sensing limit harnessing dipolar-ordered nuclear spins*

Sensing static or slowly-varying magnetic fields is of interest for many diverse applications including monitoring single neurons firing or measuring the edge currents in topological insulators. The coherence time for most sensing schemes, however, are limited by the T₂* dephasing time as opposed to the longer T₂ coherence time as any echo sequences designed to refocus dephasing will generally also cancel out sensor evolution under the magnetic field. We report on a DC magnetometry technique that is intrinsically limited by sensor spin transverse lifetime T₂’. Our approach exploits the response of spins initially prepared in a dipolar-ordered state to an externally applied AC field. In the presence of a DC or low-frequency field to be sensed, the spin response is amplified by the AC field, and can be discerned with high sensitivity. We experimentally demonstrate the technique employing hyperpolarized ¹³C nuclear spins in diamond particles as quantum sensors. Potential applications are also discussed.