Extension of Spin Dephasing Time of Continuously Excited Ensemble Nitrogen Vacancy Centers by Double‐Quantum Ramsey Magnetometry with Spin Bath Driving.

Quantum magnetic sensors based on nitrogen vacancy (NV) centers are expected to be used for various biological and medical applications. Continuously excited (CE) Ramsey magnetometry is one of the most sensitive measurement protocols and has the potential to improve the sensitivity of sensors using additional quantum manipulations to extend the spin dephasing time. In a typical ensemble NV measurement, the spin dephasing time is limited by inhomogeneous coupling. To overcome this limitation, a magnetometry that extends the CE Ramsey protocol with a double‐quantum (DQ) Ramsey sequence and spin bath driving is developed. It is demonstrated that the electric noise due to the spatial inhomogeneity of the crystal strain is cancelled out by the DQ Ramsey, and the magnetic noise due to the P1 centers was suppressed by spin bath driving. Compared to the conventional CE Ramsey, the CE‐DQ Ramsey with spin bath driving effectively doubles the gyromagnetic ratio and yields a twofold extension of T2*, corresponding to a fourfold enhancement of the photon‐shot‐noise limited sensitivity. [ABSTRACT FROM AUTHOR]