  A coherent ensemble of spins interfaced with a proxy qubit is an attractive
platform to create many-body coherences and probe the regime of collective
excitations. An electron spin qubit in a semiconductor quantum dot is a
realisation of such an interface to a dense nuclear spin ensemble consisting of
multiple high-spin species. To date, this electron has relayed properties of
its nuclear environment via the statistics of its mean-field interaction with
the total nuclear polarisation, namely its mean and variance. Here, we
demonstrate a species-selective spin-state reconstruction of a nuclear ensemble
that instead exploits its response to the electron-mediated injection of
collective nuclear excitations. For the accessible range of optically prepared
mean fields, the reconstructed populations evidence a non-thermal, correlated
nuclear state. The sum over reconstructed species-resolved polarisations
exceeds threefold our prepared mean field. This stark deviation follows from a
spin ensemble that contains coherences, and manifests the formation of a dark
many-body state.
