Quasiparticle heterodyne mixing in SIS tunnel junctions

Abstract

The rapid onset of quasiparitcle tunneling current in superconductor‐insulator‐superconductor (Josephson) junctions at voltages above 2Δ/e is being used for millimeter‐wave heterodyne mixing. Junctions with a 2‐μm diameter and R_N=50 Ω have little capacitive shunting at the signal frequency of 36 GHz. Because there is no series resistance, residual capacitance can be tuned out. Double sideband conversion efficiencies of 0.32 and mixer noise temperatures as low as T_M⩽7 K=4hν/k have been observed. The results are compared with shot‐noise‐limited mixer theory. Photon‐assisted tunneling effects are seen which indicate the approach to photon‐noise‐limited operation.