  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.
