Characterization of losses in dielectric substrates for quantum computing devices using tunable superconducting RF cavity resonator.

Among several quantum computing (QC) approaches, Josephson junction (JJ)-based superconducting quantum devices demonstrate high potential for scalability, control and, ultimately, provide a pathway to reaching practical quantum computing. However, crucial challenges need to be addressed to achieve this goal. For instance, one of the main factors limiting performance of widely used planar superconducting QC devices such as transmons is decoherence and loss due to unwanted interaction of a device with dielectric substrate. Contribution of the substrate to the total loss is even more significant in the case of 3D architectures where QC devices are placed inside 3D superconducting cavity resonators. In this case microwave EM field penetrates into the bulk of the substrate which causes significant degradation of the quality factor of the system device + resonator.

We present microwave loss characterization results for Si and Sapphire - two substrate materials widely used in fabrication of superconducting QC devices. We also demonstrate the experimental testbed that allows us to characterize various dielectric materials in sub-1K temperatures and wide frequency range from 5 - 8GHz. Our measurements of the loss tangent as a function of frequency, temperature and probing power help to identify the dominant loss mechanisms and select optimal materials and fabrication procedures for the next generation of QC devices.