  On-demand, high repetition rate sources of indistinguishable, polarised
single photons are the key component for future photonic quantum technologies.
Colour centres in diamond offer a promising solution, and the narrow line-width
of the recently identified nickel-based NE8 centre makes it particularly
appealing for realising the transform-limited sources necessary for quantum
interference. Here we report the characterisation 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 the NE8 is a far more promising source than the more commonly
studied nitrogen-vacancy centre and point the way to the realisation of a
practical diamond colour centre-based single photon source.
