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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 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''.〔http://www.cise.ufl.edu/research/revcomp/〕 A process is said to be ''physically reversible'' if it results in no increase in physical entropy; it is ''isentropic''. These circuits are also 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 energy efficiency of computers beyond the fundamental von Neumann-Landauer limit〔J. von Neumann, ''Theory of Self-Reproducing Automata'', Univ. of Illinois Press, 1966.〕 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 2000-s and thousands less in 2010-s.〔Bérut, Antoine, et al. "(Experimental verification of Landauer/'s principle linking information and thermodynamics. )" Nature 483.7388 (2012): 187-189: "''From a technological perspective, energy dissipation per logic operation in present-day silicon-based digital circuits is about a factor of 1,000 greater than the ultimate Landauer limit, but is predicted to quickly attain it within the next couple of decades''"〕) ==Relation to Thermodynamics== As was first argued by Rolf Landauer of IBM,〔R. Landauer, "Irreversibility and heat generation in the computing process," IBM Journal of Research and Development, vol. 5, pp. 183-191, 1961.〕 in order for a computational process to be physically reversible, it must also be ''logically reversible''. Landauer's principle is the loosely formulated notion that the erasure of ''n'' bits of information must always incur a cost of ''nkT'' ln(2) in thermodynamic entropy. A discrete, deterministic computational process is said to be logically reversible if the transition function that maps old computational states to new ones is a one-to-one function; i.e. the output logical states uniquely defines the input logical states of the computational operation. For computational processes that are nondeterministic (in the sense of being probabilistic or random), the relation between old and new states is not a single-valued function, and the requirement needed to obtain physical reversibility becomes a slightly weaker condition, namely that the size of a given ensemble of possible initial computational states does not decrease, on average, as the computation proceeds forwards. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Reversible computing」の詳細全文を読む スポンサード リンク
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