Certifying the quantumness of a nuclear spin qudit through its uniform precession

Spin precession is a textbook example of the dynamics of a quantum system that exactly mimics its classical counterpart. Here we challenge this view by certifying the quantumness of exotic states of a nuclear spin through its uniform precession. The key to this result is measuring the positivity, instead of the expectation value, of the x-projection of the precessing spin, and using a spin > 1/2 qudit, that is not restricted to semi-classical spin coherent states. The experiment is performed on a single spin-7/2 123Sb nucleus, implanted in a silicon nanoelectronic device, amenable to high-fidelity preparation, control, and projective single-shot readout. Using Schrödinger cat states and other bespoke states of the nucleus, we violate the classical bound by 19 standard deviation, proving that no classical probability distribution can explain the statistic of this spin precession, and highlighting our ability to prepare quantum resource states with high fidelity in a single atomic-scale qudit.

自旋进动是量子系统动力学的经典教材范例，其行为与经典对应体系完全一致。本研究通过核自旋的均匀进动验证核自旋奇异态的量子性，从而挑战这一既有认知。本研究的核心在于，测量进动自旋x方向投影的正性而非期望值，并采用自旋大于1/2的量子数位（qudit）——这类系统不受半经典自旋相干态的限制。实验对象为注入硅纳米电子器件中的单个自旋7/2的锑-123（¹²³Sb）原子核，该体系可实现高保真制备、精准操控与投影式单次读出。通过利用该原子核的薛定谔猫态（Schrödinger cat states）与其他定制量子态，我们的实验结果超出经典边界达19个标准差，证明不存在可解释该自旋进动统计特性的经典概率分布，同时验证了我们在单原子尺度量子数位中以高保真度制备量子资源态的能力。