PowerPoint Presentation

9/22/03

Digression:
Time-Reversibility and the
Physical Limits of Computation

•Irreversible logic gate loses one bit of information

•This equals entropy decrease of kT ln 2

•Therefore a conventional gate must dissipate at least kT ln 2 joules

–typical transistors dissipate about 108kT

•Reversible gates can dissipate arbitrarily little energy

•Charles H. Bennett (1973). See also Feynman Lectures on Computation, ch. 5


	I = lg N, where N = #states
	To specify position & momentum of each particle of a physical system: (BY 12-13)
	S = k ln N = k ln 2 I
	entropies in the order of 10 bits/atom are typical
	finiteness due to quantum theory
	Boltzmann constant k = 1.38 X 10^-23 J/degree K
	(Feynman 151-4)