  Quantum networks require quantum nodes with coherent optical interfaces and
multiple stationary qubits. In terms of optical properties, semiconductor
quantum dots are highly compelling, but their adoption as quantum nodes has
been impaired by the lack of auxiliary qubits. Here, we demonstrate a
functional quantum register in a semiconductor quantum dot leveraging the
dense, always-present nuclear spin ensemble. We prepare 13,000 host nuclear
spins into a single many-body dark state to operate as the register logic state
$|0\rangle$. The logic state $|1\rangle$ is defined as a single nuclear magnon
excitation, enabling controlled quantum-state transfer between the electron
spin qubit and the nuclear magnonic register. Using 130-ns SWAP gates, we
implement a full write-store-retrieve-readout protocol with 68.6(4)% raw
overall fidelity and a storage time of 130(16) $\mu$s in the absence of
dynamical decoupling. Our work establishes how many-body physics can add
step-change functionality to quantum devices, in this case transforming quantum
dots into multi-qubit quantum nodes with deterministic registers.
