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Journal of Applied Physics

Accurate Experimental and Theoretical Comparisons Between SIS Mixers Showing Weak and Strong Quantum Effects

William R. McGrath, P. L. Richards, Dean W. Face, D. E. Prober, Frances L. Lloyd

Published Jan 1, 1988• 10 citations
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IEEE Transactions on Magnetics•Mar 1, 1991•20 citations
We have made a systematic study of the gain and noise in superconductor-insulator-superconductor mixers employing Ta based, Nb based, and Pb-alloy based tunnel junctions. These junctions displayed both weak and strong quantum effects at a signal frequency of 33 GHz. The effects of energy gap sharpness and subgap current were investigated and are quantitatively related to mixer performance. Detailed comparisons are made of the mixing results with the predictions of a three-port model approximation to the Tucker theory. We have measured mixer performance with a novel test apparatus which is accurate enough to allow for the first quantitative tests of theoretical noise predictions. We find that the three-port model of the Tucker theory underestimates the mixer noise temperature by a factor of about 2 for all of our mixers. In addition, predicted values of available mixer gain are in reasonable agreement with experiment when quantum effects are weak. However, as quantum effects become strong, the predicted available gain diverges to infinity, which is in sharp contrast to the experimental results. Predictions of coupled gain do not always show such divergences.
Accurate measurements have been made of the noise and gain of superconducting-insulating- superconducting (SIS) mixers employing small-area (1 micrometers 2) Ta/Ta2O5/Pb0.9Bi0.1 tunnel junctions. The authors have measured an added mixer noise of 0.61 +/- 0.31 quanta at 95.0 GHz, which is within 25% of the quantum limit of 0.5 quanta. A detailed comparison is made between theoretical predictions of the quantum theory of mixing and experimentally measured noise and gain. The authors 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.
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.