Theories of the noise rise in Josephson paramps

A physically very interesting problem is the origin of the gain-proportional noise that distinguishes the zero-voltage Josephson parametric amplifiers. Many radically different theories have been proposed to explain the disturbingly high noise outputs from these devices. Here we collect the known experimental information about the "noise rise", and discuss the various theories in light of the experimental data. We also present some incompletely developed basic ideas which combine aspects of and expand upon the most successful of these theories.