Sunlight generates quantum entanglement for the first time — lasers no longer required
Researchers at the University of Ottawa and the Max Planck Institute for the Science of Light in Erlangen have generated entangled photon pairs using focused sunlight for the first time. The findings, published in Optica, show that lasers are not strictly necessary to produce quantum entanglement, overturning a long-held assumption.
Entangled photons are pairs of light particles with linked quantum properties. Measuring one particle can reveal information about the corresponding measurement of the other, even when they are separated. This behavior makes them useful for quantum encryption and other forms of quantum information processing.
Most systems generate such pairs through spontaneous parametric down-conversion, where a laser shines through a special crystal, producing photons with correlated quantum properties. Lasers have been favored because they emit coherent light: waves maintain a consistent phase and typically operate within a narrow range of wavelengths.
Sunlight, by contrast, is incoherent. It contains many wavelengths and arrives from different directions, making it seem an unlikely candidate for generating entangled photons. However, the authors had previously explored the question through theoretical work and experiments with light-emitting diodes, suggesting that incoherent light could generate entanglement if the relevant quantum property did not depend on wavelength or direction.
For the solar test, the light had to be concentrated tightly enough to direct it into a small crystal. Hanieh Fattahi's team at the Max Planck Institute built a glass, cone-shaped concentrator that collected light from a window-sized Fresnel lens and funneled it into a thin optical fiber.
The system was tested outdoors at the institute over three days. The resulting photon pairs achieved about 94% fidelity with a perfectly entangled state. The experiment also violated Bell's inequality, a test that helps confirm genuine quantum correlation.
The work does not suggest that sunlight can replace laser-based quantum sources, which remain more precise and produce stronger results. But it demonstrates that the light used to create entangled photons does not have to come from a laser, potentially reducing the energy demands of quantum technologies.
