Loss Dependence on Geometry and Applied Power in Superconducting Coplanar Resonators

TL;DR

This study investigates how geometry and applied power influence loss mechanisms in superconducting coplanar resonators, revealing complex loss behavior not solely due to surface two-level systems, with implications for quantum device coherence.

Contribution

It provides experimental analysis of loss dependence on geometry and power in superconducting resonators, highlighting non-uniform loss mechanisms beyond traditional models.

Findings

Loss decreases over four decades of photon number.

Loss behavior is not fully explained by uniform two-level system models.

Different geometries exhibit distinct loss characteristics.

Abstract

The loss in superconducting microwave resonators at low-photon number and low temperatures is not well understood but has implications for achievable coherence times in superconducting qubits. We have fabricated single-layer resonators with a high quality factor by patterning a superconducting aluminum film on a sapphire substrate. Four resonator geometries were studied with resonant frequencies ranging from 5 to 7 GHz: a quasi-lumped element resonator, a coplanar strip waveguide resonator, and two hybrid designs that contain both a coplanar strip and a quasi-lumped element. Transmitted power measurements were taken at 30 mK as a function of frequency and probe power. We find that the resonator loss, expressed as the inverse of the internal quality factor, decreases slowly over four decades of photon number in a manner not merely explained by loss from a conventional uniform spatial distribution of two-level systems in an oxide layer on the superconducting surfaces of the resonator.