Jiuyi Zhang

A lithium niobate crystal is like a layered cake: inside, electric fields alternate like layers of filling. If you slice such a crystal into thin films and imprint stripes of alternating polarity, you get a periodic structure that makes light change color as it passes through. This is how pairs of photons are born, entangled like twins.

Two pendulums on a stable table swing independently. If you place them on a shaky surface, vibrations synchronize their motion. The same principle worked on a silicon chip with photons. Scientists created two sources of light that emit pairs of particles.