Quantum limited quasiparticle mixers at 100 GHz

We have made accurate measurements of the noise and gain of superconducting-insulating-superconducting (SIS) mixers employing small area (1sq micron) Ta/Ta2O5/Pb(0.9)Bi(0.1) tunnel junctions. We have measured an added mixer noise of 0.61 +/- 0.31 quanta at 95.0 GHz, which is within 25 percent of the quantum limit of 0.5 quanta. We have carried out a detailed comparison between theoretical predictions of the quantum theory of mixing and experimentally measured noise and gain. We used the shapes of I-V curves pumped at the upper and lower sideband frequencies 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.