Abstract
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.
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Data availability
All data that support the plots within this paper and other findings of this study are available from the corresponding authors on reasonable request.
Code availability
All code that was used in the analysis of data shown in this paper is available from the corresponding authors on reasonable request.
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Acknowledgements
We thank N. R. Cooper and D. M. Kara for helpful discussions. We acknowledge support from the US Office of Naval Research Global (N62909-19-1-2115), ERC PHOENICS (617985), EPSRC NQIT (EP/M013243/1), EU H2020 FET-Open project QLUSTER (DLV-862035) and the Royal Society (EA/181068). Samples were grown in the EPSRC National Epitaxy Facility. D.A.G. acknowledges a St John’s College Fellowship and C.L.G. a Royal Society Dorothy Hodgkin Fellowship.
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D.A.G., C.L.G. and M.A. conceived the experiments. J.H.B., C.B., D.A.G., G.E.-M. and C.L. carried out the experiments. J.H.B., L.Z. and D.A.G. performed the data analysis. L.Z. performed the theory and simulations with guidance from C.L.G. and D.A.G. E.C. and M.H. grew the material. All authors contributed to the discussion of the analysis and the results. All authors participated in preparing the manuscript.
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Gangloff, D.A., Zaporski, L., Bodey, J.H. et al. Witnessing quantum correlations in a nuclear ensemble via an electron spin qubit. Nat. Phys. 17, 1247–1253 (2021). https://doi.org/10.1038/s41567-021-01344-7
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DOI: https://doi.org/10.1038/s41567-021-01344-7
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