For quantum internet, light needs to be delayed, like mail at a sorting center. At the core is a thin crystal plate with europium ions (the element that gives red glow to smartphone screens). Placed between two mirrors, the crystal forms a trap. Light, ricocheting inside, is absorbed by an ion and emitted back a moment later, just like a mailbox that accepts and returns letters.

Thanks to spectroscopy, ideal materials were selected. Efficiency: 80% for laser pulses and nearly 70% for single photons from optical fiber. According to the Standard Model, light always travels at the speed of light, but here it freezes.

A thin membrane allows tuning the color of captured light with a gentle press, like a mailbox with an adjustable slot. This builds on the single-photon experiments of Serge Haroche and Jeff Kimble, now embodied in chips for quantum repeaters and optical processors.