93.9% fidelity to an ideal Bell state was achieved when researchers used filtered daylight—not a pump laser—to generate polarization-entangled photon pairs in a peer-reviewed Optica experiment.
A 1-by-1.4 metre Fresnel lens collected sunlight, narrowed it to 405 nanometres, and fed a nonlinear crystal in a Sagnac interferometer, producing about 1,600 coincident pairs per second per milliwatt.
S = 2.5408 ± 0.2171 in a Bell test exceeded the classical limit of 2, while concurrence of 0.905 and purity of 0.919 indicated strongly entangled, relatively low-noise detected pairs.
The 94% figure measures state quality among detected pairs, not solar-to-photon-pair conversion; raw per-power output still trailed laser pumping until the team normalized for sunlight's much wider bandwidth.
The result suggests incoherent natural light can support high-quality polarization entanglement, but the setup still required filtering, tracking, temperature control and long acquisitions, leaving practical remote or space uses for future development.