The Berry phase and the pump flux in stochastic chemical kinetics

We study a classical two-state stochastic system in a sea of substrates and products (absorbing states), which can be interpreted as a single Michaelis-Menten catalyzing enzyme or as a channel on a cell surface.We introduce a novel general method and use it to derive the expression for the full counting statistics of transitions among the absorbing states.For the evolution of the system under a periodic perturbation of the kinetic rates, the latter contains a term with a purely geometrical (the Berry phase) interpretation.This term gives rise to a pump current between the absorbing states, which is due entirely to the stochastic nature of the system.We calculate the first two cumulants of this current, and we argue that it is observable experimentally.