X-rays: Beyond the Nobel Prize limit

Recent experiments conducted by teams at TU Wien and the University of California San Diego have surpassed the theoretical limitations on X-ray energy production. When atoms are irradiated with laser light, they emit high-frequency X-ray pulses, a discovery that earned Ferenc Krausz the 2023 Nobel Prize in Physics. However, a theoretical model predicted an upper limit, known as the energy cutoff, beyond which little X-ray emission occurs.

Now, a new study has demonstrated that helium atoms can produce X-rays with much higher energies than previously thought possible.

The key to this breakthrough lies in the interaction between the two electrons within the helium atom. Unlike most atoms, helium's valence electrons display strong quantum correlations, enabling them to release their energy simultaneously. When both electrons return to the atom at the same time, they release their combined energy as a single, higher-energy X-ray photon.

By using intense UV laser pulses and helium atoms, which can expel two electrons sequentially, researchers were able to generate coherent X-rays with frequencies well above the traditional cutoff range.

This discovery opens up new possibilities for attosecond spectroscopy, a technique that probes electron-electron correlations on extremely short timescales. The unique characteristics of the generated X-rays—such as their spectrum, energy limit, and polarization dependence—can provide valuable insights into the behavior of electrons in materials, potentially paving the way for advancements in quantum computing and the design of advanced nanomaterials.