Violation of a temporal Bell inequality for single spins in solid by over 50 standard deviations

Quantum non-locality has been experimentally investigated by testing different forms of Bell’s inequality, yet a loophole-free realization has not been achieved up to now. Much less explored are temporal Bell inequalities, which are not subject to the locality assumption, but impose a constrain on the system’s time-correlations. In this paper, we report on the experimental violation of a temporal Bell’s inequality using a nitrogen vacancy defect (NV) in diamond and provide a novel quantitative test of quantum coherence. We therefore present a new technique to initialize the electronic state of the NV with high fidelity, a necessary requirement for reliable quantum information processing and/or the implementation of protocols for quantum metrology.