  We show that quantum coherence can enhance the performance of a continuous
quantum heat engine in the Lindblad description. We investigate the
steady-state solutions of the particle-exchanging quantum heat engine, composed
of degenerate double quantum dots coupled to two heat baths in parallel, where
quantum coherence may be induced due to interference between relaxation
channels. We find that the engine power can be enhanced by the coherence in the
nonlinear response regime, when the symmetry of coupling configurations between
dots and two baths is broken. In the symmetric case, the coherence cannot be
maintained in the steady state, except for the maximum interference degenerate
case, where initial-condition-dependent multiple steady states appear with a
dark state.
