparently this works with photons, too. Cool.

By Anonymous Mike Anderson, at 8:16 PM  

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