Viewpoint Fluctuations in open systems

Fluctuation theorems (FTs) establish fundamental identities about energy exchanges between a nonequilibrium system and its environment. Interest in such relations has been boosted by the possibility of applying perturbations to mesoscopic systems undergoing irreversible processes. While most recent research focused on classical systems, in a paper in Physical Review Letters, Michele Campisi, Peter Talkner, and Peter Hänggi, at the University of Augsburg in Germany, have now [1] extended FTs to open quantum systems, paving the way for further progress in scenarios ranging from Bose-Einstein condensates to metallic nanowires. When in contact with a thermal bath, a system in thermodynamic equilibrium can be pushed off equilibrium by time-dependent external forces. For slow enough perturbations this process is quasistatic or reversible, i.e., the system evolves through a succession of equilibrium states and the total amount of work done by external forces equals the equilibrium free-energy difference between the initial and final states. In 1997, Chris Jarzynski, then at the Los Alamos National laboratory, found a remarkable result for nonequilibrium scenarios [2]. According to the second law of thermodynamics, the total work is more than the reversible work: the difference between actual work and reversible work is equal to the net heat produced by dissipative forces and transferred from a system to the bath. Let W be the mechanical work due to external agents and let ∆G denote the reversible work or free-energy difference. Jarzynski found that, for arbitrary irreversible processes,