N - V –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 ($\mathrm{N}$-$V$) 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-$\ensuremath{\mu}\mathrm{m}$-thick $\mathrm{N}$-$V$ layer in a macroscopic diamond chip, the DQ 4-Ramsey protocol provides a volume-normalized dc magnetic sensitivity of ${\ensuremath{\eta}}^{V}=34\phantom{\rule{0.2em}{0ex}}\mathrm{nT}\phantom{\rule{0.1em}{0ex}}{\mathrm{Hz}}^{\ensuremath{-}1/2}\phantom{\rule{0.1em}{0ex}}\ensuremath{\mu}{\mathrm{m}}^{3/2}$ across a $125\phantom{\rule{0.2em}{0ex}}\ensuremath{\mu}\mathrm{m}\ifmmode\times\else\texttimes\fi{}125\phantom{\rule{0.2em}{0ex}}\ensuremath{\mu}\mathrm{m}$ field of view, with about $5\ifmmode\times\else\texttimes\fi{}$ 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 enables imaging of dynamic broadband magnetic sources such as integrated circuits and electrically active cells.