Maxwell’s demon and quantum dots

James Clerk Maxwell, physicist,  has entered the history books for any number reasons but my personal favourite is Maxwell’s demon, a thought experiment he proposed in the 1800s to violate the 2nd law of thermodynamics. Lisa Zyga in her Feb. 1, 2013 article for phys.org provides an explanation,

When you open your door on a cold winter day, the warm air from your home and the cold air from outside begin to mix and evolve toward thermal equilibrium, a state of complete entropy where the temperatures outside and inside are the same. This situation is a rough example of the second law of thermodynamics, which says that entropy in a closed system never decreases. If you could control the air flow in a way that uses a sufficiently small amount of energy, so that the entropy of the system actually decreases overall, you would have a hypothetical mechanism called Maxwell’s demon.

Here’s how Maxwell describes his thought experiment along with a further explanation, from the Maxwell’s demon essay on Wikipedia (Note: I have removed links),

… if we conceive of a being whose faculties are so sharpened that he can follow every molecule in its course, such a being, whose attributes are as essentially finite as our own, would be able to do what is 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 from A to B, and only the slower molecules to pass from B to A. He will thus, without expenditure of work, raise the temperature of B and lower that of A, in contradiction to the second law of thermodynamics….

In other words, Maxwell imagines one container divided into two parts, A and B. Both parts are filled with the same gas at equal temperatures and placed next to each other. Observing the molecules on both sides, an imaginary demon guards a trapdoor between the two parts. When a faster-than-average molecule from A flies towards the trapdoor, the demon opens it, and the molecule will fly from A to B. Likewise, when a slower-than-average molecule from B flies towards the trapdoor, the demon will let it pass from B to A. The average speed of the molecules in B will have increased while in A they will have slowed down on average. Since average molecular speed corresponds to temperature, the temperature decreases in A and increases in B, contrary to the second law of thermodynamics.

Two scientists, according to a recent study published in Physical Review Letters, have proposed a way of creating Maxwell’s demon, from Zyga’s article,

In a new study, Philipp Strasberg at the Institute of Technology in Berlin, and coauthors have proposed that Maxwell’s demon can be physically implemented with two interacting quantum dots connected to thermal reservoirs, where one dot takes the role of the demon and the other that of the controlled system. The experiment doesn’t violate the second law of thermodynamics, but it provides a very simple, minimalist implementation of the demon.

Here’s the proposition,

In their proposed experiment, the scientists coupled one dot to two reservoirs, which acts as a single-electron transistor, and coupled the second dot to another reservoir. The physicists showed that the second dot can be tuned to detect the transistor’s state, which is either empty (0) or filled (1). In order to do this, the two dots must be perfectly correlated, so that when the first dot (transistor) gets filled up, the second dot (detector) gets emptied, and vice versa. When the detector performs an infinitely fast and precise feedback, then the system receives additional information during entropy production.

Here’s a citation and a link for the paper,

Thermodynamics of a Physical Model Implementing a Maxwell Demon by Philipp Strasberg, Gernot Schaller, Tobias Brandes, and Massimiliano Esposito. Phys. Rev. Lett. 110, 040601 (2013) [5 pages] DOI: 10.1103/PhysRevLett.110.040601

This paper is behind a paywall.

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  1. Pingback: INFERNOS: realizing Maxwell’s Demon | FrogHeart

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