Strong Field Spectroscopy of Many-Body Interactions in Solids

High-order harmonic generation (HHG) in solids is emerging as a versatile source of bright high-dimensional quantum light, linking the traditionally separate realms of strong-field physics and quantum optics. However, the efficiency and coherence of solid-state HHG are fundamentally limited by ultrafast dephasing of the laser-driven electron–hole polarization, an effect that rapidly destroys photon coherence and entanglement. The physical origin of this dephasing has remained elusive: introduced in theory as a phenomenological coherence time, it has largely been treated as an empirical parameter with no firm microscopic assignment. Here, we combine wavelength-dependent HHG measurements in monolayer and bulk semiconductors with ab initio many-body simulations revealing that momentum-resolved electron–phonon scattering is the dominant mechanism driving this ultrafast decoherence. We introduce a momentum-dependent dephasing time, obtained from first-principles electron–phonon scattering rates, which quantitatively reproduces the observed HHG yield scaling with wavelength. This concept bridges the long-lived carrier coherences of perturbative nonlinear optics and the ultrafast dephasing in strong-field HHG, providing a unified description of many-body dynamics in both regimes. Our findings resolve the long-standing debate concerning ultrafast many-body decoherence and open the door to mitigating decoherence in strongly driven solids – paving the way for practical quantum-optics applications of HHG, such as compact sources of entangled photons for quantum technologies.