Light in an optical fiber runs along the core, barely escaping outside. But if you get very close, you can feel a faint trembling of the field at the surface — the evanescent wave. It is in this 'light fog' above the surface of a tiny silicon ring that physicists have caught a single rubidium atom.
The trap works like an invisible hand that stops the atom's speed with a single touch. The mechanism resembles Sisyphus cooling: a photon takes energy from the atom, and it rolls into a potential well from which it can no longer escape. After a single scattering event, the atom ends up at a distance of 150–200 nanometers from the chip. That is about a thousand times thinner than a human hair. Without any continuous cooling, it stays there from fractions of a millisecond up to a full second.
Once trapped, the atom starts actively communicating with light. It emits single photons that are collected right on the chip. The Purcell effect makes it glow faster than in free space. The cooperativity, a measure of the joint work of the atom and the resonator, exceeds one. This means the atom prefers to give its energy to the resonator rather than somewhere else. Such a 'tandem' opens the way to quantum circuits where atoms and light are combined on a single crystal made with standard silicon technology.
The thickness of a human hair is about 80 micrometers. The atom hovers 200 times closer to the chip surface than that — at a distance that light crosses in half a trillionth of a second.
In Stanislaw Lem's novel 'Solaris', the ocean interacted with people at a distance imperceptible to instruments — much like here, where a single atom senses the light field without touching the chip.