Where the typical electrostatic Hamiltonian neglects electromagnetic fluctuations , Tsai and co-workers instead choose to consider an atomic system of interest within an optical cavity as a collection of charges, neutral, and coupled to a unique photonic mode. Acknowledging that a full description can be defined within a weak-coupling regime , a number of approximations can be made; a long-wavelength approximation , the length gauge , and a unitary phase transformation ; assuming the photonic mode is larger in wavelength than the atomic system of interest, and the electric displacement field is coupled to the dipole moment, yielding Equation .