QUANTUM LIMITED RESPONSIVITY OF A NB/AL2O3/NB SIS WAVE-GUIDE MIXER AT 460 GHZ

The noise and gain behaviour of a Nb/Al2O3/Nb SIS waveguide mixer, with an on chip integrated tuning element, is analyzed at 460 GHz. The receiver sensitivity of the whole system, including the beamsplitter, window and lenses is 116 K DSB. The mixer noise temperature is 35 +/- 20 K. Within the experimental error this is as low as the quantum limit at 460 GHz. This is the first measurement of quantum limited sensitivity above 400 GHz. We compare data of the pumped I-V curves with the Werthamer-Tucker theory and demonstrate an excellent agreement at 460 GHz. A comparison of the measured IF-output noise versus bias voltage with the quantum theory of mixing shows a good agreement, indicating for the first time the applicability of this theory to Nb tunnel junctions up to 500 GHz.