Sunlight Powers Quantum Photon Generation for Ghost Imaging

A team led by Wuhong Zhang and Lixiang Chen at Xiamen University has demonstrated that sunlight can serve as the sole pump source for generating quantum-correlated photon pairs using spontaneous parametric down-conversion (SPDC), a process previously reliant on stable lasers.

The experiment, detailed in Advanced Photonics on April 24, 2026, utilized an automatic sun-tracking device to continuously direct sunlight into a 20-meter plastic multimode optical fiber. This fiber guided the light into a dark laboratory environment where it illuminated a periodically poled potassium titanyl phosphate (PPKTP) nonlinear crystal, producing photon pairs with strong position correlations.

Despite the natural fluctuations in sunlight’s brightness, angle, and position, the setup achieved a ghost-imaging visibility of 90.7%, closely approaching the 95.5% visibility attained using a conventional 405 nm laser at the same pump power. The researchers first performed double-slit imaging and then reconstructed a complex two-dimensional “ghost face” image, demonstrating the system’s capacity to capture detailed spatial structures.

Traditional quantum optics experiments rely on coherent laser sources for SPDC, where laser light is directed into nonlinear crystals to generate entangled photons. However, recent findings indicate that SPDC can be driven by partially coherent light sources, transferring some coherence properties to the generated photons. This insight motivated the investigation into sunlight as a practical alternative.

Sunlight’s broad spectrum facilitates quasi-phase matching within the nonlinear crystal, enabling the generation of large quantities of position-correlated photon pairs. By extending data collection periods, the researchers enhanced signal-to-noise and contrast-to-noise ratios, maintaining stable imaging performance despite the inherent variability of solar illumination.

This work represents the first successful demonstration of sunlight-pumped SPDC combined with ghost imaging, creating a fully passive source of correlated photon pairs without the need for lasers or external electrical power. The researchers suggest that this approach could support quantum imaging and information systems in remote or space environments where traditional laser equipment is impractical.

Future improvements in sunlight collection methods, nonlinear crystal design, and image reconstruction techniques such as compressed sensing and machine learning may increase imaging speed and quality, advancing the practical application of sunlight-powered quantum imaging.