Sunlight Powers Quantum Entanglement in New Proof-of-Concept

A team led by researchers at the University of Ottawa and the Max Planck Institute for the Science of Light (MPL) has demonstrated that focused sunlight can generate pairs of entangled photons. The results, published in *Optica*, show that diffuse solar illumination, when concentrated onto a nonlinear crystal, produces photon pairs whose quantum states match a perfectly entangled configuration with a fidelity of roughly 94 percent.

Scientific American

For decades, photonic quantum-computing experiments have relied on high-energy lasers directed through special crystals—a process called spontaneous parametric down-conversion. Lasers provide coherent, single-color light, which physicists assumed was essential for creating the strong correlations required for entanglement. By contrast, sunlight comprises a broad spectrum of wavelengths arriving from many directions, and it had been unclear whether such incoherent light could support the same quantum link.

Scientific American

MPL’s co-author Hanieh Fattahi’s group built an all-glass, cone-shaped solar concentrator that channels light from a window-sized Fresnel lens into a thin optical fiber. The concentrator, mounted on a solar-tracking motor, feeds the focused beam into a nonlinear crystal housed inside a blackout tent. Single-photon detectors record the emerging pairs. Outdoor tests conducted over three days yielded entangled photons that violated Bell’s inequality, confirming genuine quantum behavior. The measured fidelity (?94 %) fell short of the best laser-driven sources; the team attributes the shortfall primarily to distortions in the optical components rather than to sunlight’s incoherence.

Scientific American

He envisions future satellite-based quantum-encryption keys generated directly from ambient solar radiation and notes that reducing the energy footprint of quantum hardware will be crucial as data-center demand grows.

Scientific American

• “What we want is really to make quantum technology more and more accessible for everybody,” — Cheng Li

Scientific American