Comparison of Nb and Ta pentoxide loss tangents for superconducting quantum devices

Superconducting transmon qubits are commonly made with thin-film Nb wiring, but recent studies have shown increased performance with Ta wiring. In this work, we compare the resonator-induced single photon, millikelvin dielectric loss for pentoxides of Nb (Nb2O5) and Ta (Ta2O5) in order to further understand limiting losses in qubits. Nb and Ta pentoxides of three thicknesses are deposited via pulsed laser deposition onto nominally identical coplanar waveguide resonators. The two-level system loss tangent is determined to be 0.0064 (0.0035) for Nb2O5 (Ta2O5) for a dielectric constant of 10. This work indicates that qubits with Nb wiring are affected by higher loss arising from the native pentoxide itself, likely in addition to the presence of suboxides, which are largely absent in Ta.