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.

These high-energy, or “hard,” attosecond X-ray pulses are invaluable for researchers studying the universe at its smallest, fastest scales. “The pulses are both energetic enough to look deep into atomic systems and fast enough to record electronic motion frozen in time,” said Agostino Marinelli, a study author and professor of photon science, particle physics and astrophysics at SLAC.

The challenge, Marinelli explained, wasn’t creating these ultrashort pulses, but rather proving that it had been done. “Over the course of several years, we’ve learned how to create these high-energy attosecond pulses, but measuring pulses this short is really, really difficult. This was the first diagnostic that proves the pulses are indeed only a few hundred attoseconds long.”