  Carnot's theorem poses a fundamental limit on the maximum efficiency
achievable from an engine that works between two reservoirs at thermal
equilibrium. We extend this result to the case of arbitrary nonthermal
stationary reservoirs, even with quantum coherence. In order to do this we
prove that a single nonthermal reservoir is formally equivalent to multiple
equilibrium ones. Finally, we discuss the possibility of realizing an engine
that works at unit efficiency by exploiting quantum coherence present in the
reservoir.
