For decades, quantum physics laboratories have shared an almost inevitable protagonist: the laser. Its light, perfectly ordered and synchronized, seemed to be the essential ingredient to generate one of the strangest phenomena in the universe: quantum entanglement. Now, An experiment shows that nature had a surprise in store: Chaotic and changing sunlight can also produce entangled particles.
The discovery, published in Opticsquestions one of the most deeply rooted ideas in quantum optics and opens the door to simpler and more energy-efficient quantum technologies. The authors, led by Cheng Li, from the University of Ottawa, lThey managed to generate entangled photons using only concentrated sunlight, without resorting to the traditional laser beam.
“Quantum entanglement is fundamental for applications such as secure communications, ultra-high precision sensors or quantum computing – explains Li -. Our work demonstrates that abundant and free natural sources of light can also be used to produce itwhich could make these technologies more accessible and energy efficient.”
Entanglement is one of the most puzzling phenomena described by quantum mechanics. When two particles become entangled, their properties remain correlated in a way that cannot be explained by classical physics. Even if they later separate, measuring one of them allows us to instantly know the status of the other, even if they are separated by a great distance. Albert Einstein called this idea “ghostly action at a distance”convinced that there must be a more conventional explanation. However, decades of experiments have confirmed that entanglement is a real phenomenon and today constitutes one of the pillars of quantum technologies.
Until now there was a very simple reason for using lasers: they produce extremely orderly light. Laser waves advance in sync, with their crests and troughs perfectly aligned, like a crowd marching in lockstep. This coherence seemed essential to generate pairs of entangled photons. Sunlight, on the other hand, represents almost the opposite extreme. It is made up of millions of different colors and comes from a multitude of different directions. It is, in physical terms, a highly incoherent source.
For a long time it was assumed that this apparent disorder would prevent entanglement from occurring. But the new study shows that nature is much more “permissive.” The key to the study is demonstrate that not all light needs to be perfectly organized; one of its properties is enough, in this case, polarization.
To understand this, we can imagine a crowd leaving a stadium. Each person walks at different speeds, takes different streets and wears different colored clothes. Everything seems chaotic. However, if they all held an umbrella pointing in exactly the same direction, there would be a common pattern hidden within that disorder. That’s precisely what Li’s team discovered.
“We designed the experiment so that differences due to different colors and propagation directions did not influence the polarization of the photons,” adds Li. “As our theory predicts, if the entanglement resides solely in the polarizationit only matters that that property remains ordered, even if the rest of the light is completely incoherent.
The next challenge was engineering. Typically, the nonlinear crystal where the entangled photons are generated measures only a few millimeters. Instead, sunlight arrives scattered from an apparent half-degree disk in the sky. To solve the problem, the team at the Max Planck Institute for the Science of Light designed an all-glass concentrator. This is a large Fresnel lens, similar in size to that of a conventional window, collected solar radiation and concentrated it within an optical fiber as fine as a human hair. That fiber finally guided the light to the tiny crystal where the quantum process took place.
Thanks to this, Li’s team not only managed to generate pairs of photons, they also demonstrated that they were truly entangled. Using quantum tomography they verified that The state obtained reached a fidelity of 94% with respect to an ideal quantum state. Furthermore, the observed correlations violated Bell’s inequalities, one of the strongest criteria to demonstrate that a phenomenon cannot be explained by classical physics. In other words, the photons produced by sunlight exhibited unequivocally quantum behavior.
Although this is still an experimental demonstration, the potential applications are numerous. The study points out, for example, satellites capable of generating quantum encryption keys simply using sunlight.reducing energy consumption and eliminating a good part of the complex laser system that current designs require.
“Since this project began, many colleagues doubted that we could even detect a single photon using sunlight, let alone entangled photons,” Li recalls. “But we were confident in our calculations, we continued to improve the experiment, and we finally showed that it was possible”.
However, perhaps the most important consequence is not technological, but conceptual. For decades it was believed that entanglement required perfectly ordered light. This work demonstrates that it is enough to preserve a small pattern of order hidden within the apparent chaos. Nature, once again, has turned out to be much less strict than we imagined. And that means that even The light that has traveled 150 million kilometers from the Sun can become, upon reaching Earth, raw material for one of the most sophisticated technologies that the human being has conceived.