Study of microwave resonances induced by bias lines of shunted Josephson junctions

TL;DR

This paper models microwave resonances caused by bias lines in shunted Josephson junctions, demonstrating how to predict and mitigate unwanted resonances in superconducting circuits.

Contribution

It introduces an electrical LCL T-model for bias line resonances and explores how resistor placement affects resonance damping in Josephson junction circuits.

Findings

Resonance observed at 230 GHz in experiments.

The LCL T-model accurately predicts microwave behavior.

Resistor placement can effectively damp unwanted resonances.

Abstract

Bias lines routed over a ground plane naturally form microstrip lines associated with the presence of a capacitance. This can lead to unwanted resonances when coupled to Josephson junctions. This work presents an electrical model of a shunted Josephson junction with its bias lines and pads, fabricated with the 1 kA/cm$^2$ RSFQ niobium process of the FLUXONICS Foundry. A compact LCL T-model is used to simulate the microwave behavior of the bias line, predict resonances and design resonance-free superconducting circuits. The I-V characteristics of three shunted Josephson junctions have been obtained from time-domain simulations done with JSIM and show a good match with the global behavior and experimentally observed resonance at 230 GHz, measured at 4.2 K. The influence of the position and value of a series resistor placed on bias lines is studied to damp unwanted resonances at the junction.