Non-linear amplification in Inelastic Cooper-pair Tunneling Amplifiers (ICTA)

The Inelastic Cooper-pair Tunneling Amplifier (ICTA) allows for linear parametric amplification with added noise close to the quantum limit. It works according to the same principle as a Josephson parametric amplifier (JPA), but replaces the pump photon by the energy of a Cooper pair tunneling through a voltage biased Josephson junction. For linear amplification, this energy corresponds to the creation of one photon in both the signal and idler mode. Using a DC bias to control the device allows to easily tune the "pump frequency" to match processes where multiple photons are created in one of the modes. These processes present gain with nonlinear characteristics which could be used advantageously in state discrimination by giving low gain at the noise background power level and higher gain at the signal power level. However, this nonlinear gain also introduces distortion of states which might be interpreted as extra noise. Here we discuss the performance and limitations of these nonlinear amplification processes in the ICTA.