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

The negatively charged nitrogen vacancy center in diamond is widely used for quantum sensing due to the sensitivity of the spin triplet in the electronic ground state to external perturbations such as strain and electromagnetic fields, which makes it an excellent probe for changes in these perturbations. The spin state can be measured through optically detected magnetic resonance, which is most commonly achieved by detecting the photoluminescence 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 efforts to optimize the magnetometer sensitivity at room temperature and without cavities. We reach a sensitivity of <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"> <a:mn>18</a:mn> <a:mspace width="0.2em"/> <a:mi>pT</a:mi> <a:mo>/</a:mo> <a:msqrt> <a:mi>Hz</a:mi> </a:msqrt> </a:math> , surpassing previously reported values, and a calculated shot-noise limit of <d:math xmlns:d="http://www.w3.org/1998/Math/MathML" display="inline"> <d:mn>5</d:mn> <d:mspace width="0.2em"/> <d:mi>pT</d:mi> <d:mo>/</d:mo> <d:msqrt> <d:mi>Hz</d:mi> </d:msqrt> </d:math> . We also report on a defect that is native to diamond grown by chemical vapor deposition and thus absent in high-pressure high-temperature diamond, the excitation of which impacts the measured singlet absorption signal.