Effects of surface treatments and packaging on transmon qubits

The last two decades have seen significant advances in the coherence times of superconducting qubits. However, further progress in transmon coherence times seems to require substantial effort in understanding the remaining limitations due to material interfaces and imperfections, which give rise to two level fluctuators [1-3]. Often, ion milling is an integral part of Josephson junction fabrication and possibly damages the material surface, but its impact on qubit coherence is not well understood and needs experimental investigation.
Here, we present our work towards understanding decoherence mechanisms in flux-tunable transmon qubits. We study the effects of qubit packaging under UHV and controlled atmospheres on the qubit coherence. In addition, we analyze the impact of surface treatments, such as UV light exposure and ion milling, on the qubit parameters. For further insight, qubit coherence and parameter fluctuations are investigated by tracking the coherence times and transition frequencies of multiple qubits over several hours. The resulting noise spectra for different packaging and treatment methods are then compared.

[1] J. M. Gambetta et al., IEEE Trans. Appl. Supercond. 27, (2016)
[2] S. De Graaf et al., Nat. Commun. 9, (2018)
[3] C. Müller et al., arXiv:1705.01108 (2017)