Pairs of correlated or entangled photons are a foundational resource in quantum optics. They are most commonly produced through spontaneous parametric down-conversion (SPDC), a nonlinear optical process that typically relies on a stable, coherent laser to pump a nonlinear crystal. Because of this requirement, SPDC has long been viewed as impractical without laboratory-grade laser systems.
Recent studies have shown that fully coherent light is not strictly necessary: partially coherent sources can also drive SPDC, with their coherence properties transferred to the generated photon pairs. This insight raises a natural and intriguing question—can sunlight, the most abundant natural light source, be used to generate correlated photon pairs?
Using sunlight for SPDC presents clear challenges. Sunlight collected from the ground is inherently unstable, with continuous changes in intensity, angle, and position that interfere with the precise illumination and photon detection required for SPDC experiments. At the same time, sunlight offers a compelling advantage: it removes dependence on lasers and external power sources, opening possibilities for photon-pair generation in remote or extreme environments.
Researchers led by Wuhong Zhang and Lixiang Chen at Xiamen University have now demonstrated a practical solution. As reported in Advanced Photonics , they developed an experimental system in which sunlight serves as the sole pump source for SPDC. An automatic sun-tracking device, similar to an equatorial telescope mount, continuously collects sunlight throughout the day and couples it into a 20‑m plastic multimode optical fiber. The fiber delivers the light into a dark indoor laboratory, where it pumps a periodically poled potassium titanyl phosphate (PPKTP) nonlinear crystal.
Despite the challenges of solar illumination, the system successfully generated photon pairs with strong position correlations. To demonstrate performance, the team used the photon pairs to perform ghost imaging—a technique in which an image is reconstructed using correlated photons rather than direct spatial detection. The sunlight-driven system achieved a ghost-imaging visibility of 90.7%, comparable to the 95.5 percent visibility obtained using a conventional 405‑nm laser at the same pump power.
In addition to double-slit imaging, the researchers reconstructed a more complex two-dimensional image—a "ghost face"—showing that the approach works for detailed spatial structures. According to the authors, the broad spectrum of sunlight enables quasi-phase matching in the nonlinear crystal, producing a large number of position-correlated photon pairs. By accumulating data over long periods, the system achieves improved signal-to-noise and contrast-to-noise ratios, demonstrating stable operation despite the variability of the solar source.
This work represents the first experimental demonstration of sunlight-pumped SPDC and its use in ghost imaging. By eliminating the need for lasers and external electrical power, the approach enables a fully passive source of correlated photon pairs.
The researchers note that such systems could be particularly valuable for space-based or remote quantum imaging and information applications. Further improvements in sunlight collection, crystal design, and reconstruction methods—such as compressed sensing or machine learning—could enhance imaging speed and quality and support future real-world deployment.
For details, see the original Gold Open Access article by Y. Xing et al., " Sunlight-excited spontaneous parametric down-conversion for ghost imaging ," Adv. Photon. 8(3), 036011 (2026), doi 10.1117/1.AP.8.3.036011
