Intermodulation gain in nonlinear NbN superconducting microwave resonators

We report the measurement of intermodulation gain greater than unity in NbN superconducting stripline resonators. In the intermodulation measurements we inject two unequal tones into the oscillator-the pump and signal-both lying within the resonance band. At the onset of instability of the reflected pump we obtain a simultaneous gain of both the idler and the reflected signal. The measured gain in both cases can be as high as 15 dB.