CC BY 4.0 UnportedStruck, TomHollmann, ArneSchauer, FloydFedorets, OlexiySchmidbauer, AndreasSawano, KentarouRiemann, HelgeAbrosimov, Nikolay V.Cywiński, ŁukaszBougeard, DominiqueSchreiber, Lars R.2021-11-292021-11-292020https://oa.tib.eu/renate/handle/123456789/7548https://doi.org/10.34657/6595We identify the dominant source for low-frequency spin qubit splitting noise in a highly isotopically-purified silicon device with an embedded nanomagnet and a spin echo decay time T2echo = 128 µs. The power spectral density (PSD) of the charge noise explains both, the clear transition from a 1/f2- to a 1/f-dependence of the splitting noise PSD as well as the experimental observation of a decreasing time-ensemble spin dephasing time, from T2*˜ 20 µs, with increasing measurement time over several hours. Despite their strong hyperfine contact interaction, the few 73Ge nuclei overlapping with the quantum dot in the barrier do not limit T2*, likely because their dynamics is frozen on a few hours measurement scale. We conclude that charge noise and the design of the gradient magnetic field are the key to further improve the qubit fidelity in isotopically purified 28Si/SiGe. © 2020, The Author(s).enghttps://creativecommons.org/licenses/by/4.0/530low-frequency spin qubit splitting noisenanomagnetpower spectral density (PSD)Article