Work production of quantum rotor engines

We study the mechanical performance of quantum rotor heat engines in terms of common notions of work. Our investigation focuses on quantum models of a mill driven by the directed heat flow from a hot to a cold working mode, and of a piston alternating the heating and cooling of a single mode in a continuous Otto cycle. We evaluate the extractable work in terms of ergotropy, the kinetic energy associated to net directed rotation, as well as the intrinsic work based on the exerted torque under autonomous operation, and we compare them to the energy output for the case of an external dissipative load and for externally driven engine cycles. Apart from signatures of angular momentum quantization, the ergotropy is found to be consistent with the unambiguous classical notion of work stored in net directed motion. It also agrees with the energy output to an external load or agent under optimal conditions.