Indistinguishability of arbitrary photons for entanglement generation in scalable quantum networks

Using a network-compatible source of single photons, we study the interference of heralded signal and idler photons that are not part of the same biphoton pair. We characterize the indistinguishability of such arbitrary single photons that are generated in independent parametric down-conversion events using a Hong-Ou-Mandel interferometer, and show non-classical coalescence, <i>C</i> = 0.85(9). Separately, we measure coalescence of multi-mode thermal fields corresponding to the signal and idler photons and characterize their indistinguishability. This source exhibits ubiquitous indistinguishability between any photons that it generates and thus enables the interchangeable use of signal and idler photons in multi-node, photon-interference-based protocols for scalable quantum networks. The indistinguishability between arbitrary photons from such sources enables requirement-driven entanglement to be generated in nodes within a quantum network.