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On-demand, high repetition rate sources of indistinguishable, polarized single photons are the key component for future photonic quantum technologies (O'Brien et al 2009 Nat. Photonics 3 687-95). Colour centres in diamond offer a promising solution, and the narrow linewidth of the recently identified nickel-based NE8 centre makes it particularly appealing for realizing the transform-limited sources necessary for quantum interference. Here, we report the characterization of dipole orientation and coherence properties of a single NE8 colour centre in a diamond nanocrystal at room temperature. We observe a single-photon coherence time of 0.21 ps and an emission lifetime of 1.5 ns. Combined with an emission wavelength that is ideally suited for applications in existing quantum optical systems, these results show that NE8 is a far more promising source than the more commonly studied nitrogen-vacancy centre and point the way to the realization of a practical diamond colour centre-based single-photon source.