Consider an electron for which l=0 is kept in a uniform magnetic field B.
For which the hamiltonian matrix is {μBB,0,0,-μBB}
now if the electron is in the state 1/√2{1,1}(e.g in the eigenstates of Sx eigenvalue ħ/2}
If this state is time evolved
1/√2{1,0}exp(-iEt/ħ)+1/√2{0,1}exp(iEt/ħ)
where...