Here’s a new way to get an answer from a quantum computer: you plug it in, turn it on, and then don’t run the program. If you’re lucky, you’ll get the answer anyway.
It sounds absurd — but Onur Hosten and colleagues show in Nature this week that it works. They set up an experiment with light beams, which enacts a quantum computation, using information encoded in the quantum states of photons (light particles) to carry out a simple database search. This implements a well-known quantum-computing algorithm, called Grover’s search algorithm.
Crucially, the researchers don’t actually run the computation. Instead, they use a trick for probing the possible states of the light-based quantum computer — including the one where the computation was carried out — without really seeming to ‘look’ at it at all. This doesn’t always give them the answer they’re looking for — the location of the item they are searching out — but sometimes it does.
This seemingly nonsensical behaviour is possible precisely because the ‘computer’ runs on quantum rules. Quantum-mechanical systems can exist in two states simultaneously — this is the origin of the great increase in computing power that quantum computers potentially offer. In the present case, it means that the computer can exist in a state in which the algorithm was run, and one in which it wasn’t. By probing the nature of such states, the researchers can reconstruct what the state would have been if the algorithm had run, purely by looking at those states for which it didn’t.
From Nature.com Press Release.
March 1st, 2006 | General Science