The Dielectric Dipper: a differential technique to measure dielectric loss tangents with high sensitivity

Dielectric loss is suspected to be a major contributor limiting state-of-the-art superconducting qubit lifetimes. Recent experiments imply upper bounds on bulk dielectric loss tangents on the order of 10^-7, but because these inferences are drawn from fully fabricated devices with many loss channels, it is difficult to know the actual dielectric loss tangents with a high degree of certainty.  We have devised a method capable of separating and resolving dielectric loss with a sensitivity on the order of 10^-8. We call our method the Dielectric Dipper, as the method involves the in-situ insertion of a dielectric sample into a high-quality cavity mode. Continuous variation of the sample’s participation in the cavity mode enables a highly selective differential measurement of dielectric loss. Our method probes the low-power behavior of dielectrics at cryogenic temperatures without the need for lithographic processes. This enables controlled comparison of isolated substrates and processing techniques. Such comparisons will inform designs and practices to better minimize dielectric loss. We present experimental comparisons of common dielectric substrates measured using this method.