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.
Maxwell's demon is named after the physicist James Clerk Maxwell, who first considered the idea in the 1800s. He proposed that the demon would act as a doorman, allowing only hot (fast-moving) molecules to go one way and cold (slow-moving) molecules to go the other way. Theoretically, the demon would be able to heat a hot reservoir and cool down a cold reservoir. As long as the demon uses a sufficiently small amount of energy, the entropy that it creates will be less than the entropy of the system that it lowers. As a result, Maxwell's demon violates the second law of thermodynamics. So far, scientists have not been able to physically create Maxwell's demon, at least not without increasing more entropy elsewhere. However, they have been trying numerous approaches, especially in the last decade. 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.
"To the best of our knowledge, our work provides for the first time a complete thermodynamic description of a minimalistic Maxwell demon," coauthor Massimiliano Esposito at the University of Luxembourg told Phys.org. "We rigorously quantify the action of the Maxwell demon on the system by identifying the information flux that he is generating. But we also clearly demonstrate that when the entropy production needed to implement the Maxwell demon mechanism is taken into account, the second law for the system together with the demon is recovered." In other words, entropy does increase somewhere, but not in the first quantum dot alone.