Eugene Demler
Developing reliable theoretical tools for analyzing strongly interacting light-matter systems is one of the primary goals of the emergent field of ultrastrong coupling waveguide QED. When the coupling strength is comparable to or even exceeds the energy of photons and matter excitations, standard theoretical approaches based on perturbative treatment of light-matter interactions or truncation in the number photon states are not applicable. This talk will review a novel approach to analyzing in- and out-of-equilibrium properties of ultrastrong coupling waveguide QED systems based on a non-perturbative unitary transformation that entangles photons and matter excitations. In the new frame of reference factorization between light and matter becomes exact for infinite interaction strength and for all interaction strengths an accurate effective model can be derived. New features of the ultrastrong coupling regime include emergence of a ladder of many-body bound states, drastic mass enhancement, and renormalization of single particle potentials.