They used a technique known as 'high-harmonic' generation to convert a 7 femtosecond infrared pulse into a 1.8 femtosecond X-ray pulse. In harmonic generation a large number of photons are combined in a noble gas - the Vienna team used neon - to create a single photon with a correspondingly reduced wavelength. By filtering the harmonics, it is possible to control the duration and wavelength of the X-ray pulse. Krausz and co-workers used such a pulse to study an laser-induced energy shift in krypton atoms with a time resolution that was shorter than a single cycle of their laser source (which lasts 2.6 fs).

Physicists may be able to use the new technique to study processes that occur on sub-femtosecond timescales such as inner-shell phenomena in atoms and ionization via 'optical tunnelling'. Drescher and colleagues are optimistic that their discovery will pave the way for ultrafast spectroscopy - the study of processes that take place on timescales of just attoseconds - 10-18 seconds.

Over the last four years, other groups have produced laser pulses with durations of around 10 femtoseconds using the high-harmonic method. However, the light in these pulses had longer wavelengths - in the visible and near-infrared regions of the spectrum. These pulses could not be shortened further because they were already approaching the fundamental lower limit for light of these wavelengths.