All quantum states have an intrinsic frequency defined by the Hamiltonian.  Photons also have intrinsic frequency that determines their color.  Entanglement of two quantum states depends strongly on the field frequency you apply.  If one starts from the ground state ($\rho_{gg} = 1$), the interaction with the field induces a transition from the ground state population given by $\rho_{gg}$ to the emitting coherence, $\rho_{ag}$. The relationship between $\rho_{gg}$ and $\rho_{ag}$ in the steady state limit is

$$\rho_{ag} \equiv c_a c_g^* = \frac{\Omega_{ag}}{2 \Delta_{ag}} \rho_{gg}$$

where $\Delta_{ag} \equiv \delta_{ag} - i \Gamma_{ag}$ and $\delta_{ag} \equiv \omega_{ag} - \omega$ is the detuning between the transition frequency and the frequency of our driving laser, $\omega$. Since normally $\frac{\Omega_{ag}}{\Delta_{ag}} << 1$, one can work in the perturbative limit where each interaction with the field changes just one of the entangled quantum states. Note that this simple equation already contains the essence of absorption spectroscopy and refraction. Remembering that absorption and refraction are described by the imaginary and real parts of the coherence, we see that $\rho_{ag} = \frac{\Omega_{ag}(\delta_{ag} + i \Gamma_{ag}) }{2 (\delta_{ag}^2 + \Gamma^2)} \rho_{gg}$ which means that $\frac{\Gamma_{ag}}{\delta_{ag}^2 + \Gamma_{ag}^2}$ and $\frac{\delta_{ag}}{\delta_{ag}^2 + \Gamma_{ag}^2}$ are the frequency dependences of absorption and refraction, respectively. The two dependences are related by the Kramers-Kronig transform.