Physicists Break a Long-Standing X-Ray Energy Limit Using Correlated Electron Pairs in Helium

Researchers at the University of California San Diego and TU Wien have observed coherent X-rays at higher energies than standard theory predicts, revealing what they describe as a new quantum regime. The results were published in Nature Photonics.

The underlying process, called high-harmonic generation, occurs when intense laser light strikes certain atoms, causing them to emit light at much higher frequencies, sometimes reaching the X-ray range. The technique produced record-setting results at TU Wien in the 1990s and later became part of the work recognised by the 2023 Nobel Prize in Physics. Standard theory predicts a firm energy cutoff beyond which X-ray production drops sharply, depending on the laser used.

'The laser tears a single electron away from the atom. The electron is then accelerated in the laser's electric field until it eventually collides with the atom again. The energy it loses in the process is emitted in the form of light,' said Prof. Tenio Popmintchev of the Institute of Photonics at TU Wien, describing the mechanism Nobel laureate Ferenc Krausz used there in the 1990s. Dimitar Popmintchev, a postdoc in Tenio Popmintchev's team, said the returning electron generates a range of frequencies with roughly equal intensity up to an upper limit: 'Frequencies above this limit are much weaker; it is simply not possible to exceed a certain maximum frequency.'

The researchers then asked what would happen if two electrons participated instead of one. Using intense ultraviolet (UV) laser pulses on helium atoms, they removed the atom's two electrons in sequence, keeping the pair quantum mechanically correlated rather than independent. 'Using UV driving pulses, we can arrange for both electrons to return to the atom at exactly the same time,' Dimitar Popmintchev said. 'The energy of two electrons is then released all at once. And when more energy is available, a single higher-energy X-ray photon with higher frequency can also be generated.'

The experiment revealed exactly that additional high-energy radiation: a second, weaker plateau in the coherent X-ray spectrum extending well beyond the previously established energy range. The effect appeared only in helium; the valence electrons in argon and neon did not produce the same behaviour, adding evidence that the correlation between helium's two electrons is responsible for the higher-energy signal.