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 \({T}_{2}^{\,\text{echo}}\) = 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 \({T}_{2}^{* }\approx\) 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 \({T}_{2}^{* }\), 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 ²⁸Si/SiGe.

我们确定了具有嵌入式纳米磁铁和自旋回波衰减时间\（{T} _ {2} ^ {\，\ text {echo}}的高度同位素纯化的硅器件中低频自旋qubit分裂噪声的主要来源。\）  = 128 µs。电荷噪声的功率谱密度（PSD）解释了分裂噪声PSD从1 / f ^2-到1 / f-依赖关系的清晰过渡以及减小的时间集合自旋移相的实验观察结果时间，从\（{T} _ {2} ^ {*} \大约\）开始到 20 µs，并且测量时间会增加几个小时。尽管它们具有很强的超精细接触相互作用，但与势垒中量子点重叠的少数⁷³ Ge核并没有限制\（{T} _ {2} ^ {** \\），可能是因为它们的动态被冻结在几个小时的测量范围内。我们得出结论，电荷噪声和梯度磁场的设计是进一步提高同位素纯化的²⁸ Si / SiGe中量子比特保真度的关键。