Si/SiGe**

The manipulation fidelity of a single-electron spin qubit gate-confined in a ²⁸Si/SiGe quantum dot has recently been drastically improved by nuclear isotope purification [1]. Here, we characterize a ²⁸Si/SiGe device with an embedded nanomagnet, a large valley splitting (> 0.2 meV [2]), and a remaining ²⁹Si concentration of only 60 ppm in the strained silicon quantum well layer, which is grown by molecular beam epitaxy. We identify the combination of charge noise and gradient magnetic field as the dominant source of low frequency qubit detuning noise. The power spectral density (PSD) of the charge noise explains the frequency-dependence of the detuning noise PSD, as well as the observation of a decreasing time-ensemble spin dephasing time with increasing measurement time over several hours [3]. We also comment on the role of the remaining nuclear spins in the SiGe barrier on the qubit dephasing and the origin of the observed large valley splitting.
[1] J. Yoneda, et al., Nat. Nanotechnol. 13, 102 (2018).
[2] A. Hollmann et al., arXiv:1907.04146.
[3] T. Struck et al., arXiv:1909.11396.