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What is Reversible Computing?

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As the densities and switching speeds of our computational devices continue to increase exponentially, the amount of energy dissipated by these devices must remain at a certain level, otherwise economically impractical cooling apparatus is required. Conventional computers perform thermodynamically irreversible logic operations, that is, it is not possible to extrapolate prior machine states based solely upon information from future states. Information, in the form of bits, is erased. This bit erasure represents entropy, which is correlated to heat dissipation.

As we employ increasingly advanced techniques to design our integrated circuits, the energy dissipation per logic operation has continually been falling. But around 2015 development will reach a fundamental barrier - the kT barrier - which represents a quantity of energy computed by multiplying the temperature of the computing environment (generally room temperature, or ~300 Kelvin) by Boltzmann's constant. The only way to penetrate this barrier is to either lower the temperature of our computers or to develop thermodynamically reversible computers which do not generate entropy and therefore do not dissipate nearly as much heat as conventional, irreversible computers.

Creating reversible computers is a significantly more attractive option than cooling because lowering the computing environment to the lowest attainable temperature (~0 Kelvin) only decreases the energy dissipation per unit volume by two orders of magnitude, whereas building reversible computers allows the energy dissipation to be reduced arbitrarily.

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By building computers that perform reversible logic operations, arbitrarily low levels of heat dissipation can be achieved. The downside is that reversible architectures can become quite complicated. As 2015 nears and the computing industry begins to approach the kT barrier, it is likely that compilers will be designed to maximize the number of thermodynamically reversible operations within conventional computing architectures. When we begin to consider computers constructed from very tiny and fast logic gates, as in nanocomputing, reversibility becomes an essential feature for keeping energy dissipation at tolerable levels.

Research in reversible computing today is being pioneered by MIT, whose Pendulum Project was specifically created to devise a fully reversible computing architecture. Since the maximum attainable computer efficiencies are necessarily made up of reversible architectures, this area of research is indispensable if the power and economy of our computers will continue to increase.

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