Towards high-sensitivity magnetometry with nitrogen vacancy centers in diamond using the singlet infrared absorption

The negatively-charged nitrogen vacancy (NV$^{-}$) center in diamond is widely used for quantum sensing since the sensitivity of the spin triplet in the electronic ground state to external perturbations such as strain and electromagnetic fields make it an excellent probe for changes in these perturbations. The spin state can be measured through optically detected magnetic resonance (ODMR), which is most commonly achieved by detecting the photoluminescence (PL) after exciting the spin-triplet transition. Recently, methods have been proposed and demonstrated that use the absorption of the infrared singlet transition at 1042 nm instead. These methods however require cryogenic temperatures or external cavities to enhance the absorption signal. Here, we report on our optimization efforts of the magnetometer sensitivity at room temperature and without cavities. We reach sensitivities of 18 pT$/\sqrt{\mathrm{Hz}}$, surpassing previously reported values. We also report on a defect that is native to CVD-grown diamond and thus absent in HPHT diamond, the excitation of which impacts the measured singlet absorption signal.