Low frequency noise in Josephson junctions

Using a high resolution SQUID voltmeter, we have measured the spectrum of low frequency voltage fluctuations across a thin-film Josephson tunnel junction biased at a constant current I greater than the junction critical current Ic. We find that the frequency dependence of the voltage spectrum may be accurately represented by the power law over the frequency range of our data: Hz. The dependence of the magnitude of the spectra at any single frequency upon the value of the bias current I and upon the sample temperature T supports our hypothesis that the observed voltage fluctuations arise from a modulation of the junction critical current Icby equilibrium, thermodynamic temperature fluctuations in the active junction volume. We are able to interpret our measurements in terms of the semi-empirical theory of Clarke and Voss for the low frequency fluctuation spectrum of systems obeying a diffusion equation. This interpretation provides design criteria which may prove useful in reducing the level of long-term drifts in systems employing Josephson tunnel junctions.