Two-level System Loss in Amorphous Nb2O5 and Ta2O5

Superconducting transmon qubits with Ta or Ta-capped paddles seem to exhibit higher relaxation times than those of Nb, [1,2] but the exact reason for this improvement is not yet known. Ta oxide consists almost exclusively of Ta2O5, while Nb oxide includes significant amounts of suboxides [3]. The decrease in total loss could reasonably be attributed to the absence of suboxides or to some difference in the pentoxides themselves. By blanket depositing amorphous films of Nb2O5 and Ta2O5 onto high Qi CPW resonators, we directly compare bulk TLS loss of the pentoxides, demonstrating that amorphous Ta2O5 exhibits lower loss than that of Nb2O5.

[1] A. P. M. Place et al., New material platform for superconducting transmon qubits with coherence times exceeding 0.3 milliseconds, Nature Communications 12, (2021).

[2] M. Bal et al., Systematic improvements in transmon qubit coherence enabled by niobium surface encapsulation, Npj Quantum Inf 10, 1 (2024).

[3] J.-S. Oh et al., Structure and Formation Mechanisms in Tantalum and Niobium Oxides in Superconducting Quantum Circuits, ACS Nano (2024).