A terrific story has appeared on The Register this evening. I can’t paraphrase it so I am going to have to quote:

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.

Wait – it get’s better:

Utilising two coupled optical interferometers, nested within a third, the 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. 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. That is at the heart of quantum interrogation schemes, and to my mind, quantum mechanics doesn’t get any more mysterious than this.”

Now you know! And now you know why I couldn’t paraphrase it. I wonder if it will put an end to the computer errors my electricity company is always having…