Ultra low noise Nb DC SQUIDs

Electron-beam lithography has been used to produce ultra low noise Nb dc SQUIDs (Superconducting QUantum Interference Devices) with Josephson elements consisting of either small area tunnel junctions or very narrow variable thickness bridges (nanobridges). Detailed voltage noise and transfer function measurements have been made as a function of temperature and flux and current bias. These measurements allow a computation of the intrinsic energy resolution ε vs bias parameters as well as comparison with models of intrinsic SQUID noise. The best ε obtained was of order Planck´s constant h. Moreover, the detailed characteristics of the SQUID voltage noise are in agreement with a small signal analysis that depends only on the current noise in each of the Josephson elements and measurable parameters.