title: Violation of a temporal bell inequality for single spins in a diamond defect center.
abstract: Quantum nonlocality 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 constraint on the system's time correlations. In this Letter, we report on the experimental violation of a temporal Bell's inequality using a nitrogen-vacancy (NV) defect in diamond and provide a novel quantitative test of quantum coherence. Such a test requires strong control over the system, and we present a new technique to initialize the electronic state of the NV with high fidelity, a necessary requirement also for reliable quantum information processing and/or the implementation of protocols for quantum metrology.
authors.affiliation: Physikalisches Institut and Research Center SCOPE, University of Stuttgart, Pfaffenwaldring 57, 70569 Stuttgart, Germany. g.waldherr@physik.uni-stuttgart.de
authors.forename: G
authors.initials: G
authors.lastname: Waldherr
authors.forename: P
authors.initials: P
authors.lastname: Neumann
authors.forename: S F
authors.initials: SF
authors.lastname: Huelga
authors.forename: F
authors.initials: F
authors.lastname: Jelezko
authors.forename: J
authors.initials: J
authors.lastname: Wrachtrup