Quantum-vacuum-protected topological edge polaritons

This paper uncovers the formation of topological edge polaritons that are protected by the presence of quantum vacuum. Such quantum-vacuum-protected edge polaritons could be achieved in a system of neutral atomic lattice under appropriate interaction with a single photonic mode. In the absence of the light-matter coupling, the system is shown to be topologically trivial, which consequently does not support edge modes. By employing Floquet theory, the system is also found to be topologically trivial in the classical light limit, i.e., at very small light-matter coupling but a very large number of photons. On the other hand, by treating both the atomic and photonic degrees of freedom quantum mechanically, the system becomes topologically nontrivial in the full (atomic and photonic) Hilbert space, which manifests itself as a pair of topological (almost) zero energy eigenstates localized near each lattice's edge and with very small mean photon number. Finally, the robustness of such quantum-vacuum-protected edge polaritons against spatial disorder and counterrotating coupling effect is explicitly demonstrated.