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 T₂\,echo T 2 echo = 128 µs. 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 T₂*≈ T 2 * ≈ 20 µs, with increasing measurement time over several hours. Despite their strong hyperfine contact interaction, the few 73 Ge nuclei overlapping with the quantum dot in the barrier do not limit T₂* 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 28 Si/SiGe.