I'm having some trouble understanding the idea of coherence. Consider the phrase: "a coherent superposition of states." Here I understand the coherence to refer to the states being coupled to each other and capable of interfering, due to a perturbing field. How does this relate to coherent light, which I understand to be light that is monochromatic and phase-correlated? What happens if you shine light that is monochromatic, but not coherent, on the atomic system?