Shot noise and single Josephson junctions

J. Delahaye, P. Hakonen, T. Heikkilä, R. Lindell, M. Paalanen, L. Roschier, M. Sillanpää,  E. Sonin, and T. Yamaguchi

We have made careful measurements of conductance versus current for solitary, resistively confined small Josephson junctions. Our results show, for the first time, that the Cooper pair blockade is strongly sensitive to the non-Gaussian nature of shot noise. In our measurements, in which shot noise was induced by quasiparticle current in a nearby SIN-junction, we find a linear decrease in zero bias conductance with increasing shot noise power as well as an asymmetry of IV curves that depends on the sign of the applied current. Our results provide evidence of a ratchet effect induced by shot noise. Both the asymmetry and the ratchet effect are consequences of the non-Gaussian nature of shot noise.

These investigations of Coulomb blockade can be applied into detection of small current noise sources at the level of 0.2 fA/. The high sensitivity is achieved thanks to the large band width, ~ 1/RC, of a detector junction. The voltage resolution, assuming perfect capacitive coupling from a noise source, is on the order of 5.10-11 V/. This sensitivity is sufficient to measure, for example, back action noise from a superconducting SET.