I present a general method to derive entanglement properties of the free massless Fermion on a lightray, based in functional analysis and singular integral equations. As an application, I will show how this method can be used to derive non-universal behaviour, such as the modular flow for states other than the vacuum. In particular, this allows to non-perturbative access to entanglement at finite temperature.
Literature: This talk presents the articles arXiv:1905.05768 [hep-th] and arXiv:1906.02207 [hep-th] by I. Reyes and myself and gives an outlook to our current work.