  We experimentally realize an incoherent microwave source driven by
voltage-controlled quantum tunneling of electrons through nanoscale
normal-metal--insulator--superconductor junctions coupled to a resonator. We
observe the direct conversion of the electronic energy into microwave photons
by measuring the power spectrum of the microwave radiation emitted from the
resonator. The demonstrated total output power exceeds that of 2.5-K thermal
radiation although the photon and electron reservoirs are at subkelvin
temperatures. Measurements of the output power quantitatively agree with a
theoretical model in a wide range of the bias voltages providing information on
the electrically-controlled photon creation. The developed photon source is
fully compatible with low-temperature electronics and offers convenient in-situ
electrical control of the photon emission rate with a predetermined frequency,
without relying on intrinsic voltage fluctuations of heated normal-metal
components nor suffering from unwanted dissipation in room temperature cables.
In addition to its potential applications in microwave photonics, our results
provide complementary verification of the working principles of the recently
discovered quantum-circuit refrigerator.
