Light falling on a crystal reflects not like from a mirror, but in distinct narrow beams. This effect, discovered by William Lawrence Bragg, is called Bragg scattering and typically requires billions of atoms arranged by nature into a regular lattice. But physicists have managed to do the same with just a handful of individual atoms. Using laser tweezers, focused beams that act like invisible hands, they placed several atoms in a perfect line. The spacings were chosen so that the atoms act like the teeth of a comb for light waves: the incoming light gets "combed" and leaves as a single thin beam.
To achieve this, the atoms had to be cooled to near absolute zero — otherwise thermal jitter would blur the lineup. Adding atoms one by one, the scientists watched the reflected peak become sharper and sharper. The most striking thing: the effect appeared with a chain of just a handful of particles, not billions. Previously, such a thing had only been seen in bulky crystals.
Such an atomic "comb" promises optical circuits that can be tuned by changing the number and arrangement of elements. This paves the way for quantum computers, ultrasensitive sensors, and mirrors made of individual atoms.
Bragg scattering is typically seen in crystals with billions of atoms; here, for the first time, physicists observed it from a chain of just a few atoms, adding them one at a time.