Physicists have for the first time succeeded in generating quantum-entangled photons using ordinary sunlight as their source. Until now, it was believed that this phenomenon required only stable, well-controlled lasers. The work, published in the journal Optica, demonstrates that even the chaotic light from the Sun can produce genuine quantum correlations.

Why this was considered impossible

Quantum entanglement is a special state in which two particles become linked so that a change in the properties of one instantly affects the other, even when they are separated by large distances. Such photon pairs are usually produced using lasers, which deliver clean, coherent light.

Sunlight, by contrast, is extremely "disorderly"—it contains a broad spectrum of wavelengths and random phases and polarisations. Many experts doubted that entangled pairs could be extracted from such a source at all. According to the study's authors, some well-known scientists even stated outright that capturing any discernible signal would most likely be impossible.

How the researchers overcame the problem

A team led by Cheng Li from the University of Ottawa, together with colleagues at the Max Planck Institute for the Science of Light, took an unconventional approach. Rather than trying to "tame" the chaotic nature of sunlight, they focused on carefully managing the polarisation of the photons.

Sunlight was collected using a Fresnel lens and channelled into an optical fibre about the width of a human hair. A specialised optical setup then extracted photon pairs with the desired properties. In the end, the resulting particles achieved a 94% correspondence to an ideal entangled state. Moreover, they violated Bell's inequality—a classic test confirming that the observed correlations cannot be explained by classical physics.

What this changes in practice

Until now, nearly all quantum technologies—from secure communications to components of quantum computers—have relied on laser sources. Lasers require energy, precise tuning, and complex maintenance. The ability to use free, ubiquitously available sunlight in their place could substantially simplify and reduce the cost of such systems.

One particularly promising application is in space-based communications. Satellites could generate quantum encryption keys directly from ambient sunlight, without depending on onboard lasers and bulky equipment. This opens the way to establishing quantum networks in environments where resources and power are severely constrained.

In brief

For the first time, physicists have succeeded in producing genuine quantum-entangled photons directly from sunlight. The particles exhibited a 94% degree of entanglement and violated Bell's inequality. The discovery proves that quantum technologies do not always require complex lasers—properly "filtered" ordinary sunlight can suffice. In the future, this could simplify secure satellite communications and other quantum systems.