Entangled photons are pairs of light particles that remain correlated in ways no classical physics can explain. They are a key resource for quantum communication, computing, and sensing. Such photon pairs are routinely produced through a process called spontaneous parametric down-conversion (SPDC), in which photons from a pump beam are converted inside a nonlinear crystal into pairs of daughter photons.
Laser loses its distinctiveness
For decades, lasers have been considered the only suitable pump source for this process, while sunlight was regarded as an unviable source. This belief was rooted in two fundamental assumptions. First, the high optical coherence of lasers was considered essential. Optical coherence means that the light waves are in a fixed, synchronized temporal or spatial relationship to one another. It was assumed that phase-stable electromagnetic fields are required for generating quantum correlations through SPDC. Second, it was assumed that lasers are the only light sources capable of delivering the optical power densities needed to drive nonlinear optical processes efficiently. Natural sunlight, however, is incoherent and much less intense than laser light. Due to these features, many scientists had dismissed sunlight as a viable pump for entanglement generation.
In a collaborative research project by the Max Planck Institute for the Science of Light (MPL), Max Planck Center for Extreme and Quantum Photonics (MPC) and the University of Ottawa, scientists have demonstrated for the first time that, contrary to the previously held scientific view, entangled photons can be generated from solar light.
Incoherence leaves room for entanglement
In optics, 'coherence' is not a single property. Light can be described by various defined physical parameters, known as degrees of freedom. Thus, light can act regularly in one degree of freedom while remaining disordered in another. For instance, a beam can be perfectly polarized yet completely incoherent in space and time. For SPDC, the pump's incoherence in one degree of freedom only limits entanglement in that same degree of freedom. For instance, position-momentum entanglement – a type of spatial entanglement – is known to vanish when the pump beam loses its spatial coherence. Such a relation does not hold when the targeted entanglement is found in a different degree of freedom than that of the pump beam’s incoherence. “As long as the pump beam is perfectly polarized, its spatial or temporal incoherence should not preclude the generation of polarization entanglement,” says Dr. Cheng Li, a PhD graduate from Prof. Robert Boyd’s group at the University of Ottawa. “The trick to harnessing sunlight is to keep different degrees of freedom of light from influencing each other during the process. This means that sunlight is perfectly capable of generating entangled photons, as long as one can concentrate enough sunlight into a nonlinear crystal to induce SPDC.”
Solar concentrator delivers sufficient pump power
The clear aperture of a nonlinear optical crystal typically spans only a few millimeters. To direct enough sunlight – which di-verges widely in the natural environment – into such a confined region, the group of Dr. Hanieh Fattahi, research group leader MPL built a sunlight concentration system. This system collects light over 1.4 square meters and funnels it down into a fiber as thin as a human hair.
The core of this system is a solar concentrator developed and made in-house at the MPL. It is a cone-shaped device made of glass. The base of the cone is placed at the focal spot of a large Fresnel lens, which is mounted on a solar-tracking motor. The focused sunlight then receives further concentration through total internal reflections as it propagates towards the tip of the cone, which couples the sunlight into a multimode fiber. The fiber can then guide the sunlight into a nonlinear crystal to drive entanglement generation via SPDC.
“We believe this work can inspire much new research in nonlinear and quantum optics”
Robert Boyd
— Full Professor, uOttawa's Department of Physics
Directly driving entanglement generation with sunlight
As predicted by Boyd’s team, sunlight-driven SPDC generated photon pairs with a very high degree of entanglement: With a fidelity of nearly 94%, this comes very close to the performance of a laser. The SPDC photons also display correlations that can violate Bell’s inequality, indicating that these correlations cannot be modelled with classical theories of physics and are in-deed features of quantum entanglement.
The team also found that, when the photon production rate is normalized against the pump power and effective bandwidth of the nonlinear process, the efficiency of sunlight-driven entanglement generation is on par with that which is laser-driven. This finding suggests that lasers may not hold as many fundamental advantages over sunlight as researchers assumed, and practical sunlight-driven quantum light sources can become increasingly realizable through technical optimizations of, for example, sunlight collection and bandwidth utilization. “Sunlight is an abundant and reliable resource in many environments, especially in space. Being able to generate quantum-entangled photons directly from sunlight could enable simpler and more resilient quantum systems for satellites and future deep-space missions,” says Fattahi.
Sunlight-driven quantum devices also offer other advantages that lasers cannot provide. The broad spectrum of sunlight could offer access to entangled photons across a wider range of wavelengths, especially where lasers are not available. More importantly, driving quantum light sources directly with sunlight eliminates the electrical-to-optical conversion entirely — no active stabilization and far less waste heat to manage. Such a system will have fewer points of potential failure. This feature is particularly appealing for deployment in strategically import-ant yet resource-constrained environments, such as satellites, interplanetary missions, and remote regions like the Arctic. A spacecraft in a Sun-synchronous orbit, for example, would enjoy nearly uninterrupted access to its pump source.
“The best part of this research is that it is only a beginning”, says Professor Robert Boyd. “In addition to SPDC, there are many other nonlinear optical approaches to generate entangled photons–four-wave mixing is one good example. For each of these nonlinear interactions, there are ways to make it more efficient. We believe this work can inspire much new research in nonlinear and quantum optics, and these researches may in turn make sunlight-driven quantum technology more practical.”
Original publication in Optica:
C. Li, J. Brar, M. Kublbock, J. Upham, H. Fattahi, R. W. Boyd, “Generating quantum entanglement from sunlight” 13, Opti-ca (2026). DOI: 10.1364/OPTICA.601797.