Bin Li

A microwave signal that doesn't jitter from interference is needed in both radar and telecommunications. Until now, a clean signal was obtained on bulky setups where lasers and resonators sat on an optical table. Now engineers have packed everything into a box the size of a cup—166 milliliters.

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 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.

In a ballroom where couples were dancing, the music changed, and the pairs lined up in rows suddenly spun into whirlpools. That is how the magnetic crystal Fe3Ge rearranges its internal pattern once the temperature climbs above 375 kelvins, which is about a hundred degrees of heat.