Microscopic origin of critical current fluctuations in large, small, and ultra-small area Josephson junctions

We analyze data on the critical current and normal-state resistance noise in Josephson junctions and argue that the noise in the critical current is due to a mechanism that is absent in the normal state. We estimate the noise produced by conventional two level systems in the insulating barrier and find that it agrees both in magnitude and in temperature dependence with the resistance fluctuations in the normal state but it is not sufficient to explain the critical current noise observed in large superconducting contacts. We propose a microscopic mechanism for the noise in the superconducting state in which the noise is due to electron tunneling between weak Kondo states at subgap energies. We argue that the noise produced by this mechanism gives temperature, area dependence, and intensity that agree with the data.