Nature, Published online: 14 January 2026; doi:10.1038/s41586-025-09911-1

Using single broadband X-ray pulses from a free-electron laser on a gaseous neon target, coherent, nonlinear four-photon interactions with core–shell electrons is demonstrated, representing a strategy for multidimensional correlation spectroscopy at the atomic scale.

Nature, Published online: 14 January 2026; doi:10.1038/s41586-025-09911-1

Using single broadband X-ray pulses from a free-electron laser on a gaseous neon target, coherent, nonlinear four-photon interactions with core–shell electrons is demonstrated, representing a strategy for multidimensional correlation spectroscopy at the atomic scale.