  The measurement of single quanta in a collection of coherently interacting
objects is transformative in the investigations of emergent quantum phenomena.
An isolated nuclear-spin ensemble is a remarkable platform owing to its
coherence, but detecting its single spin excitations has remained elusive.
Here, we use an electron spin qubit in a semiconductor quantum dot to sense a
single nuclear-spin excitation (a nuclear magnon) with 1.9-ppm precision via
the 200-kHz hyperfine shift on the 28-GHz qubit frequency. We demonstrate this
single-magnon precision across multiple modes identified by nuclear species and
polarity. Finally, we monitor the coherent dynamics of a nuclear magnon and the
emergence of quantum correlations competing against decoherence. A direct
extension of this work is to probe engineered quantum states of the ensemble
including long-lived memory states.
