Sunlight generated quantum entanglement with 94% similarity to laser-based methods, according to a study, marking a breakthrough in energy-efficient quantum technology.
In an experiment, researchers demonstrated that sunlight can produce quantum entanglement comparable to laser-based systems, achieving 94% similarity to a perfectly entangled state. This development, reported in Optica and Advanced Photonics, challenges longstanding assumptions about the need for coherent light sources in quantum technologies and opens new pathways for energy-efficient applications.
Sunlight-Driven Quantum Entanglement
Cheng Li, a recent graduate of the University of Ottawa in Canada and first author of the paper, explained that sunlight’s incoherent nature—spreading across multiple colors and directions—was once thought incompatible with quantum entanglement. However, the team showed that polarization-entangled photons could be generated by carefully controlling how sunlight interacts with a nonlinear crystal. 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,
Li said.
The experiment used a solar concentrator created by Hanieh Fattahi’s team at the Max Planck Institute for the Science of Light (MPL) in Germany. This device, featuring a Fresnel lens and optical fiber, focused sunlight onto a millimeter-sized nonlinear crystal. The setup enabled spontaneous parametric down-conversion (SPDC), a process where photons split into entangled pairs. Quantum state tomography confirmed the entanglement quality, with results 94% similar to ideal entanglement. 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,
Li noted.
Technical Innovations in Solar Concentration
The solar concentrator addressed a major challenge: directing enough sunlight onto the tiny nonlinear crystal. Traditional methods were inadequate for capturing the diffuse, incoherent light. Fattahi’s team solved this with an all-glass system that collected sunlight through a Fresnel lens and channeled it into an optical fiber. This allowed the team to maintain the polarization coherence required for entanglement despite sunlight’s spatial and temporal incoherence.
Meanwhile, a study by Wuhong Zhang and Lixiang Chen at Xiamen University demonstrated sunlight-pumped ghost imaging, a technique that reconstructs images using correlated photons. Their system used an automatic sun-tracking device to direct sunlight into a nonlinear crystal, generating photon pairs with 90.7% visibility in ghost imaging—a result close to the 95.5% visibility achieved with a conventional 405 nm laser operating at the same pump power. Sunlight’s broad spectrum supports quasi-phase matching inside the nonlinear crystal, helping generate large numbers of position-correlated photon pairs,
the team explained.
Implications for Quantum Technologies
The findings have significant implications for quantum communication, computing, and imaging. By eliminating the need for lasers, sunlight-driven entanglement could reduce energy consumption in quantum systems, a critical factor as these technologies scale. For example, satellites could use solar power to create secure encryption keys without relying on energy-intensive onboard lasers, according to Li.
The research also advances passive quantum imaging. Zhang’s team highlighted that sunlight’s variability could be mitigated through extended data collection, improving signal-to-noise ratios. By collecting data over longer periods, we improved both signal-to-noise and contrast-to-noise ratios,
the researchers said. This approach could enable quantum imaging in remote or extreme environments where traditional laser equipment may be difficult to use.
Both studies underscore a shift toward leveraging natural light sources for quantum applications. While lasers remain essential for precise control, sunlight’s abundance and low energy cost offer a compelling alternative for specific tasks. As Li noted, 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.
The next steps involve refining solar concentrators and exploring applications in space and remote sensing.
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