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.

与代理量子位接口的自旋的连贯集合是一个有吸引力的平台，可以创建多体相干性和探索集体激发的机制。半导体量子点中的电子自旋量子比特可以充当由多个高自旋原子种类组成的量子点内致密核自旋系综的界面。早期的工作表明，电子可以通过统计其平均场与总核极化的相互作用，即其均值和方差来传递其核环境的特性。在这里，我们展示了一种探测核系综的自旋状态的方法，该方法利用其对集体自旋激发的响应，实现了平均场之外的物种选择性重建。对于光学准备的平均场的可访问范围，重建的种群表明该集合处于非热相关的核状态。重建的物种分辨极化的总和超过了经典预测的三倍。这种明显的偏差源于包含粒子间相干性的自旋系综，并作为纠缠见证，证实了黑暗多体状态的形成。