Quantum sensing of photonic spin density

Nitrogen-vacancy (NV) centers in diamond have emerged as promising room-temperature quantum sensors for probing condensed matter phenomena ranging from spin liquids and magnons to hydrodynamic flow of current. Here we demonstrate that the NV center can be used as a quantum sensor for detecting the photonic spin density (PSD). PSD is the spatial distribution of light's spin angular momentum in the nanoscale. The PSD is associated with spin-momentum locking for light and topological effects such as photonic skyrmion. The spinning field of light induces an effective static magnetic field in the spin qubit of the NV centers. We experimentally detect this effective field (10s of nanotesla) with a single NV center and an ensemble of NV centers at room temperature. We use ac magnetometry techniques to reach the required sensitivity using Bloch sphere operations driven by a microwave field (XY8 protocol). This nanoscale quantum magnetometer can measure the local polarization of light in ultra-sub-wavelength volumes. The direct detection of the photonic spin density at the nanoscale opens interesting quantum metrological avenues for studying exotic phases of photons, nanoscale properties of structured light as well as future on-chip applications in and all-optical control of spin qubits.