  The nitrogen-vacancy (NV) colour centre in diamond is an important physical
system for emergent quantum technologies, including quantum metrology,
information processing and communications, as well as for various
nanotechnologies, such as biological and sub-diffraction limit imaging, and for
tests of entanglement in quantum mechanics. Given this array of existing and
potential applications and the almost 50 years of NV research, one would expect
that the physics of the centre is well understood, however, the study of the NV
centre has proved challenging, with many early assertions now believed false
and many remaining issues yet to be resolved. This review represents the first
time that the key empirical and ab initio results have been extracted from the
extensive NV literature and assembled into one consistent picture of the
current understanding of the centre. As a result, the key unresolved issues
concerning the NV centre are identified and the possible avenues for their
resolution are examined.
