Witnessing quantum correlations in a nuclear ensemble via an electron spin qubit

• Published in: Nature physics Vol. 17; no. 11; pp. 1247 - 1253
• Main Authors: Gangloff, Dorian A., Zaporski, Leon, Bodey, Jonathan H., Bachorz, Clara, Jackson, Daniel M., Éthier-Majcher, Gabriel, Lang, Constantin, Clarke, Edmund, Hugues, Maxime, Le Gall, Claire, Atatüre, Mete
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 01.11.2021; Nature Publishing Group
• Subjects: 639/624/400/482; 639/766/119/1000/1017; 639/766/483; 639/766/483/481; Atomic; Classical and Continuum Physics; Coherence; Complex Systems; Condensed Matter; Condensed Matter Physics; Electron spin; Electrons; Excitation; Mathematical and Computational Physics; Mean; Mesoscopic Systems and Quantum Hall Effect; Molecular; Nuclear spin; Optical and Plasma Physics; Particle spin; Physics; Physics and Astronomy; Quantum dots; Quantum entanglement; Qubits (quantum computing); Theoretical
• ISSN: 1745-2473; 1745-2481; 1476-4636
• DOI: 10.1038/s41567-021-01344-7

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. Atoms in a semiconductor can have non-zero nuclear spins, creating a large ensemble with many quantum degrees of freedom. An electron spin coupled to the nuclei of a semiconductor quantum dot can witness the creation of entanglement within the ensemble.