Researchers from TU Wien and UC San Diego have exceeded a long-accepted limit in X-ray generation by using ultraviolet lasers to manipulate helium atoms in a way conventional theory does not explain. The team directed intense UV laser pulses at helium, freeing both electrons from the atom in succession. Though freed at different moments, quantum correlations kept the electrons' behavior connected. By timing the laser pulses precisely, the researchers arranged for both electrons to return to the atom simultaneously and release their combined energy as a single X-ray photon.
The resulting X-rays exceeded the energy cutoff that standard theory predicts for single-electron processes. Prof. Tenio Popmintchev of TU Wien's Institute of Photonics explained that conventional understanding holds: "There is an upper limit. Frequencies above this limit are much weaker; it is simply not possible to exceed a certain maximum frequency." The new experiment broke through that boundary using two electrons instead of one.
The researchers identified a second plateau of sustained emission at much higher photon energies than predicted. This signal appeared only with helium, not with argon or neon, because helium's two electrons show particularly strong quantum correlations allowing their synchronized energy release. The findings were published in Nature Photonics.
Beyond producing higher-energy X-rays, the discovery offers a new tool to study electron interactions. The unusual coherent X-ray emission could act as a sensitive spectroscopic sensor for electron-electron correlations, allowing scientists to observe quantum processes occurring over attoseconds, or billionths of a billionths of a second. The same principle may eventually extend to molecules and solid materials where electrons interact strongly, potentially aiding quantum computing research and the design of advanced nanomaterials.