Measurement of the Low-Temperature Loss Tangent of High-resistivity Silicon Wafers with High-Q Superconducting Resonators

In this Letter, we present the first direct loss tangent measurement of high resistivity (100) silicon wafers in the temperature range ~70 mK to 1 K. The measurement was performed by means of an innovative resonant technique taking advantage of a high quality factor superconducting niobium resonator, that allows to achieve unprecedented level of accuracy. We report Si loss tangent values at the lowest temperature one order of magnitude larger than what typically expected, and we observe non-monotonic trend of the loss tangent as a function of temperature. With this study, we established a new capability to directly measure the loss tangent of insulating materials with high accuracy and precision. This technique will allow to identify substrates and materials with low losses aiming to maximize coherence in quantum devices.