Ergotropy from coherences in an open quantum system

Abstract: We show that it is possible to have non-zero ergotropy in the steady-states of an open quantum system consisting of qubits that are collectively coupled to a thermal bath at a finite temperature. The dynamics of our model leads the qubits into a steady-state that has coherences in the energy eigenbasis when the number of qubits in the system is more than one. We observe that even though the system do not have inverted populations, it is possible to extract work from the coherences and analytically show that in the high temperature limit, ergotropy per unit energy is equal to the $l_1$ norm of coherence for the two qubit case. Further, we analyze the scaling of coherence and ergotropy as a function of the number of qubits in the system for different initial states. Our results suggest that one can design a quantum battery that is charged by a dissipative thermal bath in the weak coupling regime.