Correlated and entangled photon pairs are essential tools in quantum optics. Scientists typically create these photon pairs through a process called spontaneous parametric downconversion (SPDC). In this process, a powerful and highly stable laser shines onto a nonlinear crystal. Because SPDC relies so heavily on coherent laser light, researchers have long considered the technology impractical outside of carefully controlled laboratory environments.
Recent studies have shown that SPDCs do not actually require perfectly coherent light to function. Even partially coherent light sources can generate correlated photon pairs and can also transfer some of their unique coherence properties to the generated photons. This discovery led researchers to ask interesting questions. Could sunlight itself be used to generate correlated photon pairs?
Using sunlight for quantum optics
There are significant obstacles to converting sunlight into a usable SPDC light source. The sunlight that reaches Earth constantly fluctuates in brightness, direction, and position, making it difficult to maintain the precise alignment needed for SPDC experiments and photon detection.
At the same time, sunlight also has significant benefits. Unlike lasers, it does not require electrical power or complex laboratory equipment. Solar-based systems can also operate in remote locations and space where traditional laser systems are impractical.
A research team led by Wuhong Zhang and Lixiang Chen from Xiamen University has demonstrated a practical solution. Writing in progress advanced photonicsscientists described an experimental setup that uses sunlight as the sole pumping source for SPDC.
Their system includes an automatic solar tracker similar to an equatorial telescope mount. The tracker tracks the sun continuously throughout the day, directing it into 20 meters of plastic multimode optical fiber. The fiber carries the light into a dark indoor laboratory, where it excites a periodically poled potassium titanyl phosphate (PPKTP) nonlinear crystal.
Sunlight successfully generates correlated photon pairs
Despite the instability of natural sunlight, this setup was successful in producing photon pairs with strong positional correlation. To test the system, the researchers used photon pairs for ghost imaging. Ghost imaging is a quantum imaging technique that uses correlated photons instead of direct spatial detection to reconstruct images.
The solar powered system achieved 90.7% visibility of ghost imaging. This is close to the 95.5% visibility produced by a standard 405 nm laser operating at the same pump power.
Beyond simple double-slit imaging, the researchers also reconstructed more detailed two-dimensional images called “ghost faces.” The results demonstrated that the photovoltaic system can handle more complex spatial patterns.
According to the researchers, the broad spectrum of sunlight supports quasi-phase matching inside nonlinear crystals, allowing the creation of large numbers of positionally correlated photon pairs. By collecting data over an extended period of time, the team improved both the signal-to-noise and contrast-to-noise ratios and showed that the system could maintain stable performance despite natural fluctuations in sunlight.
Completely passive quantum imaging system
This experiment marks the first successful demonstration of the combination of solar-pumped SPDC and ghost imaging. By removing the need for lasers and external power, the system creates a completely passive source of correlated photon pairs.
The researchers believe this technology could be particularly useful for future quantum imaging and quantum information systems used in remote environments and space-based applications.
They also noted that advances in solar harvesting, crystal engineering, and image reconstruction techniques, including compressed sensing and machine learning, could further improve image quality and image processing speed, while also moving the technology closer to real-world applications.
