Loss mechanisms in superconducting thin film microwave resonators

TL;DR

This paper systematically analyzes internal loss mechanisms in superconducting niobium thin film microwave resonators, focusing on interface-induced two-level state losses, quasiparticle effects at higher temperatures, and eddy current losses, providing insights for optimizing resonator performance.

Contribution

It identifies and quantifies key loss mechanisms in Nb-based superconducting resonators, especially the impact of Nb/Al interfaces and substrate backside conductivity, offering guidance for improved design.

Findings

Nb/Al interfaces can cause significant TLS losses if not at current nodes.

Quasiparticle losses become relevant above 200 mK in Al-inclusive resonators.

Eddy current losses can be reduced with thicker substrates or high-conductivity metals.

Abstract

We present a systematic analysis of the internal losses of superconducting coplanar waveguide microwave resonators based on niobium thin films on silicon substrates. In particular, we investigate losses introduced by Nb/Al interfaces in the center conductor, which is important for experiments where Al based Josephson junctions are integrated into Nb based circuits. We find that these interfaces can be a strong source for two-level state (TLS) losses, when the interfaces are not positioned at current nodes of the resonator. In addition to TLS losses, for resonators including Al, quasiparticle losses become relevant above 200 mK. Finally, we investigate how losses generated by eddy currents in conductive material on the backside of the substrate can be minimized by using thick enough substrates or metals with high conductivity on the substrate backside.