2025/03/18 by Bafia, D., Abdisatarov, B., Pilipenko, R. +4 · 1 citation
#FOS: Physical sciences #Quantum Physics (quant-ph) #Superconductivity (cond-mat.supr-con)
paper · doi:10.48550/arxiv.2503.14616
Trapped magnetic vortices in niobium introduce microwave losses that degrade the performance of superconducting resonators. While such losses have been extensively studied above 1~K, we report here their direct quantification in the millikelvin and low-photon regime relevant to quantum devices. Using a high-quality factor 3-D niobium cavity cooled through its superconducting transition in controlled magnetic fields, we isolate vortex-induced losses and find the resistive component of the sensitivity to trapped flux S to be approximately 2~nΩ/mG at 10~mK and 6~GHz. The decay rate is initially dominated by two-level system (TLS) losses from the native niobium pentoxide, with vortex-induced degradation of T1 occurring above Btrap∼~50~mG. In the absence of the oxide, even 10~mG of trapped flux limits performance Q0∼~1010, or T1∼~350~ms, underscoring the need for stringent magnetic shielding. The resistive sensitivity S decreases with temperature and remains largely field-independent, whereas the reactive component, S', exhibits a maximum near 0.8~K. These behaviors are well modeled within the Coffey-Clem framework in the zero-creep limit, under the assumption that vortex pinning is enhanced by thermally activated processes. Our results suggest that niobium-based transmon qubits can tolerate vortex-induced dissipation at trapped field levels up to several hundred mG, but achieving long coherence times still requires careful magnetic shielding to suppress lower-field losses from other mechanisms.