Operation of a High-T$_c$ Rapid Single-Flux-Quantum 4-stage Shift Register

We have designed and fabricated a single-flux-quantum(SFQ) four-stage shift register using YBCO bicrystal Josephson junctions, and tested its operations using a digital measurement set-up. The circuit consists of 4 shift register stages and a read SQUID placed next to each side of the shift register. Each SQUID was inductively coupled to the nearby shift register stage. The major obstacle in testing the circuits was the interference between the two read SQUIDs, and we could get over the problem by determining the correct operation points of the SQUID from the simultaneously measured modulation curves. Loaded data ('1' or '0') were successfully shifted from a stage to the next by a controlled current pulse injected to the bias lines located between the stages, and the corresponding correct data shifts were observed with the two read SQUIDs.