Recent advances in free-electron laser (FEL) technology have intensified interest in light-matter interactions in the XUV regime. Our group has been at the forefront of theoretical developments in this field and working closely with experimentalists, c.f. [1] and [2]. While previous studies have focused on traditional strong-coupling processes, enforcing energy conservation via the rotating-wave approximation, we explore now a distinct regime that challenges traditional approximations, namely a regime for ultrastrong coupling on ultrafast timescales, which leads to giant counter-rotating oscillations on the attosecond timescale. Additionally, the use of time symmetries in strong-coupling will be introduced to control quantum entanglement in photoionization [3].

[1] Nandi, S., et al. Observation of Rabi dynamics with a short-wavelength free-electron laser. Nature 608, 488–493 (2022).
[2] Nandi, S., et al. Generation of entanglement using a short-wavelength seeded free-electron laser. Sci. Adv.10, eado0668 (2024).
[3] Stenquist, A. and Dahlström, J. M. . Harnessing time symmetry to fundamentally alter entanglement in photoionization. Phys. Rev. Research 7, 013270 (2025).