Fundamental operation of single-flux-quantum circuits using coplanar-type high-T c SQUIDs

We have fabricated coplanar type dc SQUIDs using NdBa2Cu3Oy superconducting thin films and operated fundamental single-flux-quantum (SFQ) circuits. The Josephson junctions were made by the narrow-focused ion beam irradiation technique. For a 145 μm wide and 10 μm long logic SQUID having a critical current of 105 μA and an inductive parameter (βL) of 28, a store and a restore of the flux quantum have been demonstrated at temperatures of 4.2–30 K. These operations were performed with an input pulsewidth of 5 ns (5 ns was the shortest input pulse width available from our function generating equipment). These results show experimentally the possibility of high speed operation in all high-Tc superconducting digital circuits.