Light–Matter Response in Nonrelativistic Quantum Electrodynamics Johannes Flick Davis M. Welakuh Michael Ruggenthaler Heiko Appel Angel Rubio 10.1021/acsphotonics.9b00768.s001 https://acs.figshare.com/articles/journal_contribution/Light_Matter_Response_in_Nonrelativistic_Quantum_Electrodynamics/10072649 We derive the full linear-response theory for nonrelativistic quantum electrodynamics in the long wavelength limit and provide a practical framework to solve the resulting equations by using quantum-electrodynamical density-functional theory. We highlight how the coupling between quantized light and matter changes the usual response functions and introduces cross-correlated light-matter response functions. These cross-correlation responses lead to measurable changes in Maxwell’s equations due to the quantum-matter-mediated photon–photon interactions. Key features of treating the combined matter-photon response are that natural lifetimes of excitations become directly accessible from first-principles, changes in the electronic structure due to strong light-matter coupling are treated fully nonperturbatively, and self-consistent solutions of the back-reaction of matter onto the photon vacuum and vice versa are accounted for. By introducing a straightforward extension of the random-phase approximation for the coupled matter-photon problem, we calculate the ab initio spectra for a real molecular system that is coupled to the quantized electromagnetic field. Our approach can be solved numerically very efficiently. The presented framework leads to a shift in paradigm by highlighting how electronically excited states arise as a modification of the photon field and that experimentally observed effects are always due to a complex interplay between light and matter. At the same time the findings provide a route to analyze as well as propose experiments at the interface between quantum chemistry, nanoplasmonics and quantum optics. 2019-10-29 19:03:21 random-phase approximation matter-photon response response functions framework matter changes photon field quantum optics linear-response theory wavelength limit quantum chemistry nonrelativistic quantum electrodynamics Nonrelativistic Quantum Electrodynamics cross-correlation responses Key features ab initio spectra quantized electromagnetic field matter-photon problem quantized light equation photon vacuum quantum-electrodynamical density-functional theory cross-correlated light-matter response functions