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We demonstrate optical spin polarization of the neutrally-charged silicon-vacancy defect in diamond (SiV 0 ), an S = 1 defect which emits with a zero-phonon line at 946 nm. The spin polarization is found to be most efficient under resonant excitation, but non-zero at below-resonant energies. We measure an ensemble spin coherence time T2>100 µs at low-temperature, and a spin relaxation limit of T1 > 25 s. Optical spin state initialization around 946 nm allows independent initialization of SiV 0 and NV − within the same optically-addressed volume, and SiV 0 emits within the telecoms downconversion band to 1550 nm: when combined with its high Debye-Waller factor, our initial results suggest that SiV 0 is a promising candidate for a long-range quantum communication technology.Point defects in diamond have attracted considerable interest owing to their application for quantum information processing, communication, and metrology. The most-studied defect, the negatively-charged nitrogenvacancy (NV − ) center, possesses efficient optical spin polarization and spin-state dependent fluorescence, enabling its exploitation as an ultra-sensitive nano-scale magnetic field sensor [1][2][3]. However, the zero phonon line (ZPL) of NV − accounts for only a few percent of its total emission [4], leading to low efficiency in coherent photonic applications. The negatively-charged siliconvacancy (SiV − ) center has also received significant interest as its high Debye-Waller factor (≈ 0.8 [5]) makes it an attractive candidate for long-range quantum computation and communication. However, the exceptional optical properties of SiV − are not matched by its spin properties, where a large spin-orbit coupling in the ground state enables phonon-assisted spin-state depopulation, resulting in spin-lattice relaxation-limited coherence lifetimes of 40 ns even at 5 K [6]: efforts are ongoing to overcome this limitation by strain engineering, but currently liquid helium temperatures and below are required to access and readout SiV − spin states [7].The neutrally-charged silicon-vacancy (SiV 0 ) has a ground state electron spin S = 1. Unlike the NV center, where the nitrogen remains covalently bonded to three carbon atoms and the nitrogen-vacancy axis forms a C 3v symmetry axis, the silicon atom in SiV adopts a bond-center location, with a D 3d axis formed by the 1 1 1 joining the split-vacancy [ Fig. 1(a)]. SiV 0 has been characterized both by electron paramagnetic resonance (EPR) [8,9] and optical absorption/photoluminescence (PL) [10]. Similarly to SiV − , the neutral charge state also has a high Debye-Waller factor, with the majority of its photons emitted at the primary zero-phonon line (ZPL) at 946 nm (1.31 eV) [ Fig. 1(b)]: this transition has been shown to occur between a 3 A 2g ground state (GS) We have studied SiV 0 in two samples grown by chemical vapor deposition (CVD); silicon was introduced by adding silane to the process gasses during growth. Sample A was grown on a {1 0 0}-oriented high pressure high temperature (HPHT) substrate, ir...

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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.