The dielectric dipper: experimental comparisons of common dielectric substrates in isolation*

We have developed a differential probe of bulk dielectric loss with a sensitivity on the order of 10^-8. The method uses a 3D cavity to probe the low-power behavior of dielectrics at cryogenic temperatures without the need for lithographic processing. Using this technique, we investigate various substrate materials and processes with the aim to better quantify dielectric loss, understand its origin, and determine how it can be mitigated in fabrication of superconducting quantum devices. So far, the method has identified bulk loss as a major source of decoherence of transmon qubits on EFG sapphire. With a bulk loss tangent of 6*10^-8, bulk loss in EFG sapphire places a limit on transmon lifetime of about 800 us. We have also identified HEM sapphire and Float-zone silicon as promising materials for use as substrates to further extend superconducting qubit lifetimes.