  Incorporating time into thermodynamics allows addressing the tradeoff between
efficiency and power. A qubit engine serves as a toy model to study this
tradeoff from first principles, based on the quantum theory of open systems. We
study the quantum origin of irreversibility, originating from heat transport,
quantum friction and thermalization in the presence of external driving. We
construct various finite-time engine cycles based on the Otto and Carnot
templates. Our analysis highlights the role of coherence and the quantum origin
of entropy production.
