Radiative Losses in Superconducting Coplanar Resonators

Radiation is a potential loss mechanism in superconducting qubits. Radiation loss was studied in superconducting coplanar resonators, which are important both in coupling superconducting qubits and because they provide a simple system to quantitatively measure the resulting effects. We fabricated 8 GHz resonators and measured the resulting reduction of the high-power Q. We found it was necessary to design the resonators carefully to reduce stray coupling between the resonators so that losses would be dominated by radiation. The radiation loss is measured to be 30 times greater than predicted by a simple theoretical model, but was predicted accurately by simulation data. We attribute this to the effects of the device mount and the finite substrate height on the radiation pattern. We conclude that radiation is an unlikely decoherence mechanism for the present generation of qubits and resonators.