By controlling a type of polarized light widely used in quantum optics, researchers have transformed subtle optical patterns in our peripheral vision into vibrant, easily perceptible shapes. Led by Dusan Sarenac at the University of Waterloo, Canada, the team showed for the first time that the “Boehm’s brushes” perceived by participants can be altered by adjusting the topology of spin-orbit coupled light. Their results, reported in Proceedings of the National Academy of Sciences, could lead to promising new techniques for checking our eye health.

For the most part, human eyes are incapable of detecting polarization in visible light. But in some rare cases, structures inside the eye itself can scatter light in different ways depending on its polarization, creating subtle yet perceivable “entoptic” patterns. Perhaps the best-known examples of these are “Haidinger’s brushes”: hourglass-shaped patterns that can form in our central vision, but only when viewing linearly polarized light at blue wavelengths.

In 1940, Swiss biophysicist Gundo Boehm discovered another example: a two-lobed, bowtie-shaped entoptic pattern that forms when viewing linearly polarized light in the peripheral vision as it scatters from subcellular structures in the inner retina. Unlike Haidinger’s brushes, this lesser-known effect is driven by angular variations in scattering strength relative to the polarization direction.

But as Sarenac explains, these ethereal shapes are more than just a curiosity. “Because retinal disease may alter a person’s ability to perceive the pattern, researchers have long considered whether Boehm’s brushes could serve as a biomarker of retinal health,” he says. “However, their weak visibility has limited practical use.”

Whereas previous studies simply asked participants to passively observe the effect, Sarenac’s team considered what would happen if the light’s polarization was constantly varied. To explore this possibility, the researchers created a beam of spin-orbit coupled light, in which polarization is linked to the light’s orbital angular momentum – a property describing how the wavefront twists as the beam travels – so that the polarization direction rotates steadily across the beam.

In quantum optics, these structured waveforms are already being widely explored for their possibilities in imaging, communication and information processing. But for the first time, Sarenac’s team considered how our eyes could actually perceive this light. “We engineered the spatial polarization of the light to match the symmetry of the retinal scattering response,” Sarenac describes. “This allowed many weak local contributions to reinforce one another.”

The researchers then projected this structured light through a ring-shaped aperture, targeting specific regions in the periphery of participants’ retinas, which could also be adjusted to target varying distances from the centre of vision. Using an automated test, they measured how much contrast participants needed to reliably distinguish a Boehm’s brush from the background at these varying distances.

Based on the subject’s responses, Sarenac’s team determined that the use of spin-orbit coupled light not only made Boehm’s brushes appear far more vivid to the participants, it also transformed the bowtie shape into a multi-lobed pattern, with the number of lobes varying depending on the topology of the light’s twisting polarization.

“Across 11 participants, detection thresholds improved exponentially away from the centre of vision, and the pattern became robustly visible at approximately one degree of retinal angular distance,” Sarenac describes. These results reveal a fascinating link between human vision and quantum mechanics, which the team hope could be harnessed in practical healthcare. With further studies involving patients with retinal disease, they now envisage eye tests where retinal conditions could be diagnosed in people who can’t clearly perceive vibrant, multi-lobed Boehm’s brushes in their peripheral vision.

“In the longer term, these patterns may provide a non-invasive functional probe of retinal integrity because their shape and visibility can be measured across different retinal locations,” Sarenac says.

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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.

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.”

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Engineered light transforms Boehm’s brushes from a faint visual pattern into a much brighter one that could help