Quantum sensing of a coherent single spin excitation in a nuclear ensemble

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.