Correlated and entangled photon pairs are essential tools in quantum optics. Scientists usually create these photon pairs through a process called spontaneous parametric down-conversion (SPDC), in which a powerful, highly stable laser shines into a nonlinear crystal. Because SPDC depends so heavily on coherent laser light, researchers have long considered the technique impractical outside carefully controlled laboratory environments.
More recently, studies have shown that perfectly coherent light is not actually required for SPDC to work. Even partially coherent light sources can produce correlated photon pairs, while also transferring some of their own coherence properties to the generated photons. That discovery led researchers to ask an intriguing question: could sunlight itself be use