Autoheterodyne Characterisation for Narrowband Photon-Pair Purity

Recently, there have been several advancements in the development of sources for narrowband non-classical light (with linewidth ≲ MHz). This is motivated by applications in quantum information and sensing where quantum light is interfaced with material quanta such as atoms or molecules. For instance, photonic quantum information processing can be realised through strong photon-photon interactions mediated by non-linearity in atoms and re-quires photon-pairs in which both photons are resonant and matched in bandwidth to an atomic transition. Applications in quantum networking, e.g. entanglement-swapping with memory-compatible photons, require the photons to be pure and indistinguishable in addition to being matched in frequency and bandwidth to quantum memories.