Noise measurements in multiloop superconducting interferometers

Noise measurements on multiloop dc SQUIDs, fabricated with Nb‐NbOx‐PbAuIn and Pb‐alloy Josephson tunnel junctions and well coupled with the external world by an input coil, we performed in flux locked loop mode. To achieve this task, we built a reliable low‐cost control electronics to detect the voltage output of the SQUID. The results show that for multiloop dc SQUIDs with single‐junction critical currents larger than 20 μA and an inductive parameter βL less than 4.5, an intrinsic white‐energy spectral density of about 100h can be obtained independently of the technology used to fabricate the junctions. Furthermore, the Nb‐NbOx‐PbAuIn dc SQUIDs have a lower 1/f noise than the Pb‐alloy ones