  Photonic spin density (PSD) in the near-field gives rise to exotic phenomena
such as photonic skyrmions, optical spin-momentum locking and unidirectional
topological edge waves. Experimental investigation of these phenomena requires
a nanoscale probe that directly interacts with PSD. Here, we propose and
demonstrate that the nitrogen-vacancy (NV) center in diamond can be used as a
quantum sensor for detecting the spinning nature of photons. This room
temperature magnetometer can measure the local polarization of light in
ultra-subwavelength volumes through photon-spin-induced virtual transitions.
The direct detection of light's spin density at the nanoscale using NV centers
in diamond opens a new frontier for studying exotic phases of photons as well
as future on-chip applications in spin quantum electrodynamics (sQED).
