A glowing spot in a crystal is like a lighthouse in fog: the longer it shines, the more reliable the signal. In quantum devices, ions of rare-earth metals embedded in a transparent lattice play the role of such a lighthouse. They absorb energy and release it as a flash of light. But if there are atoms with magnetic moments nearby, the flash quickly dies out.

Researchers took cerium oxide — a substance where all atomic nuclei are "magnetically silent." They added two types of ions to this film: erbium and thulium. Erbium glowed for 2.9–5.3 milliseconds, like a lamp that stays on while you read a paragraph. Thulium flashed and faded in 14–68 microseconds — faster than an eye blink.

Calculations confirmed: in thulium, the electron clouds of oxygen and thulium overlap like two circles in a diagram. In erbium, the circles barely intersect. That is why erbium in cerium oxide is a promising candidate for quantum memory, where a photon must be stored for as long as possible.