Ultra low noise DC SQUIDS

Three different Josephson weak link technologies have been used to successfully fabricate ultra low noise dc SQUIDs: (1) all-Nb tunnel junctions using electronbeam lithography to define minimum linewidths < 1 μm; (2) Pb alloy tunnel junctions using photolithography to define minimum linewidths approx. equal to 2.5 μm; and (3) Nb weak links (nanobridges) formed from electron-beam contamination lithography with minimum linewidths approx. <= 30 nm. New techniques were developed which, for the first time, directly record SQUID voltage noise and small signal flux sensitivity as a function of both current and flux bias. In this paper, detailed measurements on the three types of SQUIDs will be presented that demonstrate that the intrinsic energy resolution PHIsub(n)2/2L (PHIsub(n) is the flux noise and L is the SQUID inductance) can approach the limit of order Planck's constant h set by the uncertainty principle. The use of these devices as practical sensors will be discussed. Fabrication details of the junctions and nanobridges are described elsewhere. (orig.)