emon and Landauer's principle The QIT story begins with a strange machine devised in 1871 for a thought experiment by the physicist J. C. Maxwell. Such a machine - if one were ever able to construct it - would violate the second principle of thermodynamics! As Maxwell describes in his theory of heat:1 One of the best established facts in thermodynamics is that it is impossible in a system enclosed in an envelope which permits neither change of volume nor passage of heat and in which both the temperature and the pressure are everywhere the same, to produce any inequality of temperature or of pressure without the expenditure of work. This is the second law of thermodynamics, and See http://en.wikipedia.org/wiki/Maxwell%27s_demon;http://users.ntsource.com/~neilsen/papers/demon/ node3.html. Reversible computation T''>T Figure 15.1 Maxwell's demon thought experiment: (a) initially, the gas molecules are evenly spread between two parts A and B (dark = fast, clear = slow), with homogeneous temperature T, and (b) after the demon sorted molecules according to speed, showing lower temperature T' in part A and higher temperature T" in part B. it is undoubtedly true as long as we can deal with bodies only in mass, and have no power of perceiving or handling the separate molecules of which they are made up. But if we conceive a being, whose faculties are SO sharpened that he can follow every molecule in its course, such a being, whose attributes are still as essentially finite as our own, would be able to do what is at present impossible to us. For we have seen that molecules in a vessel full of air at uniform temperature are moving with velocities by no means uniform, though the mean velocity of any great number of them, arbitrarily selected, is almost exactly uniform. Now let us suppose that such a vessel is divided into two portions, A and B, by a division in which there is a small hole, and that a being, who can see the individual molecules, opens and closes this hole, SO as to allow only the swifter molecules to pass fro