High Sensitivity Loss Tangent Measurement of C-Plane Sapphire at mK with a High Q Nb Superconducting Radio-frequency Cavity

We present results on the dielectric loss tangent of c-plane sapphire at the temperatures and fields relevant for the superconducting quantum computing regime. We utilize an ultra-high quality factor Nb superconducting resonant frequency cavity which enables unprecedented accuracy. To identify potential mechanisms that drive RF losses, the power and temperature dependence of the sapphire dielectric loss tangent are investigated. We find that the sapphire substrate degrades the quantum coherence times of our current 2-D and 3-D quantum devices, motivating further studies into identifying minimum material specifications which further reduce RF losses.