where |E n ⟩ is the singly excited state for the n th molecule located at x n , |k α ⟩ is the 1-photon-dressed ground state with wave-vector k ∥ = k α , and |G⟩ = n |g n ⟩ α |0 kα ⟩ represents the ground state. We assume identical loss rates Γ c for all cavity modes k α , which is consistent with angle-resolved reflectance measurements of typical FP cavity and previous theoretical work, and define the cavity quality factor at normal incidence (k