The first term in Equation 9.25 comes from the , and the second from . Thus dropping the...

The first term in Equation 9.25 comes from the , and the second from . Thus dropping the first term is formally equivalent to writing , which is to say,

(The latter is required to make the Hamiltonian matrix Hermitian-or, if you prefer, to pick out the dominant term in the formula analogous to Equation 9.25 for ) Rabi noticed that if you make this so-called rotating wave approximation at the beginning of the calculation, Equation 9.13 can be solved exactly, with no need for perturbation theory, and no assumption about the strength of the field.