Researchers have demonstrated that natural sunlight can generate quantum entanglement between photons, challenging the long-held assumption that coherent laser light is required for quantum optics. Led by teams at the University of Ottawa and the Max Planck Institute for the Science of Light, the breakthrough could significantly reduce energy demands for future quantum technologies and space communications.
Quantum technologies rely heavily on powerful lasers to generate photon entanglement, which is crucial for secure communication, ultra-precise sensing, and high-performance computation. However, these systems consume significant amounts of energy, raising concerns about their electricity requirements as the technology scales. Researchers have now demonstrated that sunlight itself can serve as an alternative pump source to produce entanglement comparable to laser-based techniques when input bandwidth differences are factored in.
Challenging Assumptions About Quantum Light Sources
Scientists have traditionally believed that producing the strong correlations needed for photon entanglement requires coherent light, where wave peaks and valleys follow a predictable pattern. Because lasers generate highly coherent light concentrated at a single color, they have been the standard tool for this work. Earlier theoretical predictions and experiments from Robert Boyd’s team at the University of Ottawa began challenging that assumption by using an LED to produce polarization-entangled photons.
The new research pushes that concept further by replacing LEDs with natural sunlight. Sunlight presents a much greater challenge because it spreads across many directions and contains a wide spectrum of colors. To overcome this, the researchers relied on spontaneous parametric down-conversion (SPDC), an established optical process where a pump beam enters a nonlinear crystal and individual photons split into pairs that may become quantum entangled. The team supplied the system with sunlight that was strongly polarized while remaining highly incoherent across both space and time.
Cheng Li from the University of Ottawa explained that they had designed their experimental setup to ensure that differences introduced by varying colors and propagation directions did not influence the photons’ polarization, noting that as their theory predicted, if the entanglement resided solely in polarization, it should depend only on the orderliness of the pump’s oscillation direction rather than its direction or color, which ultimately enabled them to produce high-quality polarization entanglement from highly spatially and temporally incoherent sunlight.
Overcoming the Millimeter-Sized Crystal Challenge
Getting enough sunlight onto an extremely small nonlinear crystal—which measures only about a millimeter in size—presented a major physical obstacle. Ordinary methods of collecting sunlight were impractical for such a scale. To solve this, Hanieh Fattahi’s team at the Max Planck Institute for the Science of Light (MPL) in Germany designed an all-glass solar concentrator.
The cone-shaped system collects sunlight using a Fresnel lens roughly the size of a household window. It then channels that light into an optical fiber about as wide as a human hair, directing the concentrated solar energy onto the tiny nonlinear crystal.
Parallel Breakthroughs in Quantum Ghost Imaging
In related research, a team led by Wuhong Zhang and Lixiang Chen at Xiamen University also demonstrated sunlight-pumped SPDC, utilizing the technique to perform quantum ghost imaging. Their experimental setup used an automatic sun-tracking device similar to an equatorial telescope mount to direct collected light into a 20-meter plastic multimode optical fiber.
That fiber carried the sunlight into a dark indoor laboratory to illuminate a periodically poled potassium titanyl phosphate (PPKTP) nonlinear crystal. The Xiamen University system achieved a ghost-imaging visibility of 90.7%, coming close to the 95.5% visibility achieved by a conventional 405 nm laser operating at identical pump power. The researchers demonstrated double-slit imaging and reconstructed a two-dimensional ghost face
image, proving the system could capture detailed spatial structures despite the natural variability of solar illumination.
Implications for Space Satellites and Quantum Computing
By removing the need for lasers and dedicated external electrical power sources, sunlight-driven quantum generation creates a fully passive source of correlated and entangled photon pairs. This approach could support quantum technologies in remote or extreme environments where conventional equipment is difficult to operate.
According to Cheng Li, first author of the University of Ottawa study, the technology could eventually enable satellites to create secure encryption keys using abundant space sunlight, reducing the need for onboard lasers and supporting hardware. Furthermore, sunlight-driven entanglement generation could supply the necessary ingredient to scale up quantum computing without adding to the energy burden.
