Quantifying surface losses in superconducting aluminum microwave resonators

TL;DR

This study quantifies surface two-level system losses in superconducting aluminum resonators, revealing that native aluminum oxide significantly limits coherence, with surface treatments only partially mitigating these effects.

Contribution

It provides a detailed analysis of surface TLS-induced losses in aluminum resonators and compares their impact to that in tantalum-based qubits, highlighting the role of native oxide.

Findings

Surface TLSs in native AlOx dominate aluminum resonator losses.

HF treatment removes AlOx but oxide regrows quickly, limiting improvements.

Surface TLSs contribute approximately 27% to qubit relaxation rates.

Abstract

The recent realization of millisecond-scale coherence with tantalum-on-silicon transmon qubits showed that depositing the Al/AlOx/Al Josephson junction in a high purity, ultrahigh vacuum environment was critical for achieving lifetime-limited coherence, motivating careful examination of the aluminum surface two-level system (TLS) bath. Here, we measure the microwave absorption arising from surface TLSs in superconducting aluminum resonators, following methodology developed for tantalum resonators. We vary film and surface properties and correlate microwave measurements with materials characterization. We find that the lifetimes of superconducting aluminum resonators are primarily limited by surface losses associated with TLSs in the 2.7 nm-thick native AlOx. Treatment with 49% HF removes surface AlOx completely; however, rapid oxide regrowth limits improvements in surface loss and long term device stability. Using these measurements we estimate that TLSs in aluminum interfaces contribute around 27% of the relaxation rate of state-of-the-art tantalum-on-silicon qubits that incorporate aluminum-based Josephson junctions.