Researchers generate quantum entanglement from sunlight using solar concentrator

Scientists from the Max Planck Institute for the Science of Light (MPL), the Max Planck Center for Extreme and Quantum Photonics, and the University of Ottawa have shown for the first time that natural sunlight can generate quantum-entangled photon pairs. The findings, published in the journal Optica, challenge the long-standing assumption that only highly coherent laser light can drive the process needed for entanglement (Optica).

Quantum entanglement, in which pairs of photons remain correlated in ways that classical physics cannot explain, is essential for secure quantum communication, precision sensing, and quantum computing. Entangled photons are typically created via spontaneous parametric down-conversion (SPDC), in which a pump beam interacts with a nonlinear crystal, splitting photons into entangled pairs. Lasers have been the standard pump source because of their high optical coherence—synchronized light waves—and ability to deliver high power densities (MPL).

Sunlight, by contrast, is incoherent, spreads in many directions, and spans a broad spectrum of wavelengths, leading many researchers to dismiss it as unviable. Earlier work by Robert Boyd’s group at the University of Ottawa had shown that incoherent light from an LED could still produce polarization-entangled photons, provided the entanglement is confined to a degree of freedom (polarization) unaffected by the pump’s spatial or temporal disorder. The new study extends this to sunlight (ScienceDaily).

To overcome the intensity challenge, Hanieh Fattahi’s team at MPL developed an all-glass, cone-shaped solar concentrator. A Fresnel lens roughly the size of a household window, mounted on a solar-tracking motor, collects sunlight over about 1.4 square meters. The cone further concentrates the light through total internal reflection into an optical fiber the width of a human hair, which then directs it onto a millimeter-sized nonlinear crystal (phys.org).

In an outdoor experiment at MPL, the polarized but spatially and temporally incoherent sunlight drove SPDC. Quantum state tomography revealed that the resulting polarization-entangled photons had a fidelity of nearly 94% relative to a perfect entangled state. The photon correlations also violated Bell’s inequality, confirming genuine quantum entanglement rather than classical correlations. When photon production rates were normalized for pump power and the effective bandwidth of the nonlinear process, the efficiency matched that of laser-driven systems (The Quantum Insider).

First author Cheng Li, a recent University of Ottawa graduate, noted that the setup was designed so differences in color and propagation direction did not affect polarization entanglement. “As our theory predicts, if the entanglement lives only in polarization, then it should only depend on the pump’s orderliness in its oscillation direction,” Li said in statements released with the paper. The team is now improving brightness and entanglement quality toward a field-deployable system. The approach could extend to other nonlinear processes such as four-wave mixing (Optica).

Potential applications include simpler quantum systems for satellites, where sunlight is abundant and continuous in certain orbits, reducing the need for onboard lasers, power systems, and cooling. Fattahi highlighted advantages for resource-constrained environments such as deep-space missions or remote regions, noting the elimination of electrical-to-optical conversion and associated heat and failure points. Boyd described the result as a starting point that could inspire further research in nonlinear and quantum optics to make sunlight-driven sources more practical (MPL).