Light can hold an atom in place, just like a jet of air holds a lightweight ball. But a dysprosium atom is no ordinary ball: inside it has many "gears" (electrons) that respond differently to light of different colors. To catch such an atom and keep it from slipping away, you need to know exactly how strongly the light pulls on it.

Scientists used a laser with a wavelength of 530.306 nanometers: that's green light, slightly more blue than the color of fresh grass. They directed it at a dysprosium atom and measured how the atom's energy changes depending on the light's polarization (how the light wave "twists"). They got two numbers: one shows the overall attraction, the other shows the difference for different states of the atom. These numbers matched calculations based on the theory of atomic structure.

Why does this matter? Such measurements allow building optical tweezers — traps where each atom sits in its own "nest" of light. If you know the force of attraction in advance, you can arrange atoms neatly like pieces on a chessboard and make them interact with each other. This is a step toward quantum computing, where atoms act as qubits.

Now engineers have verified numbers for green light, and they can design traps for dysprosium more precisely than before.