The slave boson method is a technique for dealing with models of strongly correlated systems, providing a method to second-quantize valence fluctuations within a restrictive manifold of states. In the 1960s the physicist John Hubbard introduced an operator, now named the "Hubbard operator" to describe the creation of an electron within a restrictive manifold of valence configurations. Consider for example, a rare earth or actinide ion in which strong Coulomb interactions restrict the charge fluctuations to two valence states, such as the Ce (4f) and Ce (4f) configurations of a mixed-valence cerium compound. The corresponding quantum states of these two states are the singlet | f 0 ⟩ ⟩ state and the magnetic | f 1 : σ σ ⟩ ⟩ state, where σ σ =↑ ↑ , ↓ ↓ is the spin. The fermionic Hubbard operators that link these states are then