Correlated electrons extend X-ray high-harmonic generation beyond the single-electron limit

High-harmonic generation underpins attosecond science. For over three decades, high-harmonic upconversion has been framed within the confines of a single-active-electron light-matter interaction featuring a well-defined cutoff photon energy. Here we demonstrate experimentally that correlated electrons can propel high-harmonic emission beyond this single-active-electron limit, markedly increasing the generated photon energies. We observe a weak secondary plateau that extends the conventional cutoff beyond 120 eV up to the water window at 280 eV. This phenomenon arises from double-electron recombination of strongly correlated electron pairs, resulting in a new cutoff scaling of up to 5.5 times the ponderomotive energy of the rescattering electrons, which notably deviates from the conventional scaling factor of 3.2. These findings reshape our fundamental understanding of the high-harmonic upconversion process and position high-harmonic generation as a potent photonic probe of attosecond-to-femtosecond electron correlations in quantum systems, opening new pathways for advanced ultrafast spectroscopy, novel attosecond source development and the exploration of strongly correlated quantum materials.