Weak-coupling bound states in semi-infinite topological waveguide QED

Abstract: A striking feature of cavity quantum electrodynamics is the existence of atom-photon bound states, which typically form when the coupling between the atom and its environment are strong enough that after de-excitation the atom can ``grab'' an emitted photon and re-absorb it, resulting in a virtual cloud surrounding the atom. Here we will demonstrate the existence of bound states that instead form in the case of weak coupling. Specifically, we show that when a quantum emitter is weakly coupled to a structured reservoir exhibiting topologically-protected surface states, hybridizations between these states and the emitter can form, resulting in mid-gap bound states. We illustrate this using a semi-infinite extension of the Su-Schrieffer-Heeger (SSH) model as our reservoir. First, we diagonalize the bare semi-infinite SSH chain and reveal a winding number that predicts only the edge state on the finite side of the chain survives the semi-infinite extension. Then, after coupling the quantum emitter to this end of the chain, we analyze the modified emitter spectrum and reveal the existence of bound states in three parameter regions. Two of these represent the usual strong-coupling bound states, while the third gives the weak-coupling bound states with eigenvalue appearing in the SSH band gap and which exhibit partial sublattice localization. We demonstrate that oscillations between the weak-coupling bound states can be used to transfer the particle from the emitter into the lattice in a predictable and reversible manner.