Accessing an ensemble of coherently interacting objects at the level of single quanta via a proxy qubit is transformative in the investigations of emergent quantum phenomena. An isolated nuclear spin ensemble is a remarkable platform owing to its coherence, but sensing its excitations with single spin precision has remained elusive. Here we achieve quantum sensing of a single nuclear-spin excitation (a nuclear magnon) in a dense ensemble of approximately 80,000 nuclei. A Ramsey measurement on the electron proxy qubit enables us to sense the hyperfine shift induced by a single nuclear magnon. We resolve multiple magnon modes distinguished by atomic species and spin polarity via the spectral dependence of this hyperfine shift. Finally, we observe the time-dependent shift induced by collective Rabi oscillations, revealing the competition between the buildup of quantum correlations and decoherence in the ensemble. These techniques could be extended to probe the engineered quantum states of the ensemble such as long-lived memory states.

通过代理量子比特在单个量子水平上访问一组相干相互作用的物体对于新兴量子现象的研究具有变革性。由于其相干性，孤立的核自旋系综是一个了不起的平台，但以单自旋精度感知其激发仍然难以捉摸。在这里，我们在大约 80,000 个核的密集集合中实现了单个核自旋激发（核磁振子）的量子传感。对电子代理量子比特的 Ramsey 测量使我们能够感知由单个核磁振子引起的超精细位移。我们通过这种超精细位移的光谱依赖性解决了由原子种类和自旋极性区分的多个磁振子模式。最后，我们观察到由集体拉比振荡引起的时间相关偏移，揭示了量子相关性的建立与系综中的退相干之间的竞争。这些技术可以扩展到探测整体的工程量子态，例如长寿命的记忆态。