Materials loss characterization using superconducting microwave resonators

Continued improvements in the relaxation and coherence times of niobium and tantalum superconducting qubits have been recently demonstrated. Nonetheless, the device performance is still limited by materials loss, such as two-level system defects at poorly controlled interfaces. In this work, we characterized Ta/Nb lumped element resonators with varying material composition to compare different loss mechanisms between tantalum and niobium circuits. We also employed resonator design with diverse impedance to identify the dominant regime of defect coupling. Critical parameters such as surface participation ratio at interfaces are extracted through two-step finite-element simulation and correlate with the results from extensive materials characterization.