Conversion gain in mm-wave quasiparticle heterodyne mixers

Abstract

We report the observation of heterodyne mixing in superconductor‐insulator‐superconductor tunnel junctions with significant conversion gain and with a noise temperature comparable to the photon noise limit. (Double sideband L⁻¹=1.400.14, T_M≲1.5 K at 36 GHz.) The mixing arises from the strong nonlinearity in the quasiparticle tunneling currents at voltages comparable to the full energy gap. Gain is observed when the onset of quasiparticle current is sufficiently sharp that quantum effects are important. The observed gain is in good quantitative agreement with calculations based on the work of Tucker. It should make possible the construction of photon‐noise‐limited microwave heterodyne receivers.