Identifying TLS losses from Amorphous Oxides using Aluminum Superconducting Resonator

Tantalum-based superconducting qubits have demonstrated record-breaking coherence times (T₁) but continue to be limited by saturable two-level systems (TLSs) that reside in multiple parts of the devices [1]. From the several possible sources of TLSs, amorphous oxides are believed to host multiple mechanisms for TLS loss [2]. Apart from the bulk sapphire substrate, microwave properties of these oxides can be studied at four interfaces: metal-air, substrate-air, metal-substrate, and most importantly, the Josephson junction. Each interface has a different participation ratio, manifesting in loss tangents that present themselves as different limitations on Q_int, the internal quality factor of the device. Understanding how disordered oxides play a role in microwave loss in superconducting resonators at each of these interfaces is paramount to increasing the coherence in superconducting qubits. Here we study the role of structure and chemical composition of the amorphous oxide used in superconducting qubits, native aluminum oxide, and link these properties to microwave loss. We can then improve the native oxide through fabrication and post-processing methods to decrease losses in Al-based Josephson junctions.

[1] Place, A.P.M., Rodgers, L.V.H., Mundada, P. et al. New material platform for superconducting transmon qubits with coherence times exceeding 0.3 milliseconds, Nat. Commun. 12, 1779 (2021).

[2] Crowley, K.D, McLellan, R.A., Dutta, A. et al. Disentangling Losses in Tantalum Superconducting Circuits, Phys. Rev. X 13, 041005 (2023).