QUANTUM LIMITED RESPONSIVITY OF A NB AL2O3/NB SIS WAVE-GUIDE MIXER AT460 GHZ/

The noise and gain behaviour of a Nb/Al2O3/Nb SIS waveguide mixer, wit h an on chip integrated tuning element, is analyzed at 460 GHz. The re ceiver sensitivity of the whole system, including the beamsplitter, wi ndow 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 sensitivi ty above 400 GHz. We compare data of the pumped I-V curves with the We rthamer-Tucker theory and demonstrate an excellent agreement at 460 GH z. A comparison of the measured IF-output noise versus bias voltage wi th the quantum theory of mixing shows a good agreement, indicating for the first time the applicability of this theory to Nb tunnel junction s up to 500 GHz.