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 (JJ). We designed a superconducting frequency divider that uses a single NbN JJ as a pulse generator, several NbN Josephson transmission lines, an NbN Toggle Flip-flop, and a load. By comparing the bias voltage V in of the pulse generator with the output voltage V out of the load, we determined if the circuit was working correctly and calculated the maximum operating frequency f max . Additionally, we employed JSICsim software for simulations to analyze the impact of key parameters of the NbN JJ, such as critical current density J c , gap voltage V g , characteristic voltage V c , quality factor Q , and specific capacitance C s , on f max . The simulation results demonstrate that f max increases with increasing J c and Q . Specifically when J c exceeds 300 kA cm −2 and Q surpasses 4, the superconducting frequency divider can achieve a f max of 1 THz. Furthermore, we successfully fabricated a superconducting frequency divider using a 10 kA cm −2 process and tested a f max of 260 GHz, with a deviation of approximately 6% from the simulation results.