Quantum Limited Millimeter Wave Mixers and Detectors

We report operation of superconductor-insulator-superconductor quasiparticle tunnel junctions as mixers and detectors at millimeter wavelengths. The voltage scale of the nonlinearity in the I-V curve of such junctions can be small enough that classical device theory breaks down. Calculations based on photon assisted tunneling theory are then necessary to understand the observed performance. The performance predicted in this novel regime is sufficiently good that practical applications appear very promising. The best mixer result observed at 36 GHz is a double side-band mixer noise temperature T
m
≤ 7 K which is about 4 times the photon noise limited value of hv/k. When operated as a direct detector, a current responsivity of 3,500 A/W was observed. This is half the quantum limited value e/hv, which corresponds to one tunneling electron for each incident microwave photon. The best observed NEP was
3 \times 10^{-16}\mathrm{W}\sqrt{\mathrm{Hz}}
.