Near-field photon entanglement in total angular momentum

Photons can carry angular momentum, which is conventionally attributed to two constituents—spin angular momentum (SAM), which is an intrinsic property related to the polarization, and orbital angular momentum (OAM), which is related to the photon spatial distribution. In paraxial optics, these two forms of angular momentum are separable¹, such that entanglement can be induced between the SAM and the OAM of a single photon^2,3 or of different photons in a multi-photon state⁴. In nanophotonic systems, however, the SAM and the OAM of a photon are inseparable^5,6, so only the total angular momentum (TAM) serves as a good quantum number^{7, 8–9}. Here we present the observation of non-classical correlations between two photons in the near-field regime, giving rise to entanglement related to the TAM. We entangle those nanophotonic states by coupling photon pairs to plasmonic modes and use quantum imaging techniques^10,11 to measure their correlations. We observe that entanglement in TAM leads to a completely different structure of quantum correlations of photon pairs, compared with entanglement related to the two constituent angular momenta. This work paves the way for on-chip quantum information processing using the TAM of photons as the encoding property for quantum information.