Heat fluxes in a two-qubit cascaded system due to coherences of a non-thermal bath

Abstract

We study a cascaded system of two subsystems repeatedly interacting with a bath prepared in various non-thermal states. Effects of different types of bath coherence on dynamics of the system heat flux are explored. For a single-qubit bath with the displacement coherence, heat flux of each subsystem is divided into dissipative and coherent parts. In the stationary regime, the dissipative heat fluxes are the same for the two subsystems, but the coherent parts are not due to the cascaded feature. In the transient regime, heat fluxes occur even when the system and the bath have the same temperature and, more surprisingly, heat flow reversal appears when temperatures of the system and the bath differ. For a two-qubit bath we consider two types of coherence: the squeezing and the heat-exchange ones. In the stationary regime, the squeezing coherence establishes correlations between the subsystems, each of which is evolving to its thermal state with a temperature identical to the bath’s. The squeezing coherence slightly influences only the dynamics of the subsystem that interacts with the bath later. As for the heat-exchange coherence, it forces the total system to thermalize to a state with the temperature related to both magnitude and relative phase of the coherence. Particularly, depending on the relative phases, the heat-exchange coherence can amplify or suppress the heat flux during the time evolution.