mpf@eng.fsu.edu

If we hope to compute with better energy-efficiency than this, then we must perform bit operations *adiabatically*, that is, without transforming much of the typical *kT* ln *N* bit energy into heat. Highly adiabatic (or thermodynamically reversible) operations must also be *logically* reversible, that is, they must perform one-to-one transformations of the local digital state. It has been known since at least 1963 that reversible operations are still computationally universal. Although various technologists have occasionally expressed vague doubts about whether a practical computer based on reversible logic principles could ever be built, ongoing research over the last twenty years has shown that these doubts are ill-founded.

For example, in the late 1990s at MIT, in the DARPA-funded Pendulum project, we successfully designed, built and tested a variety of CMOS VLSI chips (including a complete RISC-style microprocessor) that consisted entirely of adiabatic logic circuits, as a proof of concept. Many physical simulations, validated by empirical studies, have confirmed the long-term potential of reversible computing to circumvent the *kT* limit, and comprehensive analytical studies show that it can be cost-effective, despite its overheads. Active research towards practical reversible computing continues today at the University of Florida and at the FAMU-FSU College of Engineering, under a grant from the Semiconductor Research Corporation (a consortium of all the major chipmakers).

Although a number of significant research challenges do still remain, in the areas of high-quality resonators, low-resistance switching devices, power/clock signal distribution, and reversible logic synthesis and optimization, these problems which we are tackling in reversible computing today are, arguably, much less severe and more straightforward to solve than are the wide range of very fundamental physical barriers that presently threaten to halt the progress of traditional semiconductor technology, perhaps in only the next few years. To proceed very far beyond the limits of MOSFET technology *requires* that we implement reversible computing, in one form or another, as a prerequisite. But, to solve the remaining research problems will take time, so a more intense level of effort directed towards them needs to begin soon.

Thus, the high-performance computing community would be well-advised to turn an increasing amount of attention to reversible computing over the next few years, since it is really the *only* avenue (consistent with fundamental physics) that might possibly allow us to extend the historical power-performance trends out far enough to achieve our ambitions for extreme (Zettaflops-scale) supercomputing. In short, reversible computing is possible, it is necessary, and the time to start on it is now.