The effect of the quantum susceptance on the gain of superconducting quasiparticle mixers

A detailed analysis of the effects of the quantum susceptance on the performance of superconductor-insulator-superconductor (SIS) mixers is performed. It is found that the principal effects of the quantum susceptance are to change the DC bias at which optimum coupling of the signal to the mixer occurs, and to change the output admittance at the IF frequency, thus changing the available gain. It is shown that at the signal port the principal effect is to change the bias voltage at which optimum coupling to the mixer occurs. A more dramatic effect occurs at the IF port, where the quantum susceptance leads to negative values of the output conductance, which causes the available gain to be infinite.