Cheng Yu

An ordinary free-electron laser is like a musical instrument with a single string: it can play only one note until you change its design. To get a different wavelength, you have to retune magnets or accelerate electrons harder. That's expensive and bulky.

A doctor stares at a gray-white smudge on the screen. It's the face of an unborn child. One shadow in the right place, and you can plan surgery in time. But not everyone sees that shadow: cleft lip or palate is rare, and a young specialist might go years without ever spotting one.

A shooter aims at a target, pulls the trigger, and hears two shots: one loud, one quiet. That's how the new setup works, except instead of bullets, there's an electron bunch, and instead of shots, there are two flashes of light. One flash is X-ray, the other is terahertz. They fly to the same point with perfect timing.