A flashlight beam in fog spreads out within a couple of meters. Light particles scatter in all directions, like a crowd in a square. But sometimes they can march in formation — without spreading, without mixing. That's exactly the kind of march achieved in a tiny perovskite crystal.

The crystal was placed between two mirrors. Light bounces back and forth inside, and the crystal slightly bends its path, like rails guiding a train. As a result, the stream of particles, which scientists call polaritons, lines up in a single file. The beam becomes twenty times narrower than it would be without the crystal.

The most interesting part is that the beam can be controlled. By changing the size of the pump spot, scientists make the light either converge to a point or fan out. No lenses, mirrors, or other optics, just light, a crystal, and the spot size.

This is like a water faucet: turn the handle, and the stream changes shape. Only here, the handle is the pump beam, and the stream is a flow of quantum particles. Such a trick could underpin future optical computers, where beams of light replace wires and tiny crystals replace transistors.