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Witnessing quantum correlations in a nuclear ensemble via an electron spin qubit
Nature Physics  (IF20.034),  Pub Date : 2021-10-14, DOI: 10.1038/s41567-021-01344-7
Dorian A. Gangloff, Leon Zaporski, Jonathan H. Bodey, Clara Bachorz, Daniel M. Jackson, Gabriel Éthier-Majcher, Constantin Lang, Edmund Clarke, Maxime Hugues, Claire Le Gall, Mete Atatüre

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 can act as such an interface to the dense nuclear spin ensemble within the quantum dot consisting of multiple high-spin atomic species. Earlier work has shown that the electron can relay properties of its nuclear environment through the statistics of its mean-field interaction with the total nuclear polarization, namely its mean and variance. Here, we demonstrate a method to probe the spin state of a nuclear ensemble that exploits its response to collective spin excitations, enabling a species-selective reconstruction beyond the mean field. For the accessible range of optically prepared mean fields, the reconstructed populations indicate that the ensemble is in a non-thermal, correlated nuclear state. The sum over reconstructed species-resolved polarizations exceeds the classical prediction threefold. This stark deviation follows from a spin ensemble that contains inter-particle coherences, and serves as an entanglement witness that confirms the formation of a dark many-body state.