We consider the infrared (IR) aspects of the gauge invariant S-matrix in Quantum electrodynamics (QED). I introduce the dressed state formalism to obtain IR finite S-matrix elements. I explain that the conservation of the asymptotic charge is a necessary condition to obtain IR finite S-matrix elements, and this condition requires appropriate dressed states rather than conventional Fock states. I also explain that IR divergences are also necessary to prohibit non-conservation of the asymptotic charges. We also argue a decoherence problem in the inclusive computations that sum over the final soft photons. We show a relation between this problem, IR divergences, and the asymptotic charges. This talk is based on 1901.09935, 2009.11716, and a paper to appear in collaboration with Hayato Hirai.

世話人　物理学系　伊藤飛鳥(内線2075）