For the quantum internet, we need to learn how to hold light — much like mail is sorted at intermediate nodes. The technology is based on a thin crystal plate doped with europium ions, a rare-earth element that, incidentally, is responsible for the red glow of smartphone screens. The crystal sits between two mirrors, forming an optical cavity — a trap for photons. Light bounces around inside, is absorbed by a europium ion, and after a short time is re-emitted. This analogy of a mailbox that receives and returns letters simplifies the essence: in reality, it's about controlled interaction of light with atom-like systems, where the photon's energy is temporarily stored in the ion's excited state.

Using spectroscopy, a method that analyzes how light interacts with matter, the ideal materials for such a trap were identified. Experiments showed capture efficiencies of 80% for laser pulses and nearly 70% for single photons arriving from optical fiber. According to the Standard Model of particle physics, light in vacuum always travels at the speed of light, but here, inside the crystal, it effectively "freezes"—more precisely, its group velocity drops to zero through repeated cycles of absorption and re-emission.

The thin membrane in the design allows tuning the color (wavelength) of the captured light with a gentle mechanical press — similar to how the width of a mailbox slot changes. This technology builds on the legacy of experiments by Serge Haroche and Jeff Kimble with single photons, now realized in chips for quantum repeaters and optical processors — key components of future quantum networks.