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High-harmonic generation (HHG) is one of the fundamental processes at the heart of attosecond physics. Traditionally viewed as an effective single-particle effect, recent advances have focused on contributions to the harmonic spectrum beyond this single-particle picture [1, 2], as well as on probing electron correlations through HHG in atoms, molecules, and solids [3, 4]. In this work, we quantify the influence of time-dependent electron-electron correlations on HHG using correlation measures from quantum information theory. By explicitly solving the time-dependent Schrödinger equation with the multi-configurational time-dependent Hartee-Fock method [5], we obtain fully correlated results for several multi-electron atoms. When comparing different atomic species, i.e. noble gas atoms (He, Ne) with alkaline-earth atoms (Be, Mg) we find the subcycle variation of correlation parameters to differ markedly for noble gases and alkaline-earth atoms. We provide an intuitive explanation of these surprising effects based on the dynamics of natural orbitals and discuss the effect of this ultrafast correlation dynamics on the HHG spectrum [6].
[1] A. D. Shiner et al., Nature Phys 7, 464–467 (2011).
[2] A. De Las Heras et al., Phys. Rev. Res. 2, 033047 (2020).
[3] D. R. Baykusheva et al., Phys. Rev. X 12, 011013 (2022).
[4] R. E. F. Silva et al., Nat. Photon. 12, 266 (2018).
[5] T. Sato et al., Phys. Rev. A 94, 023405 (2016).
[6] K. Buczolich et al., Phys. Rev. Res. 7, 043322 (2025).