Pairs of correlated or entangled photons are a foundational resource in quantum optics. They are most commonly produced through spontaneous parametric down-conversion (SPDC), a nonlinear optical process that typically relies on a stable, coherent laser to pump a nonlinear crystal. Because of this requirement, SPDC has long been viewed as impractical without laboratory-grade laser systems.
Recent studies have shown that fully coherent light is not strictly necessary: Partially coherent sources can also drive SPDC, with their coherence properties transferred to the generated photon pairs. This insight raises a natural and intriguing question—can sunlight, the most abundant natural light source, be used to generate correlated photon pairs?
Using sunlight for SPDC presents clear challenges. Sunlight collected from the ground is inherently unstable, with continuous changes in intensity, angle, and position that interfere with the precise illumination and photon detection required for SPDC experiments. At the same time, sunlight offers a compelling advantage: it removes dependence on lasers and external power sources, opening possibilities for photon-pair generation in remote or extreme environments.








