Studying magnetic-field resilience of 3D transmons with thin-film AlOx Josephson junctions

Magnetic-field-resilient transmons enable sensing applications and hybrid architectures involving spin or topological qubits, as well as studying flux noise and quasiparticle loss. We investigate the effect of in-plane magnetic fields up to 1 T on the spectrum and coherence times of thin-film 3D aluminum transmons. Using a copper cavity, which is unaffected by strong magnetic fields, we can purely probe the magnetic-field response of the transmon. Our study includes single-junction and SQUID transmons. The latter allows for both careful alignment of the magnetic field and a flux-noise sensitivity analysis. As expected, qubit frequencies decrease with increasing fields, dominantly due to a suppression of the superconducting gap. Nevertheless, our thin-film transmons show enhanced magnetic-field resilience: Direct qubit operation is possible up to 650 mT, and SQUID oscillations remain visible in the cavity frequency up to 900 mT. Energy-relaxation times T1 remain at the micro-second level for the entire measurable range. Flux-sensitivity analysis of T2* and T2e shows a change but no clear freeze-out of flux noise at high fields.