NV-Diamond Magnetic Microscopy using a Double Quantum 4-Ramsey Protocol

We introduce a double quantum (DQ) 4-Ramsey measurement protocol that enables wide-field magnetic imaging using nitrogen vacancy (NV) centers in diamond, with enhanced homogeneity of the magnetic sensitivity relative to conventional single quantum (SQ) techniques. The DQ 4-Ramsey protocol employs microwave-phase alternation across four consecutive Ramsey (4-Ramsey) measurements to isolate the desired DQ magnetic signal from any residual SQ signal induced by microwave pulse errors. In a demonstration experiment employing a 1-$\mu$m-thick NV layer in a macroscopic diamond chip, the DQ 4-Ramsey protocol provides volume-normalized DC magnetic sensitivity of $\eta^\text{V}=34\,$nTHz$^{-1/2} \mu$m$^{3/2}$ across a $125\,\mu$m$\,\times\,125\,\mu$m field of view, with about 5$\times$ less spatial variation in sensitivity across the field of view compared to a SQ measurement. The improved robustness and magnetic sensitivity homogeneity of the DQ 4-Ramsey protocol enable imaging of dynamic, broadband magnetic sources such as integrated circuits and electrically-active cells.