Reversible computing

Reversible computing is a model of computing where the computational process to some extent is reversible, i.e., time-invertible. In a computational model that uses deterministic transitions from one state of the abstract machine to another, a necessary condition for reversibility is that the relation of the mapping from states to their successors must be one-to-one. Reversible computing is generally considered an unconventional form of computing.

There are two major, closely related, types of reversibility that are of particular interest for this purpose: physical reversibility and logical reversibility.

A process is said to be physically reversible if it results in no increase in physical entropy; it is isentropic. There is a style of circuit design ideally exhibiting this property that is referred to as charge recovery logic, adiabatic circuits, or adiabatic computing. Although in practice no nonstationary physical process can be exactly physically reversible or isentropic, there is no known limit to the closeness with which we can approach perfect reversibility, in systems that are sufficiently well-isolated from interactions with unknown external environments, when the laws of physics describing the system's evolution are precisely known.

Probably the largest motivation for the study of technologies aimed at actually implementing reversible computing is that they offer what is predicted to be the only potential way to improve the computational energy efficiency of computers beyond the fundamental von Neumann-Landauer limit of kT ln(2) energy dissipated per irreversible bit operation. (Though even the Landauer limit was millions of times below the energy consumption of computers in 2000s and thousands less in 2010s.)

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