A laser is tuned to a frequency at which a rubidium atom normally does not get excited. But if an excited atom is already nearby, it shifts the energy levels of its neighbor, and the neighbor starts absorbing light. So the pair of atoms acts like a duo: one helps the other light up.

Physicists tested this with different atomic states — S, P, and D. For each state, they measured how many atoms got excited when the laser was detuned to the blue or red side. It turned out that with repulsive interactions, blue detuning works, and with attractive interactions, red detuning works.

This is like two friends in a crowd: one raises a hand, and the other, even without hearing the command, does the same. In the microworld, such "friendship" could become the basis for quantum computing, where atoms need to act in concert.