  The nitrogen-vacancy (NV) centre in diamond is a unique optical defect that
is used in many applications today and methods to enhance its fluorescence
brightness are highly sought after. We observed experimentally an enhancement
of the NV quantum yield by up to 7% in bulk diamond caused by an external
magnetic field relative to the field-free case. This observation is
rationalised phenomenologically in terms of a magnetic field dependence of the
NV excited state triplet-to-singlet transition rate. The theoretical model is
in good qualitative agreement with the experimental results at low excitation
intensities. Our results significantly contribute to our fundamental
understanding of the photophysical properties of the NV defect in diamond and
may enable novel NV centre-based magnetometry techniques.
