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 YBa2Cu3Ox ( YBCO) bicrystal Josephson junctions and demonstrated the circuit's low erro r operation by using a computer controlled digital measurement set-up, The circuit was operated at 65 K, Binary data sequences of "1000", "1010", "101 1," 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 se nse all the individual data states. By operating the circuit with the prope r current pulses, we observed no errors during 16 hours, which is equivalen t to 21,000 error-free data shifts, We also found that temperature dependen t inductance and junction critical currents limit the operating temperature range of the circuit, and the effective thermal noise temperature can be l ower than 100 K.