Comparison of Nb and Ta Pentoxide Loss Tangents for Superconducting Quantum Devices

TL;DR

This study compares the dielectric loss of Nb2O5 and Ta2O5 pentoxides in superconducting qubits, revealing that Nb2O5 exhibits higher loss, which impacts qubit performance.

Contribution

It provides a direct comparison of Nb and Ta pentoxide loss tangents, highlighting the lower loss in Ta2O5 for superconducting quantum devices.

Findings

Nb2O5 has about 30% higher TLS loss than Ta2O5.

Ta wiring may lead to lower dielectric loss in qubits.

Native Nb2O5 contributes significantly to qubit loss.

Abstract

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 identical coplanar waveguide resonators. The two-level system (TLS) loss in Nb2O5 is determined to be about 30% higher than that of Ta2O5. 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.