Laser threshold magnetometry

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We propose a new type of sensor, which uses diamond containing the optically active nitrogen-vacancy (NV-) centres as a laser medium. The magnetometer can be operated at room-temperature and generates light that can be readily fibre coupled, thereby permitting use in industrial applications and remote sensing. By combining laser pumping with a radio-frequency Rabi-drive field, an external magnetic field changes the fluorescence of the NV- centres. We use this change in fluorescence level to push the laser above threshold, turning it on with an intensity controlled by the external magnetic field, which provides a coherent amplification of the readout signal with very high contrast. This mechanism is qualitatively different from conventional NV---based magnetometers which use fluorescence measurements, based on incoherent photon emission. We term our approach laser threshold magnetometry (LTM). We predict that an NV---based laser threshold magnetometer with a volume of 1mm³ can achieve shot-noise limited d.c. sensitivity of 1.86 fT/√Hz and a.c. sensitivity of 3.97fT/√Hz.