Effects of environment correlations on the onset of collective decay in waveguide QED

We calculate the dynamics of one and two two-level atoms interacting with the electromagnetic vacuum field in the vicinity of an optical nanofiber without making either the Born or the Markov approximations. We use a constant dielectric function and the Drude-Lorentz model, observing deviations from the standard super- and sub-radiant decays. Despite the non-trivial environment correlations, we discuss the validity of approximating the speed of atom-atom communication to the group velocity of the guided field. Our work presents a deeper understanding of the validity of commonly used approximations in recent platforms for quantum optics applications in the context of waveguide QED.