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

TL;DR

This paper demonstrates a method to probe and reconstruct the quantum state of a nuclear spin ensemble in a quantum dot using an electron spin qubit, revealing non-thermal, correlated states with entanglement.

Contribution

It introduces a species-selective reconstruction technique that detects inter-particle coherences and confirms the formation of a dark many-body entangled state.

Findings

Reconstructed nuclear populations indicate non-thermal, correlated states.

Sum of species-resolved polarizations exceeds classical predictions threefold.

Evidence of inter-particle coherences serving as an entanglement witness.

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 polarisation, 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 polarisations 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.