UC San Diego physicists have shown that two electrons can combine their energy into a single X-ray photon, breaking a theoretical ceiling in high-harmonic generation.

The university announced the findings Thursday, Aug. 27. The study, published Aug. 7 in Nature Photonics, was led by Tenio Popmintchev, assistant professor of physics at UC San Diego. Researchers from TU Wien in Austria and the University of Salamanca in Spain co-authored the paper.

The technique at the heart of the work, high-harmonic generation, helped establish attosecond physics and contributed to the 2023 Nobel Prize in Physics, according to SciTechDaily. Standard theory predicted an upper energy limit, called the cutoff, beyond which atoms could not produce X-rays. Popmintchev's team proved otherwise.

"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," Popmintchev said in a UC San Diego statement. "That gives us an X-ray fingerprint of electron correlation …"

Here is how the process works: an intense ultraviolet laser strikes a helium atom, pulling two electrons free. The laser's electric field accelerates both electrons, which remain quantum-mechanically correlated and entangled from the moment they are freed. Using UV driving pulses, the team arranged for both electrons to recombine with the same ion at exactly the same instant, releasing their combined energy as one higher-energy X-ray photon.

The team calls this double-electron recombination. It is the reverse of a known process in which a single photon ejects two electrons at once, something that can happen only because the electrons are correlated.

The effect appeared only in helium, where electron correlations are especially strong. Argon and neon did not produce the same behavior, which the researchers say supports the conclusion that helium's two-electron interaction drives the higher-energy signal.

In the coherent X-ray spectrum, the team observed a second, weaker plateau extending well beyond the previously established energy range.

UC San Diego co-authors on the paper include Siyang Wang, Jieyu Yan, Sirius Song, Aleksander Prodanov and Zhihan Wu. Funding came in part from the Alfred P. Sloan Foundation, the European Research Council, UC San Diego Startup Funds and UC San Diego Academic Senate Grants.

The researchers say the findings could advance quantum computing, where electron correlation and entanglement are the resources being engineered, and the design of advanced nanomaterials. The team has raised the question of whether the secondary plateaus they observed could serve as an all-optical quantum sensor of paired-electron correlations in condensed matter.