Two laser beams cross at a tiny angle, and an invisible lattice appears in a cloud of rubidium atoms: some atoms are excited, others are not. Then this lattice begins to melt, like a pattern in the sand under the wind. Atoms collide with surrounding gas molecules and drift apart.
The scientists measured this speed for six gases: helium, neon, nitrogen, argon, krypton, and xenon. It turned out that quantum calculations predict the result perfectly when all the subtleties of collisions are taken into account. Now engineers can more precisely choose the gas mixture for magnetometers and pressure sensors, where it matters how quickly atoms leave the working region.
The diffusion coefficient of rubidium in helium is nearly five times larger than in xenon — light helium atoms slow down heavy rubidium less.