Researchers at the Linac Coherent Light Source (LCLS) at the U.S. Department of Energy’s SLAC National Accelerator Laboratory have measured the shortest high-energy X-ray pulses ever recorded at an X-ray free-electron laser (XFEL). The pulses had photon energies of about 9 kiloelectronvolts (keV) and clocked in at durations of 100 to 400 attoseconds—mere billionths of a billionth of a second. The results are published in Nature Photonics.