Low-frequency spin qubit energy splitting noise in highly purified 28Si/SiGe

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 T-2(echo) = 128 mu s. The power spectral density (PSD) of the charge noise explains both, the clear transition from a 1/f(2)- 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 T-2* approximate to 20 mu s, with increasing measurement time over several hours. Despite their strong hyperfine contact interaction, the few Ge-73 nuclei overlapping with the quantum dot in the barrier do not limit T-2*, 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 Si-28/SiGe.