Low error operation of a 4 stage single flux quantum shift register built with Y-Ba-Cu-O bicrystal Josephson junctions

We fabricated a 4 stage single flux quantum shift register with YBa₂Cu₃O_x (YBCO) bicrystal Josephson junctions and demonstrated the circuit´s low error operation by using a computer controlled digital measurement set-up. The circuit was operated at 65 K. Binary data sequences of “1000”, “1010”, “1011,” and “1111” were successfully loaded and shifted in the circuit. The two read SQUIDs placed next to each side of the shift register were used to sense all the individual data states. By operating the circuit with the proper current pulses, we observed no errors during 16 hours, which is equivalent to 21,000 error-free data shifts. We also found that temperature dependent inductance and junction critical currents limit the operating temperature range of the circuit, and the effective thermal noise temperature can be lower than 100 K