Scientists just broke a textbook rule of physics
Scientists have defied a long-standing rule of laser physics by generating X-rays with much higher energies than conventional theory predicts. The surprising discovery reveals how two electrons working together can overcome a previously accepted limit, opening new possibilities for quantum research.
Key points
- Researchers at TU Wien and UC San Diego produced X-rays with higher energies than standard theory predicts.
- The effect relies on two electrons from helium atoms releasing their energy together.
- The findings were published in Nature Photonics.
Scientists have pushed X-ray generation past a long-accepted limit in laser physics, observing a quantum effect that conventional theory does not explain. By directing powerful ultraviolet lasers at helium atoms, researchers produced X-rays with substantially higher energies than the standard model predicts. The work was a collaboration between TU Wien and the University of California San Diego and was published in Nature Photonics.
The established theory predicts a maximum energy for X-ray emissions from laser-driven atoms, known as the energy cutoff. Beyond this boundary, X-ray production becomes extremely weak. The new experiment shows this limit can be exceeded when two electrons interact in a particular way.
How the two-electron process works
In the conventional process, a laser tears a single electron from an atom, accelerates it, and the electron returns to release energy as light. The team used intense ultraviolet laser pulses on helium atoms so that both of helium's electrons were removed in succession. Although freed at different moments, the electrons remained connected through quantum correlations, and the researchers arranged for both to return to the atom simultaneously.
Dimitar Popmintchev, a postdoc in Tenio Popmintchev's team at TU Wien, said the combined energy is released all at once. The coordinated release produced X-ray photons exceeding the conventional energy cutoff. The spectrum showed a second region of sustained emission at much higher photon energies than the standard theory predicts, weaker than the primary emission but extending well beyond the established range.
The energy of two electrons is then released all at once.
— Dimitar Popmintchev
Helium shows the effect
The unexpected signal appeared only with helium. Similar experiments with argon and neon did not produce the same behavior in their outer electrons. Helium has particularly strong correlations between its two electrons, which the researchers say allows the simultaneous release of energy.
The discovery could also give scientists a sensitive way to study electron-electron correlations, which occur over attoseconds, with one attosecond equal to one billionth of a billionth of a second. The principle might eventually extend to molecules and solid materials with strongly interacting electrons.
