Maxwell's demon, rectifiers, and the second law: Computer simulation of   Smoluchowski's trapdoor
  We have simulated numerically an automated version of Maxwell's demon inspired by Smoluchowski's ideas of 1912. Two gas chambers of equal volume are connected via an opening that is covered by a trapdoor. The trapdoor can open to the left but not to the right, and is intended to rectify naturally occurring fluctuations in density between the two chambers. Our results confirm that though the trapdoor behaves as a rectifier when large density differences are imposed by external means, it can not extract useful work from the thermal motion of the molecules when left on its own. 
P.A. Skordos and W.H. Zurek