April 22, 2026 -- High harmonic generation (HHG), a highly non-linear phenomenon in which a system (for example, an atom) absorbs many photons from an incoming laser and emits a single photon of much higher energy (a harmonic of the absorbed photons), was long thought to only produce classical light. Recent discoveries, in contrast, have shown that it is also possible to produce photons with quantum features, such as squeezing and entanglement. Yet controlling and analyzing these remains far from trivial.

Researchers at ICFO, Dr. Javier Rivera-Dean, Lidija Petrovic, Prof. Dr. Maciej Lewenstein, and Philipp Stammer, have introduced a new approach to control the quantum state of light in the process of high-harmonic generation. Published in Reports on Progress in Physics, the proposed scheme allows researchers to extract and tune the quantum characteristics of the harmonics in the XUV regime, where conventional methods reach their limitations.  

The approach drives the high-harmonic generation by mixing a strong laser field with a quantum (instead of the traditional classical) light source that exhibits squeezing. By varying the phase delay between the laser and the quantum source, the researchers can shape the quantum signatures of the emitted harmonics. The same technique can also be used to reconstruct their quantum state (a process known as quantum state tomography), recovering the quantum imprints on the harmonics; an approach they have named “attosecond quantum tomography” (AQT).

This discovery could be applied in scenarios where conventional tomography schemes fail, such as when the generated harmonics have XUV frequencies. “This is because in standard tomography the experiment is limited by the wavelength and intensity of the state to be reconstructed, so the short wavelength of XUV light becomes a big limitation,” explains Philipp Stammer, lead researcher of the article. “In AQT, there is no strict limit because the reconstruction is done in a fundamentally different way, using the non-linear HHG dynamics itself.”

This conceptually new approach, which completely merges HHG with quantum optics, could help combine attosecond temporal resolution with genuinely non-classical light sources.