David DiVincenzo at the IBM Thomas J. Watson Research Center offers this response: "All current computer device technologies are indeed limited by the speed of electron motion. This limitation is rather fundamental, because the fastest possible speed for information transmission is of course the speed of light, and the speed of an electron is already a substantial fraction of this. Where we hope for future improvements is not so much in the speed of computer devices as in the speed of computation. At first, these may sound like the same thing, until you realize that the number of computer device operations needed to perform a computation is determined by something else--namely, an algorithm. "A very efficient algorithm can perform a computation much more quickly than can an inefficient algorithm, even if there is no change in the computer hardware. So further improvement in algorithms offers a possible route to continuing to make computers faster; better exploitation of parallel operations, pre-computation of parts of a problem, and other similar tricks are all possible ways of increasing computing efficiency. "These ideas may sound like they have nothing to do with 'physical restrictions,' but in fact we have found that by taking into account some of the quantum-mechanical properties of future computer devices, we can devise new kinds of algorithms that are much, much more efficient for certain computations. We still know very little about the ultimate limitations of these 'quantum algorithms.' " ADVERTISEMENT Seth Lloyd, an assistant professor in the mechanical engineering department at the Massachusetts Institute of Technology, prepared this overview: "The speed of computers is limited by how fast they can move information from where it is now to where it has to go next and by how fast that information can be processed once it gets here. An electronic computer computes by moving electrons around, so the physical restrictions of an electron moving through matter determine how fast such computers can run. It