Maxwell demons, feedback control, and fluctuation theorems

As illustrated by the Maxwell demon and its sequels, feedback can be utilized to convert information into useful work. The recently developed fluctuation theorems turn out to be a powerful tool to analyze the energetics of feedback controlled systems. Using these theorems, we devise a method for designing optimal feedback protocols for thermodynamic engines that extract all the information gained during feedback as work. Our method is based on the observation that in a feedback-reversible process the measurement and the time-reversal of the ensuing protocol both prepare the system in the same probabilistic state. We illustrate the utility of our method with two examples of the multi-particle Szilard engine.