Janka Biznárová, Avdelningen för Kvantteknologi

Sammanfattning: In this work I present a systematic improvement of relaxation times in superconducting transmon qubits fabricated on silicon substrates using aluminium thin films. I demonstrate qubit relaxation times as long as 500 μs, with an averaged value of 270 μs for the best qubit. I achieve this improvement by mitigating the dielectric loss caused by parasitic two-level systems (TLS) present at the material interfaces within the device. This reduction of dissipation is mirrored in quality factor measurements of coplanar waveguide resonators, which also confirm that the coherence of our devices is still limited by the presence of TLS dielectric loss.

TLS are often observed in native oxides at material interfaces. Curiously, despite having stripped the silicon wafer surface of oxide by chemical treatment, prior to the aluminium deposition, I detect a distinct presence of oxygen at the substrate-metal interface through time-of-flight secondary ion mass spectrometry (ToF-SIMS) analysis. Given the high loss contribution of the substrate-metal interface, a reduction of dielectric loss in this area has a high impact on the quantum coherence of these devices. I demonstrate how modifying the film growth parameters can lead to a diminished presence of oxygen (and aluminium oxide) within the aluminium film. My experimental results show a correlation between the amount of oxygen at the silicon-aluminium interface, and qubit coherence.

[*] Janka Biznárová et al. (2023) "Mitigation of interfacial dielectric loss in aluminum-on-silicon superconducting qubits.” Preprint

Supervisors: Jonas Bylander, Anita Fadavi Roudsari, and August Yurgens
Discussion leader: Prof. Sergey Kubatkin