Researchers at the University of California San Diego, TU Wien, and the University of Salamanca have overcome a long-held theoretical limit in X-ray production using helium atoms. Standard models predicted an energy cutoff for laser-induced X-rays, but the team achieved a higher energy range by harnessing the quantum properties of electrons. The experiment revealed that when freed by laser pulses, the two electrons within helium atoms remain quantum-mechanically correlated and entangled until they recombine, enabling the release of combined energy as a single, more powerful X-ray photon.
Similar secondary plateaus have also been reported in quantum materials, raising the question of whether these features constitute a unique fingerprint of strongly correlated dynamics, and thus an all-optical quantum sensor of paired-electron correlations reading them out with ultrafast precision. “For the first time, we can see two entangled electrons return to the same ion at the same instant and give up their energy as a single X-ray photon,” said Tenio Popmintchev, UC San Diego Assistant Professor of Physics.
Helium Experiment Overcomes X-ray Energy Cutoff
Intense ultraviolet lasers enabled researchers to surpass a long-held theoretical limit in X-ray generation, a feat previously considered impossible according to established models. This breakthrough wasn’t simply about achieving higher energy; it revealed a fundamental aspect of how electrons behave within atoms during the process. The experiment hinged on the unique quantum properties of helium’s electrons, which are not independent entities but remain quantum-mechanically correlated and entangled even when liberated by the laser pulses.
This entanglement proved crucial, as the researchers leveraged it to precisely time the recombination of both electrons with the same ion at exactly the same instant. This synchronized return allowed the electrons to release their combined energy as a single, higher-energy X-ray photon, effectively circumventing the established energy limit. Siyang Wang, Jieyu Yan, Sirius Song, Aleksander Prodanov, Zhihan Wu and Tenio Popmintchev of UC San Diego were instrumental in conducting the experiment and analyzing the results.
Beyond the technical achievement, the findings suggest a novel approach to quantum sensing. The researchers observed a secondary plateau in the X-ray spectrum, a region of sustained high-energy emission beyond the traditional cutoff. This plateau, they propose, could serve as a fingerprint for detecting paired-electron correlations, not only in gaseous helium but also within the more complex structures of condensed matter.
Popmintchev explained that this provides “an X-ray fingerprint of electron correlation—the physics underlying both quantum computing and, potentially, the design of advanced nanomaterials.” The implications extend to fields reliant on understanding electron behavior, including the development of quantum computers and advanced materials. The ability to read these correlations with ultrafast precision offers a new tool for characterizing and optimizing materials at the quantum level.
The study, published August 7, 2026 in Nature Photonics, received partial funding from the Alfred P. Sloan Foundation (FG-2018-10892) and the European Research Council (XSTREAM-716950), highlighting the collaborative nature of this scientific endeavor and the international investment in exploring the frontiers of quantum physics.
For the first time, we can see two entangled electrons return to the same ion at the same instant and give up their energy as a single X-ray photon. That gives us an X-ray fingerprint of electron correlation – the physics underlying both quantum computing and, potentially, the design of advanced nanomaterials.
Tenio Popmintchev, Assistant Professor of Physics at UC San Diego
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