The dependence of operating frequency of NbN toggle flip-flop on parameters of NbN josephson junctions

Abstract

We performed a comprehensive investigation of the relationship between the maximum operating frequency of the NbN superconducting frequency divider and the key parameters of the NbN Josephson junction. We designed a superconducting frequency divider that uses a single NbN Josephson junction as a pulse generator, several NbN Josephson transmission lines (JTLs), an NbN Toggle Flip-flop (TFF), and a load. By comparing the bias voltage Vin of the pulse generator with the output voltage Vout of the load, we determined if the circuit was working correctly and calculated the maximum operating frequency fmax. Additionally, we employed JSICsim software for simulations to analyze the impact of key parameters of the NbN Josephson junction, such as critical current density Jc, gap voltage Vg, characteristic voltage Vc, quality factor Q, and specific capacitance Cs, on fmax. The simulation results demonstrate that fmax increases with increasing Jc and Q. Specifically when Jc exceeds 300 kA/cm2 and Q surpasses 4, the superconducting frequency divider can achieve a fmax of 1 THz. Furthermore, we successfully fabricated a superconducting frequency divider using a 10 kA/cm2 process and tested a fmax of 260 GHz, with a deviation of approximately 6% from the simulation results.