Quantum entanglement has a reputation for needing vast, power-hungry laser systems. A new experiment shows the Sun can do the job.

Scientists from the University of Ottawa and the Max Planck Institute for the Science of Light (MPL) have generated entangled photon pairs directly from sunlight, publishing their results in the journal Optica. Using a process called spontaneous parametric down-conversion — in which a pump beam splits into pairs of entangled photons inside a nonlinear crystal — the team replaced the usual laser pump with concentrated sunlight.

The main challenge was scale: the crystal is only about a millimeter across. Hanieh Fattahi's team at MPL built an all-glass, cone-shaped solar concentrator that collects sunlight with a Fresnel lens roughly the size of a household window and channels it into an optical fiber as wide as a human hair. In an outdoor experiment, quantum state tomography showed the sunlight-generated entanglement was about 94% similar to a perfectly entangled state, and the photons displayed correlations that violate Bell's inequality — proof of genuine quantum entanglement.

First author Cheng Li, a recent University of Ottawa graduate, says the approach could one day let satellites create secure encryption keys using the sunlight already abundant in space, and could help scale quantum computing without adding to its energy burden. The team is now working on increasing brightness and improving entanglement quality.

Sources