Quantum optics experiments typically rely on carefully controlled lasers to create correlated or entangled photon pairs. These photon pairs are produced through a process known as spontaneous parametric down-conversion (SPDC), in which laser light is directed into a nonlinear crystal. Because the process depends on stable, coherent illumination, scientists have generally assumed that sunlight would not work as a practical alternative.
That assumption has started to change in recent years. Researchers have found that SPDC does not require perfectly coherent light sources. Even partially coherent light can drive the process, while passing some of its coherence properties to the generated photons. This raised an intriguing possibility: could sunlight itself produce correlated photon pairs?
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