Physicists at TU Wien and the University of California San Diego have broken a long-accepted limit in laser physics by generating X-rays with substantially higher energies than standard theory predicts. Published in Nature Photonics, the breakthrough relies on an unusual quantum interaction between two electrons that releases their energy simultaneously.
The Standard Energy Cutoff in Laser Physics
For decades, researchers have understood that directing laser light at specific atoms causes them to emit extremely short pulses of high-frequency light, including X-rays. Prof. Tenio Popmintchev from the Institute of Photonics at TU Wien describes the mechanics of this well-known process. The laser tears a single electron away from the atom, accelerates it within the electric field, and drives it back into the atom. The energy lost during this collision is emitted as light.
Nobel laureate Ferenc Krausz used this mechanism in pioneering experiments during the 1990s. Today, Tenio Popmintchev and his team conduct their investigations at the very same TU Wien institute where Krausz performed his past research. Dimitar Popmintchev, a postdoc on the team, notes that while the returning electron generates a broad range of frequencies with roughly equal intensity, a strict upper limit exists. Traditional single-electron theory dictates that frequencies beyond this threshold drop off sharply in intensity, rendering it unfeasible to surpass this specific maximum frequency boundary.
How Two Correlated Electrons Bypass the Limit
To determine if this ceiling could be broken, the research team targeted helium atoms with intense ultraviolet laser pulses. Unlike standard setups involving a single electron, this experiment successfully stripped away both of helium’s electrons in succession. Quantum correlations maintained a connection between the two freed particles, preventing their motions from acting independently.
By precisely controlling the ultraviolet driving pulses, the researchers orchestrated the simultaneous return of both electrons to the parent atom. Dimitar Popmintchev explains that releasing the combined energy of two electrons all at once makes it possible to generate a single higher-energy X-ray photon with a higher frequency, exceeding the conventional energy cutoff.
Observing the Plateau in Helium Emissions
Experimental evidence confirmed the two-electron process through the emission spectrum of the coherent radiation. Researchers identified a secondary region of sustained emission—known as a plateau—at photon energies significantly higher than standard theory dictates. This unexpected signal appeared exclusively in helium, which possesses particularly strong electron-electron correlations.
Similar experiments conducted with argon and neon did not produce the same behavior in their outer electrons. This divergence confirmed that helium’s unique quantum properties enabled the simultaneous release of energy.
Probing Quantum Systems on Attosecond Timescales
The discovery provides a sensitive spectroscopic sensor for investigating electron-electron correlations inside atoms and other materials. By analyzing the shape of the emitted spectrum, its maximum energy, and its response to laser polarization, scientists can observe interacting electrons on attosecond timescales, where one attosecond equals one billionth of a billionth of a second.
Observing processes at this speed helps clarify how quantum particles move and influence one another. Researchers aim to extend the principle beyond individual helium atoms to molecules and solid materials characterized by strong electron interactions, ultimately contributing to quantum computing research and the design of advanced nanomaterials.
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