CONSTANT VOLTAGE STEPS IN JOSEPHSON JUNCTION SERIES ARRAYS AT 10 K.

NIST is collaborating with Japan's Electrotechnical Institute, which has recently become part of the National Institute of Advanced Industrial Science and Technology (AIST), to develop a higher temperature programmable voltage standard, so that this system can operate with a practical cryocooler. NIST has provided fabrication development advice and circuit designs and circuit testing. After considerable improvements in their fabrication process over the last two years, including the addition of two planarization steps and the addition of NbN wiring, the Japanese group has made the first all-NbN arrays with useful operating margins. Arrays of 4096 junctions have sufficient uniformity at a 10 K operating temperature to generate constant voltage steps.

NIST hopes to continue this collaboration with AIST and begin stacking the junctions for lumped arrays. The uniformity is also good enough to begin designing circuits with 33 000 (unstacked) junctions for 10 K programmable voltage standard circuits.