Experimental Investigation Of The Threshold Curves Of A 4-Junction Squid

We have studied experimentally the variation of critical current with control current of superconducting quantum interference devices (SQUIDs). We used an integrated SQUID consisting of four Josephson junctions with nominally equal critical currents. One current (the bias current) is fed directly to the four junctions in parallel, while a second current (the control current) is coupled inductively into the SQUID loops. The fabricated SQUID functions properly as designed and exhibits the threshold curves expected from calculation. By suitable design of the SQUID, we have realized for the first time threshold curves with more than two visible minor lobes. This was previously predicted but not observed.