, µ ϕ is eigen-dipole value for |ϕ⟩, and Ûµ is the unitary operator that changes the basis to the eigenbasis of μ. We notice that the photon field is described as displaced Harmonic oscillator that is centered around -q 0 ϕ (R). This displacement can be viewed as a polarization of the photon field due to the presence of the molecule-cavity coupling, such that the photon field corresponds to a non-zero (hence polarized) vector potential, in contrast to the vacuum photon field.