Which physicist's 'demon' thought experiment challenges entropy?

James Clerk Maxwell proposed it in 1867 to show that the second law of thermodynamics is statistical rather than absolute. Imagine a tiny creature at a frictionless trapdoor between two boxes of gas, letting only fast molecules through one way and slow ones the other. It does no work on the gas. Yet after a while one box is hot and the other cold, which is a temperature difference made from nothing, and you can run an engine on it.

Killing the demon took a century and the answer changed physics. Leo Szilard showed in 1929 that the act of measuring which molecules are which cannot be free. Rolf Landauer worked out in 1961 what the actual charge is: not measurement but forgetting. Erasing one bit of information must dissipate a minimum amount of heat, kT ln 2, no matter how the erasure is done. Charles Bennett then closed the loop in 1982 - the demon needs a memory, the memory is finite, and eventually it must be wiped. The entropy the demon appears to remove from the gas turns up in the cost of clearing its own notes.

That is a strange and durable result. It says information is physical, and it makes a bridge between thermodynamics and computing that nobody in 1867 could have anticipated. Landauer's limit was measured in the laboratory in 2012 and it is a real floor under the energy consumption of every computer ever built.