Chaotic noise in superconducting microbridge 4-photon,X-band parametric amplifier

The anomalous noise rise observed in nearly all types of parametric amplifiers based on Josephson junctions has been an intriguing as well as annoying problem for many years. This phenomenon has been most spectactular in microbridge amplifiers. Here we present measurements on externally pumped single microbridge 4-photon unbiased amplifiers, where the slit with the bridge is used as a slotline resonantly coupled to the waveguide in an exceptionally simple coupling scheme. This scheme may be of interest in itself, particularly if the noise problem can be overcome, and in other connections. Up to 16 dB gain was obtained at the top of the waveguide. However, the noise rise was observed as usual. An analog computer study on a model including an input/output circuit was performed. The results are in very good agreement with the experiments. The amplification is heralded by a seemingly chaotic noise rise. this noise is then amplified linearly when gain occurs. Amplification is found to take place very close to where the supercurrent is completely suppressed by the pump. The cause of the noise rise has previously been interpreted as loss of phaselock. However, the power spectra of the time-derivative of the phase show this still to be locked in the region of positive gain. Furthermore, computations of the Lyapunov exponents show one to be positive in the region where gain occurs, reaching a maximum value at the parameters corresponding to maximum gain. We therefore conclude that chaotic noise is indeed present in Josephson junction parametric amplifiers where low-impedance devices such as microbridges with negligible capacitance are used as the active elements.