where Ω Ω = ℏ ℏ − − 1 d → → eg ⋅ ⋅ E → → 0 is the Rabi frequency and Ω Ω ~ ~ := ℏ ℏ − − 1 d → → eg ⋅ ⋅ E → → 0 ∗ ∗ is the counter-rotating frequency. To see why the Ω Ω ~ ~ terms are called counter-rotating consider a unitary transformation to the interaction or Dirac picture where the transformed Hamiltonian H 1 , I is given by