| Quantum computation has the potential to provide an efficient means of solving certain problems which are intractable on classical machines. Superconducting electronics may enable a solid state quantum computer of macroscopic dimensions. The first step is to demonstrate macroscopic quantum coherence in a single superconducting rf-SQUID qubit, manifest as the tunneling of magnetic flux in and out of the SQUID loop, or equivalently, the phase across the junction alternately increasing and decreasing by 2π. This will likely require improvements in the current fabrication technology, as it will require submicron junctions with extremely low leakage current.; We propose a scheme to experimentally demonstrate macroscopic quantum coherence in the rf-SQUID qubit by speeding up the time evolution of the system by momentarily suppressing the junction critical current through the application of a series of single flux quantum (SFQ) pulses. The precise timing of the perturbing pulses and the subsequent measurement of the qubit state would be performed using rapid single flux quantum (RSFQ) circuitry.; Conventional digital electronics based on the single flux quantum offers very low power dissipation and high operating frequency. These same features, however, make timing of clock and data signals difficult. Circuits with recurrent data paths, such as the circular shift register (CSR) have the most stringent timing requirements.; A 64-bit CSR composed of approximately 425 Josephson junctions was designed and successfully demonstrated. The CSR has an area of 0.8 mm2 and consumes approximately 80 μW of power. The circuit showed correct operation at low speed with a critical margin of ±7.5%, and operated correctly at frequencies up to 18 GHz.; The bit-error rate (BER) of the 64-bit CSR, i.e. the probability that an error in circuit operation will occur during any single circulation of the data through all 64 register stages, was measured as a function of clock frequency, and the currents supplied to the clock and data paths. The error-rate curves thus obtained suggest an effective operating temperature of 8–9 K and timing jitter of approximately 290 fs per Josephson transmission line (JTL) stage. |
