  Nitrogen vacancy (NV) centers in diamond have emerged as a leading quantum
sensor platform, combining exceptional sensitivity with nanoscale spatial
resolution by optically detected magnetic resonance (ODMR). Because
fluorescence-based ODMR techniques are limited by low photon collection
efficiency and modulation contrast, there has been growing interest in infrared
(IR)-absorption-based readout of the NV singlet state transition. IR readout
can improve contrast and collection efficiency, but it has thus far been
limited to long-pathlength geometries in bulk samples due to the small
absorption cross section of the NV singlet state. Here, we amplify the IR
absorption by introducing a resonant diamond metallodielectric metasurface that
achieves a quality factor of Q ~ 1,000. This "plasmonic quantum sensing
metasurface" (PQSM) combines localized surface plasmon polariton resonances
with long-range Rayleigh-Wood anomaly modes and achieves the desired balance
between field localization and sensing volume to optimize spin readout
sensitivity. From combined electromagnetic and rate-equation modeling, we
estimate a sensitivity below 1 nT/Hz$^{1/2}$ per um$^2$ of sensing area using
numbers for present-day NV diamond samples and fabrication techniques. The
proposed PQSM enables a new form of microscopic ODMR sensing with infrared
readout near the spin-projection-noise-limited sensitivity, making it appealing
for the most demanding applications such as imaging through scattering tissue
and spatially-resolved chemical NMR detection.
