A modular source of entangled photon pairs in femtosecond-laser written waveguide circuits

Sources of entangled photons are cornerstones of photonic quantum technologies, providing the basic resources for a wide range of quantum protocols for computing, simulation and sensing. A typical technique to generate entangled photon pairs exploits spontaneous parametric down conversion (SPDC) in non-linear crystals. In this context, an integrated-optics approach based on non-linear waveguides [1] can boost source brightness due to the enhanced mode confinement, and can take great advantage from the unique compactness and stability of on-chip devices. However, integrated devices are often specialized to produce a given quantum state