Nonperturbative waveguide quantum electrodynamics

Understanding physical properties of quantum emitters strongly interacting with quantized electromagnetic modes is one the primary goals in emergent field waveguide electrodynamics (QED). When light-matter coupling strength comparable to or even exceeds energies elementary excitations, conventional approaches based on perturbative treatment interactions, two-level description matter and photon-number truncation are no longer sufficient. Here we study out equilibrium QED such nonperturbative regimes basis a comprehensive rigorous theoretical approach using an asymptotic decoupling unitary transformation. We uncover several surprising features ranging from symmetry-protected many-body bound states continuum strong renormalization effective mass potential; latter may explain recent experiments demonstrating cavity-induced changes chemical reactivity as well enhancements ferromagnetism superconductivity. To illustrate our general results concrete examples, use formalism model coupled cavity arrays, which relevant superconducting qubits microwave resonators atoms photonic crystals. examine relation between delocalization-localization transition spin-boson model; notably, point that reentrant can occur where becomes dominant energy scale. also discuss applications other problems different fields, including optics, condensed physics, chemistry.