  Coherent excitation of an ensemble of quantum objects underpins quantum
many-body phenomena, and offers the opportunity to realize a quantum memory to
store information from a qubit. Thus far, a deterministic and coherent
interface between a single quantum system, e.g. a qubit, and such an ensemble
has remained elusive. We first use an electron to cool the mesoscopic
nuclear-spin ensemble of a semiconductor quantum dot to the nuclear
sideband-resolved regime. We then implement an all-optical approach to access
these individual quantized electronic-nuclear spin transitions. Finally, we
perform coherent optical rotations of a single collective nuclear spin
excitation corresponding to a spin wave called a nuclear magnon. These results
constitute the building blocks of a dedicated local memory per quantum-dot spin
qubit and promise a solid-state platform for quantum-state engineering of
isolated many-body systems.
