Researchers report generating soft‑x‑ray light pulses lasting roughly 100 attoseconds (1 attosecond = 10^-18 seconds) with peak power on the order of 10 terawatts at the European XFEL. Each pulse carries millijoule‑level energy, an intensity scale far above what table‑top attosecond sources typically deliver. This combination of very short duration and very high peak power opens a new operating point for attosecond experiments.

To make these pulses the team used the soft‑x‑ray undulator line of the European XFEL together with an unusually strong scheme for compressing the electron bunches that drive the laser. The accelerator ran at an electron energy of 14 GeV. A relatively large charge bunch (350 pC) was compressed in several stages to achieve ultrahigh peak currents. The x‑ray light itself is produced by the usual free‑electron laser process called self‑amplified spontaneous emission (SASE). In SASE, tiny density fluctuations in the relativistic electron beam grow inside the undulator and produce intense, short x‑ray pulses.

A key technical step was exploiting the facility layout and a collimator magnet as an extra compression stage. The authors fully compressed the bunch in their last nominal compressor (BC2) and then used collective effects that occur downstream — mainly longitudinal space charge (forces between electrons because of their spacing) and coherent synchrotron radiation (radiation produced when the beam bends) — which change the energy pattern along the bunch. By deliberately offsetting the beam energy (an example offset of +1.5% is discussed) the team turned a normally second‑order effect into an effective first‑order compression. This produced the very short lasing slices needed for attosecond pulses.

Why this matters: table‑top high‑harmonic generation sources have already produced attosecond pulses, but they are limited in pulse energy and peak power. The European XFEL demonstration pushes attosecond pulses into the soft‑x‑ray regime with many times higher peak power. That stronger, shorter light can drive and probe electron motion in atoms, molecules and solids in new, nonlinear regimes that were previously out of reach.