Modern X-ray light sources are like an orchestra without a conductor: electrons in the beam fly every which way, and the radiation comes out blurry. The new idea is to make electrons line up in neat rows with a spacing of a few nanometers. Then the light they emit becomes coherent, like a laser beam.

To do this, two weak laser beams are crossed in plasma (a gas of charged particles). Their interference creates a density wave that, like a comb, rakes electrons into tidy groups. Then a powerful laser pulse accelerates these groups, and they keep their structure.

This method does not require giant accelerators: everything happens in a small volume of plasma. This opens the way to compact sources of ultra-bright X-rays, which could be useful in medicine, materials science, and for capturing ultrafast processes.