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.