The neutrally-charged silicon vacancy in diamond is a promising system for quantum technologies that combines high-efficiency, broadband optical spin polarization with long spin lifetimes (T2 ≈ 1 ms at 4 K) and up to 90 % of optical emission into its 946 nm zero-phonon line. However, the electronic structure of SiV 0 is poorly understood, making further exploitation difficult. Performing photoluminescence spectroscopy of SiV 0 under uniaxial stress, we find the previous excited electronic structure of a single 3 A1u state is incorrect, and identify instead a coupled 3 Eu − 3 A2u system, the lower state of which has forbidden optical emission at zero stress and so efficiently decreases the total emission of the defect: we propose a solution employing finite strain to form the basis of a spin-photon interface. Isotopic enrichment definitively assigns the 976 nm transition associated with the defect to a local mode of the silicon atom.
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