Sunlight used to generate quantum entanglement in outdoor experiment

Scientists from the University of Ottawa and the Max Planck Institute for the Science of Light have generated quantum entanglement between photons using sunlight instead of energy-intensive lasers. The experiment, conducted outdoors at the Max Planck Institute in Germany, achieved entanglement fidelity of about 94 percent relative to a perfectly entangled state.

The work builds on theoretical predictions from Robert Boyd's team at the University of Ottawa that incoherent light sources could produce polarization entanglement. Earlier experiments had used LEDs, but sunlight presents greater challenges because it is highly divergent and spans a broad spectrum of colors.

To overcome these challenges, the researchers used spontaneous parametric down-conversion, a process in which a pump beam interacts with a nonlinear crystal to create entangled photon pairs. They designed the setup so that only the pump light's polarization orderliness mattered, not its spatial or temporal coherence.

A key innovation was a cone-shaped all-glass solar concentrator developed by Hanieh Fattahi's team. The device collects sunlight with a household window-sized Fresnel lens and focuses it into an optical fiber the width of a human hair, delivering sufficient intensity to the millimeter-sized nonlinear crystal.

Quantum state tomography confirmed the entanglement quality, and the photon correlations violated Bell's inequality, confirming their quantum nature. First author Cheng Li, a recent University of Ottawa graduate, noted the approach could eventually allow satellites to generate secure encryption keys using abundant sunlight in space.

The researchers are now working to improve the brightness and entanglement quality toward a field-deployable system. They say the sunlight-driven method could extend to other nonlinear optical processes such as four-wave mixing, potentially opening new directions in quantum photonics.