Maxwell's demon is a thought experiment conceptualized by James Clerk Maxwell and involves a 'being' (or demon) guarding a tiny hole between two gas reservoirs at the same temperature; the demon can measure the speed of individual molecules and let through only the fast ones, which would create a temperature difference between the two reservoirs without doing any work, in apparent violation of the second law of thermodynamics. So far, all realizations of a Maxwell's demon have required an external control of its functions, making it hard to quantitatively evaluate the thermodynamic parameters — heat, entropy and information transfer — involved in the process. Jukka Pekola and colleagues at Aalto University have now made a Maxwell's demon that works without external intervention.
The system is composed of two aluminium superconducting wires functioning as reservoirs of conducting electrons. The reservoirs communicate through a copper island to form a single-electron transistor. Applying a voltage bias ensures that electrons tunnel only in one direction. Coupled to the island is a second single-electron transistor — the demon. The demon senses the presence or the absence of an electron inside the island by Coulombic interaction and quickly responds by applying a feedback mechanism that forces electrons to tunnel to and from the island always against a potential energy barrier. The net result of this uphill motion is a decrease in entropy of the system, measured as a drop in temperature between the left and right reservoirs. The price to pay, however, is in an increase in entropy of the demon, measured as a temperature rise. Because there is no exchange of heat between the system and the demon, the entropy increase is linked to the amount of information about the system acquired by the demon.