Bell's theorem states that no local hidden-variable model is compatible with quantum mechanics. Surprisingly, even if we release the locality constraint, certain nonlocal hidden-variable models, such as the one proposed by Leggett, may still be at variance with the predictions of quantum physics. Here, we report an experimental test of Leggett's nonlocal model with solid-state spins in a diamond nitrogen-vacancy center. We entangle an electron spin with a surrounding weakly coupled ${}^{13}\mathrm{C}$ nuclear spin and observe that the entangled states violate Leggett-type inequalities by more than four and seven standard deviations for six and eight measurement settings, respectively. Our experimental results are in full agreement with quantum predictions and violate Leggett's nonlocal hidden-variable inequality with a high level of confidence.

中文翻译：

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固态自旋的Leggett不等式的实验测试

贝尔定理指出，没有任何局部隐变量模型与量子力学兼容。令人惊讶的是，即使我们释放了局部性约束，某些非局部隐藏变量模型（例如莱格特提出的模型）仍可能与量子物理学的预测有所不同。在这里，我们报告了在钻石氮空位中心进行固态旋转的Leggett非局部模型的实验测试。我们将电子自旋与周围的弱耦合纠缠在一起${}^{13}\mathrm{C}$核自旋，并观察到纠缠态分别针对六种和八种测量设置违反了莱格特型不等式超过四个和七个标准偏差。我们的实验结果与量子预测完全吻合，并以较高的置信度违反了莱格特的非局部隐变量不等式。