Scientists have turned ordinary sunlight into a source of quantum entanglement, opening a surprising path toward more energy-efficient quantum technology.
Quantum technologies often depend on powerful lasers that consume significant amounts of energy. As these systems grow larger and more widespread, their energy requirements could become an increasingly important concern. Now, researchers have demonstrated that sunlight itself can be used to generate quantum entanglement between photons, potentially offering a more energy-efficient alternative.
“Quantum entanglement is crucial for applications such as secure communication, ultra-precise sensing and high-performance computation,” said Cheng Li, a recent graduate of the University of Ottawa in Canada. “Our work shows that abundant natural light sources can be used for quantum entanglement, opening the possibility of more energy-efficient and accessible quantum technologies.”
Published in Optica, Optica Publishing Group’s journal for high-impact research, the study found that sunlight could produce entanglement comparable to laser-based methods once differences in the bandwidth of the incoming light were taken into account. The work brought together theoretical advances from Robert Boyd’s team at the University of Ottawa and a new solar concentrator created by Hanieh Fattahi’s team at the Max Planck Institute for the Science of Light (MPL) in Germany.
“This technology could one day enable satellites to create secure encryption keys using the sunlight already abundant in space, reducing the need for onboard lasers and much of the supporting hardware,” said Li, first author of the paper. “Sunlight-driven entanglement generation could also provide the crucial ingredient needed to scale up quantum computing without adding to the energy burden.”
Rethinking the Light Needed for Quantum Entanglement
Scientists have traditionally believed that creating the strong correlations needed for photon entanglement requires coherent light. In coherent light, the waves remain synchronized, with their peaks and valleys following a predictable relationship. Lasers are therefore commonly used because they produce highly coherent light concentrated around a single color.
Earlier research from Boyd’s team challenged that assumption. The researchers predicted theoretically and then demonstrated experimentally that incoherent light could also generate quantum entanglement. Using an LED, which produces incoherent light, they successfully created polarization-entangled photons.
That work showed that light does not have to be orderly in every respect to produce entanglement. For example, photons can travel in disorganized directions while still becoming entangled through another characteristic, such as polarization.
The new Optica study pushes that concept further by replacing the LED with sunlight. Sunlight presents a greater challenge because it spreads in many directions and contains a wide range of colors.
Creating Entangled Photons From Sunlight
To generate entanglement, the researchers used spontaneous parametric down-conversion (SPDC), an established optical process involving a nonlinear crystal. When a pump beam interacts with the crystal, individual photons can split into pairs that may become quantum entangled.
Normally, the pump beam comes from a laser. In this experiment, the team instead used sunlight that had been strongly polarized while remaining highly incoherent across space and time. This meant the overall light field oscillated in the same direction even though it contained photons of many colors traveling along different paths.
“We designed our experimental setup so that differences introduced by the different colors and propagation directions didn’t influence the photons’ polarization,” said Li. “As our theory predicts, if the entanglement lives only in polarization, then it should only depend on the pump’s orderliness in its oscillation direction and not on its direction or color. This allowed us to produce high-quality polarization entanglement from highly spatially and temporally incoherent sunlight.”
Focusing Sunlight Onto a Tiny Crystal
A major practical challenge was concentrating enough sunlight onto a nonlinear crystal only a few millimeters in size.
To overcome this problem, Fattahi’s team at MPL developed an all-glass solar concentrator. The cone shaped device collects sunlight gathered by a Fresnel lens roughly the size of a household window and directs it into an optical fiber approximately as thin as a human hair. From there, the concentrated light can be focused onto the tiny nonlinear crystal responsible for generating entanglement.
Putting Sunlight-Based Entanglement to the Test
The researchers tested their approach in an outdoor experiment at MPL. They used quantum state tomography to analyze the quantum state produced by the setup and found that the entanglement generated from sunlight was about 94% similar to a perfectly entangled state.
The photons also displayed correlations that violate Bell’s inequality. Such correlations cannot be explained by classical physics, providing evidence that the photons were genuinely quantum entangled.
With the proof of principle now established, the researchers are working toward a system that could eventually be deployed outside the laboratory. Their next steps include increasing the brightness of the system and improving the quality of the entanglement it generates.
The researchers also note that the concept may extend beyond SPDC. Other nonlinear optical techniques, including four-wave mixing, could potentially use similar approaches, opening additional possibilities for quantum photonics.
A Quantum Idea That Faced Early Skepticism
The researchers say the project initially met considerable skepticism because sunlight seemed like an unlikely source for producing useful quantum effects.
“Since the inception of this project, our idea has met with repeated doubt and pushback,” said Li. “Some world-renowned researchers in the field even questioned whether it would be possible to detect any photons — not to mention entangled photons — from sunlight-driven nonlinear optical processes. However, we trusted our calculations, continued improving the experimental setup, and eventually showed that it was possible.”
Reference: “Generating quantum entanglement from sunlight” by Cheng Li, Jasvinder Brar, Michael Küblböck, Jeremy Upham, Hanieh Fattahi and Robert W. Boyd, 19 August 2026, Optica.
DOI: 10.1364/OPTICA.601797
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