Study of Microwave Resonances Induced by Bias Lines of Shunted Josephson Junctions

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 article presents an electrical model of a shunted Josephson junction with its bias lines and pads, fabricated with the 1 kA/cm
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rapid single flux quantum niobium process of the FLUXONICS foundry (J. Kunert, 2013), (J. Kunert, 2017). A 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 Josephson SIMulator (JSIM) (E. S. Fang and T. Van Duzer, 1989) and show a good match with the experimentally observed resonance at 230 GHz up to the normal resistance branch, 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.