where Âel and Bph are any one-body operators in the electronic and photonic sub-spaces, respectively. Here, Âel or Bph may be the dipole, excitation number, total density, transition density, etc. operators from the respective sub-spaces. For example, one can compute the exciton-polariton absorption spectra, A(E), shown in Fig. and Fig. . Here, the polaritonic transi-tion dipole matrix element µ pl 0a can be computed using Eq. 146 as μpl = Âel ⊗ Bph = μel ⊗ 1ph where 1ph is the identity operator in the photonic sub-space. With this expression, the absorption spectra can be written as