Non-equilibrium equalities with unital quantum channels

Abstract

A general tool for the description of open quantum systems is given by the formalism of quantum operations. The most important of these are trace-preserving maps, also known as quantum channels. We discuss those conditions on quantum channels under which the Jarzynski equality and related fluctuation theorems hold. It is essential that the representing quantum channel be unital. Under this condition, we first derive the corresponding Jarzynski equality. For a bistochastic map and its adjoint, we further formulate a theorem of Tasaki–Crooks type. In the context of unital channels, some notes on heat transfer between two quantum systems are given. We also consider the case of a finite system operated on by an external agent with feedback control. When unital channels are applied at the first stage and, for a mutual-information form, at the further ones, we obtain quantum Jarzynski–Sagawa–Ueda relations. These are extensions of the previously given results to unital quantum operations.