Quantum computers would be able to process information in ways that standard computers cannot by tapping the unusual properties of quantum mechanics, but an analysis suggests that quantum computers would outclass conventional machines only by a slight degree for most computing problems, writes MIT professor Scott Aaronson. Evidence now indicates that quantum machines would be susceptible to many of the same algorithmic restrictions as classical computers, and these restrictions are totally independent of the practical problems of constructing quantum computers. A solid quantum computer algorithm would guarantee that computational paths leading to an incorrect answer neutralize while paths reading to a right answer reinforce, Aaronson says. The discovery of an efficient quantum algorithm to solve NP-complete problems remains elusive despite much effort, but one definite finding is that such an algorithm would have to efficiently take advantage of the problems' structure in a manner that is outside the capabilities of present-day methods. Aaronson points out that physicists have yet to come up with a final theory of physics, which gives rise to the possibility that a physical way to efficiently solve NP-complete problems may one day be revealed by a future theory. "People speculate about yet more powerful kinds of computers, some of which would make quantum computers look as pedestrian as vending machines," he notes. "All of them, however, would rely on speculative changes to the laws of physics." Aaronson projects that the difficulty of NP-complete problems will someday be perceived as a basic principle that describes part of the universe's fundamental nature.