Demonstration of a 120 GHz single-flux-quantum shift register circuit based on a 10 kA cm⁻²Nb process

Abstract

Designs and test results for a single-flux-quantum (SFQ) eight-bit shift register circuit operating at frequencies above 100 GHz are described. The high-speed performance was realized by introducing a planarized 10 kA cm−2 Nb fabrication process as an advanced process and by adopting middle-damped junctions with McCumber parameters βc of 1.8–2.9 in the circuit. The middle-damped junctions were used to reduce the repulsion between SFQ pulses and to adjust the timing. The circuit was designed using a cell-based design method and was tested by constructing an on-chip test system with a ladder-type four-bit high-frequency clock generator. We confirmed its correct operations up to 120 GHz.