IDENTIFYING THE SOURCE OF 1/F NOISE IN SQUIDS MADE FROM HIGH-TEMPERATURE SUPERCONDUCTORS

We compare model predictions for 1/f noise in dc SQUIDs with experimental data and show that the 1/f noise in the our devices results from fluctuations in the magnitude of the critical current of the Josephson junctions that form the SQUID. Operating the SQUID while the bias current direction was being alternated canceled the 1/f noise power from these fluctuations and reduced the output noise power by two orders of magnitude. Using this process, we have measured the 1/f noise in several SQUIDs to be 1 x 10(-7) PHI-0(2)/Hz at 0.1 Hz and 77 K, the lowest value reported to date.