Design and Fabrication of Low-Power Single-Flux-Quantum Circuits Toward Quantum Bit Control

we report low-power single-flux-quantum (SFQ) circuits fabricated with a lowered critical current density process and low-voltage design. We selected 250 A/cm^{2} to obtain 2-10 μA Josephson junctions (JJs) easily using conventional facilities. Our numerical simulation shows that the expected operating frequency is 10 GHz and that we can reduce the bias voltage to 0.1-0.5 mV; power consumption can be reduced to around 1 nW per JJ. We newly developed a fabrication process featuring four planarized Nb layers and a Pd resistor layer. We obtained the correct operation of several basic SFQ circuit elements designed with the lowered critical currents to 1/2 of the conventional circuits at 4.2 K. We also evaluated the Josephson junction critical currents, specific capacitance, resistance, and inductance at 3 K and 300 mK. We observed an increase of 7% in the critical currents at 300 mK, while the other parameters were unchanged. We demonstrated a 0.5-mV Josephson transmission line where we lowered the critical currents to 1/10. We expect such low-power SFQ circuits to operate at 20 mK, providing classical, general-purpose digital processing near quantum bits.