Producing pairs of correlated or entangled photons usually requires a complex laser system. Now, however, researchers at China’s Xiamen University have shown that these crucial ingredients in quantum optics experiments can come from a far more basic source: sunlight. The discovery could simplify optical systems that rely on a process known as spontaneous parametric down-conversion (SPDC), allowing the technology to be deployed in space and other locations with limited or no access to electricity.
SPDC occurs when a short-wavelength photon passes through a nonlinear crystal and gets converted into twin photons with a longer wavelength. Traditionally, the initiating (pump) photon in this paired-photon source comes from a coherent laser. However, recent research suggested that fully coherent light sources might not be strictly necessary, and that partially coherent sources might also be able to drive SPDC.
A Sun-tracking system
In the new work, researchers led by Wuhong Zhang and Lixiang Chen took this idea even further. Could sunlight – which is inherently incoherent – also drive SPDC?
In principle, Zhang and Chen suspected the answer was “yes”. After all, a ray of sunlight, like a laser beam, is just a stream of photons. The problem is that unlike photons from a laser, the brightness and incidence angle of solar photons is constantly changing. This makes it difficult to collect enough pump photons to produce correlated photon pairs at high rates.
To overcome this problem, the researchers installed a Sun-tracking system on the roof of their laboratory building. This was essentially a telescope mount that moves with the Sun to collect light continuously throughout the day. They then worked out how to efficiently couple this collected sunlight into a multi-mode fibre and transmit it into their laboratory. There, they used the light to pump a nonlinear crystal made of periodically poled potassium titanyl phosphate (PPKTP). Within this crystal, the SPDC process converted pump photons into correlated photon pairs, demonstrating that it is indeed possible to use sunlight to generate photon pairs with strong position correlations.
The team faced numerous challenges along the way. The low spatial coherence and temporal instability of sunlight needed to be mitigated, and the coupling into the fibre needed to be very efficient. However, Chen notes that sunlight does have one advantage over traditional laser sources. “Sunlight is inherently broadband in its spectrum, so it can precisely provide any favourable wavelength,” he says. “This means it could be adapted to diverse application scenarios.”
Towards laser-free and electricity-independent SPDC light sources
According to Zhang, the team’s work shows that laser-free and electricity-independent SPDC light sources are possible. Potential applications could include correlation-enhanced sensing in remote areas and space-based quantum key distribution and teleportation, and he tells Physics World that one of the team’s next goals is to test the system in outdoor environments.
New protocol transmits quantum information in complex states of light
Chen adds that the system, which is described in Advanced Photonics, could also become a platform for fundamental studies of how the coherence of light affects the photon-splitting process in SPDC. He and his colleagues are now focusing on improving the efficiency of their sunlight collection system; optimizing the design of their nonlinear crystal so that it is better adapted to the Sun’s broadband spectrum; and implementing advanced image reconstruction techniques such as compressed sensing. “We think that introducing AI technologies such as artificial neural networks and deep learning into such imaging systems will be a key priority in going forward,” Chen says. “They will inform us on how to exploit sunlight more efficiently to implement various advanced quantum information protocols.”
