This figure depicts the equivalent signal impedance of the flux-pumped SQUID in the three-wave nondegenerate case. The constituent inductances are given by Eq. (66). In the limit that the ac-flux is small such that , the inductance is simply the quiescent Josephson inductance and . The acquires an imaginary component due to the external (real) admittance at the idler frequency. A positive, imaginary inductance is a negative, real impedance, which may therefore provide gain.