Sunlight-powered generation of correlated photon pairs

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.