Light enters a tiny cavity with mirrored walls and hits an erbium atom hidden in a crystal. The atom absorbs the light and flips its nucleus into another state, like a toggle switch that clicks when pressed. But this switch is not ordinary: it obeys the laws of the quantum world and can be in two positions at once. Such a nucleus is a ready-made quantum bit, or qubit, for future networks where information is carried not by electricity but by single particles of light.
Previously, working with such nuclei was like operating in the dark: to learn their state, researchers had to average signals from many atoms or perform complex indirect measurements. Now, the team placed the erbium-doped crystal in an optical cavity — a pair of mirrors between which light bounces many times, enhancing its interaction with the atom. The cavity is tuned so narrowly that it "sees" only one type of transition in the nucleus, leaving neighboring ones untouched. Thanks to this, they could determine the state of the nucleus in a single measurement with 91% accuracy.
The main bonus is that erbium emits light in the same range as standard optical fiber used for the internet. This means such qubits can be directly connected to existing communication lines, building quantum networks between cities. Erbium nuclei in a crystal are like neat memory cells that are read out with light and do not lose data during transmission.
Erbium is a rare-earth element also used in lasers and optical amplifiers. Its "favorite" wavelength of 1.5 micrometers coincides with the transparency window of optical fiber, so the signal barely attenuates over kilometers.
In Hannu Rajaniemi's novel The Quantum Thief, quantum states are used as currency and weapons; here, erbium nuclear spins become reliable cells for such "quantum cash."