Shaping and reconstructing quantum harmonics
Researchers at ICFO have proposed a novel protocol called Attosecond Quantum Interferometry (AQI) to both tune and reconstruct the quantum features of light emitted through high harmonic generation, even in regimes where conventional methods reach their limitations. The protocol, recently published in Reports on Progress in Physics, performs in-situ attosecond measurements of quantum properties of light, further bridging the fields of attoscience and quantum optics.
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.
Reference:
Javier Rivera-Dean et al 2026 Rep. Prog. Phys. 89 047901
Acknowledgements:
P S acknowledges funding from: The European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie Grant Agreement No 847517. ICFO-QOT group acknowledges support from: European Research Council AdG NOQIA; MCIN/AEI (PGC2018- 0910.13039/501100011033, CEX2019-000910-S/10.13039/501100011033, Plan National STAMEENA PID2022-139099NB and FUNQIP PID2022-139658NB-I00, Project funded MCIN and by the ‘European Union NextGenerationEU/PRTR’ (PRTR-C17.I1), FPI); QUANTERA DYNAMITE PCI2022-132919, QuantERA II Programme co-funded by European Union’s Horizon 2020 program under Grant Agreement No 101017733; Ministry for Digital Transformation and of Civil Service of the Spanish Government through the QUANTUM ENIA Project call—Quantum Spain Project, and by the European Union through the Recovery, Transformation and Resilience Plan—NextGenerationEU within the framework of the Digital Spain 2026 Agenda; MICIU/AEI/10.13039/501100011033 and EU (PCI2025-163167); Fundació Cellex; Fundació Mir-Puig; Generalitat de Catalunya (European Social Fund FEDER and CERCA program; Barcelona Supercomputing Center MareNostrum (FI-2023-3-0024); Funded by the European Union (HORIZON-CL4-2022-QUANTUM-02-SGA, PASQuanS2.1, 101113690, EU Horizon 2020 FET-OPEN OPTOlogic, Grant Nos 899794, QU-ATTO, 101168628), EU Horizon Europe Program (No 101080086 NeQSTGrant Agreement 101080086—NeQST). J R-D acknowledges funding from UK Engineering and Physical Sciences Research Council (EPSRC) Funding, Grant UKRI2300 – Attosecond Photoelectron Imaging with Quantum Light.