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 echo 2 = 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 T Ã 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 T Ã 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.

Valley splitting is a key feature of silicon-based spin qubits. Quantum dots in Si/Si x Ge 1−x heterostructures reportedly suffer from a relatively low valley splitting, limiting the operation temperature and the scalability of such qubit devices. Here, we demonstrate a robust and large valley splitting exceeding 200 μeV in a gate-defined single quantum dot, hosted in molecular-beam-epitaxy-grown 28 Si/Si x Ge 1−x. The valley splitting is monotonically and reproducibly tunable up to 15% by gate voltages, originating from a 6-nm lateral displacement of the quantum dot. We observe static spin relaxation times T 1 > 1 s at low magnetic fields in our device containing an integrated nanomagnet. At higher magnetic fields, T 1 is limited by the valley hotspot and by phonon noise coupling to intrinsic and artificial spin-orbit coupling, including phonon bottlenecking.