Quantum Coherence in a Quantum Heat Engine

We identify that quantum coherence is a valuable resource in the quantum heat engine, which is designed in a quantum thermodynamic cycle assisted by a quantum Maxwell's demon. This demon is in a superposed state. The quantum work and heat are redefined as the sum of coherent and incoherent parts in the energy representation. The efficiency of the heat engine potentially can exceed the Carnot efficiency due to the coherence consumption of the demon. In addition, we also discuss a heat engine driven by quantum coherence. This resource-driven viewpoint provides a direct and effective way to clarify the thermodynamic processes where the coherent superposition of states cannot be ignored.