  We give a novel derivation of Holevo's bound using an important result from
nonequilibrium statistical physics, the fluctuation theorem. To do so we
develop a general formalism of quantum fluctuation theorems for two-time
measurements, which explicitly accounts for the back action of quantum
measurements as well as possibly non-unitary time evolution. For a specific
choice of observables this fluctuation theorem yields a measurement-dependent
correction to the Holevo bound, leading to a tighter inequality. We conclude by
analyzing equality conditions for the improved bound.
