Reduced density matrix description of high-harmonic generation in multielectron atoms: Exploring subcycle correlation effects

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, as well as on probing electron correlations through HHG in atoms, molecules, and solids. In this paper, we introduce a reduced density matrix description to explore and to quantify correlation effects on a subcycle timescale and apply this approach to prototypical multielectron atoms. By comparing noble gas atoms (He, Ne) with alkaline-earth atoms (Be, Mg) exposed to driving fields with similar Keldysh parameters, we show that the subcycle variation of correlation parameters differs 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.