AN ANALYTIC INVESTIGATION OF THE SUPERCONDUCTOR QUASI-PARTICLE MIXER IN THE LOW-POWER LIMIT

The superconductor quasiparticle mixer can in principle approach quantum-noise-limited sensitivity if an ideal superconductor-insulator-superconductor (SIS) junction is used in the limit of zero local oscillator (LO) power; it can have infinite conversion gain in the same limit. Here a more physically realistic situation is studied. The properties of an SIS mixer using a slightly nonideal junction, with finite LO power, are determined by analytic expansion of the equations of the quantum theory of mixing. The result is that the conversion gain using any reasonably good quality SIS junction is essentially equal to that using a perfect junction. Quite to the contrary, the minimum noise temperature is controlled by the leakage current of the junction: The increase in the minimum added-noise temperature over the quantum limit T(Q) = h-omega/2k is given by delta-T(Q) = (h-omega/k) square-root I0I2/I1; this minimum requires that the LO voltage across the junction is V(LO) = (64I0/I2)1/4h-omega/e. (I(n) is the current measured on the unpumped I-V curve of the junction at voltage V0 + nh-omega/e.) Therefore, although an SIS mixer using an ideal junction can reach the quantum noise limit only in the limit of zero LO power, even the most nearly ideal junctions made today require a considerable LO for best sensitivity.