  One of the fundamental questions in quantum thermodynamics concerns the
decomposition of energetic changes into heat and work. Contrary to classical
engines, the entropy change of the piston cannot be neglected in the quantum
domain. As a consequence, different concepts of work arise, depending on the
desired task and the implied capabilities of the agent using the work generated
by the engine. Each work quantifier---from ergotropy to non-equilibrium free
energy---has well defined operational interpretations. We analyse these work
quantifiers for a heat-pumped three-level maser and derive the respective
engine efficiencies. In the classical limit of strong maser intensities the
engine efficiency converges towards the Scovil--Schulz-DuBois maser efficiency,
irrespective of the work quantifier.

Quantum Physics