A stable atomic narrow-band X-ray laser has reached the shortest wavelength ever: 1.5 Å, and about 10 times shorter than had been previously achieved.

Hitoki Yoneda of the University of Electro-Communications in Tokyo and the RIKEN SPring-8 Center in Hyogo, Japan, and colleagues used a photoionisation-pumping scheme proposed back in 1967, where an X-ray photon from a pump pulse removes an inner shell 1s electron from a copper atom, effectively making it an excited medium in which amplified spontaneous emission (ASE) can occur, with photons emitted from 2p electrons that fall to 1s. A second "seed" X-ray photon, tuned to the correct wavelength, is then sent in to be amplified. This is a proof of principle, with a rather low (2%) efficiency, but wavelength stability, which could make this sort of laser competitive with X-ray free-electron lasers in the future.