Sunlight generates entangled photon pairs for the first time

Entanglement, the quantum property that leads to correlations beyond what is classically attainable, is widely known to be fragile. Far less discussed is the extremely high energy cost one must pay to create and maintain such an elusive quantum phenomenon. Generating entangled photons, for instance, typically requires a laser source, which wastes a considerable amount of energy with stabilization, temperature control, and heat dissipation. In an era of rising energy demands, especially within the information technology sector, the quest for sustainable quantum technologies is quickly becoming a pressing need.

Yet lasers have remained the primary source of photonic entanglement for decades, as they are coherent and powerful enough to drive the so-called Spontaneous Parametric Down-Conversion (SPDC), a widely used method for generating entangled photon pairs. They were almost universally believed to be indispensable until, very recently, researchers at Max Planck Institute for the Science of Light (MPL), Max Planck Center for Extreme and Quantum Photonics (MPC) and the University of Ottawa challenged that view. In a study published in Optica, they have demonstrated for the first time that sunlight —highly incoherent and much less intense than laser light— can also generate entangled photon pairs through SPDC, ruling out any physical impediment to entanglement generation from this natural light source.

While the system cannot yet be deployed as a real-life technological asset, these findings point toward a sustainable alternative for the generation of entangled photon pairs. This could be particularly useful in environments where resources are limited, such as the Arctic or satellites in space. Future satellites, for instance, could leverage sunlight, which is already abundant in space, to create quantum secure keys, bypassing the need for energy-intensive laser stabilization and thermal management.

Beyond logistics, the approach may offer unexpected performance advantages. Sunlight’s inherent robustness against atmospheric turbulence may be beneficial in free-space links for quantum key distribution, and its broad spectral bandwidth could enable access to entangled photons over wider wavelength ranges for quantum communication applications in general.

Overcoming sunlight’s low coherence and intensity

A highly coherent light source is necessary to drive SPDC. Lasers, with their high coherence across space, time, and polarization, have therefore been the traditional choice. But the team, inspired by previous studies that used LEDs (a polarized but spatiotemporally incoherent source) to generate entanglement, realized a crucial insight: creating entanglement in one degree of freedom does not require coherence in another. Sunlight, though highly incoherent in space and time, can be polarized. That made it a viable candidate for generating polarization entanglement via SPDC.

Intensity posed an additional challenge. SPDC is a nonlinear process and, as such, requires a sufficiently strong light source to occur. Laser intensity is well above the threshold; sunlight’s is not. To overcome this, the group of Dr. Hanieh Fattahi at MPL built a sunlight concentration system that collects light over an area of 1.4 square meters and funnels it into an optical fiber thinner than a human hair. This concentrated light was then directed to the nonlinear crystal responsible for SPDC and entanglement generation.

The result? Photon pairs whose polarization correlations systematically violated the Bell test, confirming their non-classical nature. Remarkably, their degree of entanglement was comparable to that of laser-based SPDC. “The best part of this research is that it is only a beginning,” says Prof. Robert W. Boyd, group leader at MPC in the press release. “We believe this work can inspire much new research in nonlinear and quantum optics, and this research may in turn make sunlight-driven quantum technology more practical.” 

Reference:

Cheng Li, Jasvinder Brar, Michael Küblböck, Jeremy Upham, Hanieh Fattahi, and Robert W. Boyd, "Generating quantum entanglement from sunlight," Optica 13, 1508-1514 (2026).