  We analyze the photovoltaic current through a double quantum dot system
coupled to a high-quality driven microwave resonator. The conversion of photons
in the resonator to electronic excitations produces a current flow even at zero
bias across the leads of the double quantum dot system. We demonstrate that due
to the quantum nature of the electromagnetic field in the resonator, the
photovoltaic current exhibits a double peak dependence on the frequency
$\omega$ of an external microwave source. The distance between the peaks is
determined by the strength of interaction between photons in the resonator and
electrons in the double quantum dot. The double peak structure disappears as
strengths of relaxation processes increases, recovering a simple classical
condition for maximal current when the microwave frequency is equal to the
resonator frequency.
