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- ItemLow-frequency spin qubit energy splitting noise in highly purified 28Si/SiGe(London : Nature Publ. Group, 2020) Struck, Tom; Hollmann, Arne; Schauer, Floyd; Fedorets, Olexiy; Schmidbauer, Andreas; Sawano, Kentarou; Riemann, Helge; Abrosimov, Nikolay V.; Cywiński, Łukasz; Bougeard, Dominique; Schreiber, Lars R.We 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).