A double-flux-quantum amplifier with a single flux-biasing line

A Double-Flux-Quantum (DFQ) amplifier can multiply its input voltage with quantum accuracy using DFQ generation at under-damped Josephson junctions. We conducted research on digital-to-analogue converters that were implemented using DFQ amplifiers for metrological applications. Conventional DFQ amplifiers were usually operated using two flux-biasing lines. In this study, we redesigned the circuit layout to realize operation using a single flux-biasing line. We designed a new layout of a 20-fold DFQ amplifier and fabricated it using a 2.5-kA/cm² Nb/AlO_x/Nb process. The measurement was performed in liquid He. The Single-Flux-Quantum (SFQ) pulse train was fed using an over-biasing method, and the input-output characteristics are acquired using an oscilloscope. When the magnetic flux bias current I _fb1 was not supplied, the maximum input voltage V _inmax was 68 μV, which was 4.2 times larger than the value of 17 μV that was observed in our previous study. V _inmax was enhanced to 120 μV when we supplied an I _fb1 of 1.00 mA. (paper)