The quantum coherence and gate fidelity of electron spin qubits in semiconductors are often limited by nuclear spin fluctuations. Enrichment of spin-zero isotopes in silicon markedly improves the dephasing time T2*, which, unexpectedly, can extend two orders of magnitude beyond theoretical expectations. Using a single-atom ³¹P qubit in enriched ²⁸Si, we show that the abnormally long T2* is due to the freezing of the dynamics of the residual ²⁹Si nuclei, caused by the electron-nuclear hyperfine interaction. Inserting a waiting period when the electron is controllably removed unfreezes the nuclear dynamics and restores the ergodic T2* value. Our conclusions are supported by a nearly parameter-free modeling of the ²⁹Si nuclear spin dynamics, which reveals the degree of backaction provided by the electron spin. This study clarifies the limits of ergodic assumptions in nuclear bath dynamics and provides previously unidentified strategies for maximizing coherence and gate fidelity of spin qubits in semiconductors.

半导体中电子自旋量子位的量子相干性和栅极保真度通常受到核自旋波动的限制。硅中自旋零同位素的富集显着改善了移相时间Ť2*，这出乎意料地可以超出理论预期两个数量级。使用富集²⁸ Si中的单原子³¹ P量子位，我们发现异常长Ť2*这是由于电子-核超精细相互作用引起的残留²⁹ Si核动力学的冻结所致。在电子被可控地去除时插入等待时间会冻结核动力学并恢复遍历Ť2*值。我们的结论得到了²⁹ Si核自旋动力学几乎无参数的建模的支持，该模型揭示了电子自旋提供的反作用程度。这项研究阐明了遍历性假设在核浴动力学中的局限性，并提供了以前无法确定的策略来最大化半导体中自旋量子位的相干性和门保真度。