The synchronized return of two quantum-entangled electrons to a helium atom has made it possible to generate ultra-high-energy X-ray photons, bypassing standard theory, infohub.kz reports.
A team of physicists from TU Wien (Vienna University of Technology) and the University of California, San Diego has overturned a fundamental rule of laser physics that capped the maximum energy of X-ray radiation produced through high-harmonic generation. The researchers reported the experimental breakthrough in the journal Nature Photonics, according to ScienceDaily.
In the traditional three-step model of laser radiation, an electric field strips a single electron from an atom, accelerates it and drives it back into its parent nucleus. On its return, the particle releases the accumulated kinetic energy as a high-frequency photon — up to the soft X-ray range. Until now, a strict theoretical ceiling on the maximum energy of such quanta, known as the "cutoff limit," was thought to exist: it depends on the laser parameters and the ionization potential of a single electron. Above this limit, the radiation intensity dropped sharply.
In the new experiment, the researchers directed powerful ultraviolet pulses at helium atoms and succeeded in knocking out both of its electrons in sequence. Strong quantum correlations kept the particles entangled, and precise tuning of the laser field allowed them to be returned to the atom simultaneously. At the moment of collision, both electrons released their energy together, producing single X-ray photons with energy and frequency substantially exceeding the single-electron limit.
In the helium emission spectrum, the scientists recorded a second "plateau" of high-energy harmonics beyond the classical cutoff limit. Notably, experiments with argon and neon produced no such effect, confirming the key role of helium's strong inter-electron correlation. The discovery opens the way to ultra-sensitive spectroscopic sensors for studying quantum electron interactions on attosecond timescales, which is in demand for the design of quantum processors and nanomaterials.