Experimental Test of Leggett's Inequalities with Solid-State Spins

TL;DR

This paper reports an experimental test of Leggett's nonlocal hidden variable inequalities using solid-state spins in diamond, confirming quantum mechanics predictions and violating Leggett's inequalities with high confidence.

Contribution

First experimental violation of Leggett's inequalities using solid-state spins, demonstrating incompatibility of certain nonlocal hidden variable models with quantum mechanics.

Findings

Violates Leggett's inequalities by over four standard deviations

Uses entangled electron and nuclear spins in diamond

Results align with quantum mechanical predictions

Abstract

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 $^{13}C$ 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.