NbN-based tunnel-type π-junctions for low-power half-flux-quantum circuits.

Abstract We have developed half-flux-quantum (HFQ) circuits using all-π-junctions formed from an NbN/AlN/PdNi/NbN (SIFS) structure. The circuits were fabricated using a novel process that incorporated a ground plane on top of the chip, enabling the epitaxial growth of NbN-based junctions from the substrate. The π-state of the junctions was demonstrated through a half-flux-quantum shift. Notably, these π-junctions exhibited self-overdamped current-voltage characteristics, enabling them to function as switching components without the need for shunt resistors. The elimination of shunt resistors and the high sheet inductance of NbN are expected to enhance the density of HFQ circuits. To evaluate the performance and power consumption of the all-π-junctions HFQ circuits, we designed and fabricated an HFQ toggle flip-flop (HFQ-TFF) circuit utilizing π-π-π SQUIDs as the fundamental components. Our findings reveal that the NbN-based HFQ-TFF circuit correctly operates as a frequency divider while consuming only around 30% of the power compared to single-flux-quantum TFF (SFQ-TFF) circuits. These results suggest that the HFQ circuit using SIFS-π-junctions has promising potential for integrated circuits requiring low-power consumption at cryogenic temperatures, such as qubit control.