In a molecule resembling chlorophyll and built from carbon, a metal atom at the center acts as a toggle. Shifting it by fractions of a nanometer can make the molecule glow, quench, or boost its brightness a hundredfold. This was demonstrated using a technique combining spectroscopy and photometry with atomic precision — essentially a light microscope that sees individual atoms.
By placing two molecules side by side, physicists turned them into a three-position switch: both off, one glowing, or both working in sync, amplifying each other many times over. Moreover, between different molecules, directed energy transfer can be switched on simply by changing the position of the atom-switch. Feynman dreamed of manipulating individual atoms — now such toggles promise optical circuits where single atoms command light.
The same molecule used in laser printers and CD-R discs now works as an atomic light switch.