Cryogenic microwave source based on nanoscale tunnel junctions

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.