Scientists report the development of a new experimental system that could lead to a breakthrough resource in quantum optics by successfully generating correlated photon pairs using sunlight.
The new system relies on nature’s most abundant light source as the main driver of a nonlinear optical process known as spontaneous parametric down-conversion (SPDC), which normally requires a laser to “pump” a nonlinear crystal.
The breakthrough achievement was reported in Advanced Photonics.
Entangled Photons in Correlated Pairs
In the world of quantum optics, the phenomenon of pairs of correlated or entangled photons is an important asset, despite being a seemingly obscure concept for most of us.
Under normal circumstances, optical scientists rely on spontaneous parametric down-conversion (SPDC), a nonlinear optical process in which devices such as coherent lasers are the primary means of “pumping” a nonlinear crystal. Given that they require the kinds of lasers typically found only in top laboratories, the practical use of SPDC is nonviable under normal conditions.
Finding a practical, real-world substitute has long been an intriguing idea, which prompted researchers at Xiamen University in China to determine whether similar processes could be achieved using the most abundant source of light on Earth: sunlight.
A Challenging Process
This is easier said than done, since sunlight, unlike lasers, is generally unstable due to changes in intensity caused by environmental or atmospheric factors (think clouds, for instance) as well as changes in angle and position that occur naturally throughout the day.
All these factors compromise the precision required for SPDC. Still, the practicality of sunlight, as well as the energy it provides, has continued to make it a potentially feasible alternative that scientists hope might liberate SPDC from its reliance on lab-grade coherent lasers.
If it could be harnessed for such purposes, using sunlight to fuel SPDC would also mean that photon-pair generation could be achieved in remote areas where researchers had never previously considered it possible.
A Solution to SPDC Beyond the Lab?
According to the Xiamen University research team, a new experimental system has been developed that uses sunlight as the only pump source for this process, employing a device that tracks the sun, similar to how equatorial mounts allow astronomers to follow the movement of celestial objects as the Earth spins.
The device, according to researchers, harnesses sunlight at the proper angles throughout the day, which is then fed through a length of optical fiber to an indoor lab. From there, the light is used to pump a potassium titanyl phosphate (KTP) nonlinear crystal.
Periodically Poled Potassium Titanyl Phosphate (PPKTP) crystals are a variety of engineered nonlinear optical crystals that researchers use for high-efficiency frequency conversion and other quantum optics applications, especially for creating entangled photon pairs. They work by altering qualities of light that include its color, phase, or frequency by forcing it to pass through a specially engineered component or structure.
While using sunlight as the sole source of illumination for such processes is complex, the team found that its system successfully produced photon pairs that exhibited strong correlations.
Ghost Imaging for Photon Pair Production
Next came the demonstration phase, where the team used the photon pairs generated by their new system to perform “ghost imaging,” a process that uses correlated photons to produce imagery rather than spatial detection.
While conventional laser-based systems can achieve better than 95 percent visibility at comparable pumping power levels, the team’s sunlight-powered technology achieved ghost imaging visibility of 89.7 percent, well within the range of lab-based systems. To further illustrate the system’s use with more detailed spatial structures, the team also used it to produce, appropriately enough, a two-dimensional image of a ghostly face.
Overall, the team says quasi-phase matching in the PPKTP crystal was achievable with the broad spectrum of sunlight, enabling them to generate an abundance of position-correlated photon pairs. Additionally, the team reports that their system yields better signal-to-noise and contrast-to-noise ratios, even given the challenges posed by sunlight variability when used as a primary energy source.
Practical Use Beyond the Lab
“Our research holds substantial significance as it expands the range of viable illumination sources,” the team writes in their recent study, “including scattered light and nontraditional artificial incoherent light—for imaging applications.”
They add that among the potentially promising uses for their technology, space-based quantum information systems may be particularly beneficial, since the team’s new method “enables operation independent of laser sources.”
The team’s new paper, “Sunlight-excited spontaneous parametric down-conversion for ghost imaging,” appeared in Advanced Photonics on April 24, 2026.
Micah Hanks is the Editor-in-Chief and Co-Founder of The Debrief. A longtime reporter on science, defense, and technology with a focus on space and astronomy, he can be reached at micah@thedebrief.org. Follow him on X @MicahHanks, and at micahhanks.com.