Quantum limited quasiparticle mixers at 100 GHz

Accurate measurements of the noise and gain of superconducting-insulating-superconducting (SIS) mixers using small-area (1 mu m/sup 2/) Ta/Ta/sub 2/O/sub 5//Pb/sub 0.9/Bi/sub 0.1/ tunnel junctions were made. The authors measured an added mixer noise of 0.61+or-0.31 quanta at 95.0 GHz, which is within 25% of the quantum limit of 0.5 quanta. Detailed comparison between theoretical predictions of the quantum theory of mixing has been carried out, and noise and gain have been experimentally measured. The shapes of the I-V curves pumped at the upper and lower sideband frequencies were used to deduce values of the embedding admittances at these frequencies. Using these admittances, the mixer noise and gain predicted by quantum theory are in excellent agreement with experiment.