Scott Aaronson writes in the New York Times:
In everyday life, it would be silly to speak of a “minus 30 percent chance of rain tomorrow,” much less a “square root of minus 1 percent chance.” However, quantum mechanics is based on numbers called amplitudes, which are closely related to probabilities but can also be negative (in fact, they are complex numbers). Crucially, if an event (say, a photon hitting a screen) can happen one way with positive amplitude, and a different way with negative amplitude, then the two amplitudes can “interfere destructively” and cancel each other out, so that the event never happens at all. The goal in quantum computing is to choreograph a computation so that the amplitudes leading to wrong answers cancel each other out, while the amplitudes leading to right answers reinforce.
Contrary to a widespread misconception, it’s for only a few specialized types of problem that researchers know how exploit that trick to obtain dramatic speed increases over today’s computers. To date, the two main examples are simulating the behavior of atoms and molecules, and breaking certain cryptographic codes — including, by unlucky coincidence, most of the codes used in modern electronic commerce. Of course, both problems can be tackled with conventional computers, too — but, though no one has proved it, it’s plausibly conjectured that any conventional algorithm would need a huge amount of time. While code-breaking understandably grabs the headlines, it’s the more humdrum application of quantum computers — simulating quantum physics and chemistry — that has the potential to revolutionize fields from nanotechnology to drug design.
dailymeh posted this on December 6, 2011
