Like almost always in many-body physics or quantum optics, it is most convenient to apply the second-quantization formalism to describe the physics involved. For example, a light field is then described through Boson creation and annihilation operators B ^ ^ q † † and B ^ ^ q , respectively, where ℏ ℏ q defines the momentum of a photon. The "hat" over B signifies the operator nature of the quantity. When the many-body state consists of electronic excitations of matter, it is fully defined by Fermion creation and annihilation operators a ^ ^ λ λ , k † † and a ^ ^ λ λ , k , respectively, where ℏ ℏ k refers to particle's momentum while λ λ is some internal degree of freedom, such as spin or band index.