Fundamental bound on the power of quantum machines

Giving a universal upper bound on the power output of heat engines is a long-standing open problem. We tackle this problem for generic quantum machines in self-contained formulation by carefully including the switching process of the interaction. In this way, we show a fundamental upper bound on the power associated with the energy-time uncertainty principle. As a result, the energy fluctuation of the external controller is verified as a necessary resource for producing the power. This bound implies a trade-off between the power and `noise' for work extraction, which yields an estimation on the time scale to obtain detectable work extraction. Ideal clock-driven model of autonomous quantum machine gives a concrete demonstration of our bound.