  Negatively charged nitrogen-vacancy centres in diamond are promising quantum
magnetic field sensors. Laser threshold magnetometry has been a theoretical
approach for the improvement of NV-centre ensemble sensitivity via increased
signal strength and magnetic field contrast. In this work we experimentally
demonstrate laser threshold magnetometry. We use a macroscopic high-finesse
laser cavity containing a highly NV-doped and low absorbing diamond gain medium
that is pumped at 532nm and resonantly seeded at 710nm. This enables
amplification of the signal power by stimulated emission of 64%. We show the
magnetic-field dependency of the amplification and thus, demonstrate
magnetic-field dependent stimulated emission from an NV-centre ensemble. This
emission shows a record contrast of 33% and a maximum output power in the mW
regime. These advantages of coherent read-out of NV-centres pave the way for
novel cavity and laser applications of quantum defects as well as diamond NV
magnetic field sensors with significantly improved sensitivity for the health,
research and mining sectors.
