where p is the photon creation/annihilation operator. Here, the Hamiltonian terms for the atomic system of interest within the cavity, of the photon, and of the system-photon interaction are labelled, where E g is the photonic frequency, g are the number of photon modes within the cavity, V is the effective mode volume, λ is the two transverse polarisation directions, μ is the dipole moment operator, and e pp is the overall photonic polarisation. The effective mode volume can also be used to quantify the degree of light-matter coupling directly .