Nanocomputers and Reversible Logic

An overview of nanocomputers is given, including a discussion of reversible computing techniques and an explanation of why they are necessary. 1 Nanocomputers I usually begin this talk by trying to justify why we need nanocomputers in the rst place. As Caxton Foster once said of associative memory, nanocomputers would seem to hold out the promise of sugar tomorrow to people drowing in honey today. However, I think that having an audience of people interested in molecular simulation makes that justiication at least somewhat superruous; present practice is straining quite rmly at the bounds of available computation. Instead I'll just point out a few of the qualitita-tively diierent uses nanocomputers will make economical. In the mid-sixties, IBM sold a small computer for what was then the average price of a house. Today, single-chip micros of roughly the same computational power cost less than $5 and are used as controllers in toaster-ovens. Similarly, we can imagine putting a nanocomputer in each particle of pigment to implement \intelligent paint", or at each pixel location in an artiicial retina to implement image understanding algorithms. This last is a point worth emphasizing. With to-day's processing technology, robots operating outside a rigid, tightly controlled environment are extremely expensive and running at the ragged edge of the state of the art. Even though current systems can, for example , drive in traac at highway speeds, no one is going to replace truck drivers with them until their cost comes down by some orders of magnitude. EEec-tive robotics depends on enough computational power to perform sensory perception; nanocomputers should make this cost-eeective the way microcomputers did text processing. Beyond providing robots with the processing power humans already have, there is the opportunity of extending those powers themselves. Nanocomputers would make high delity virtual reality possible and economical. One imagines the wearable ooce. As is slowly being understood in the world of existing computers , the interface to the human is easily the most computationally intensive task of all. Last but not least { in some sense, the most important application for nanocomputers { is as the controllers for nanomechanical devices. In particular , molecular assemblers will need nanocomputers to control them; and we will need assemblers to build nanocomputers. 1.1 What's a nanocomputer? Currently, the feature sizes in state-of-the-art VLSI fabrication are on the order of half a micron, i.e. 500 nanometers. In fteen years, using nothing …