A general solution to the two-level problem

Previous results obtained for transition probabilities in two-level systems are simplified and generalised. It was shown in an earlier paper by Robinson (1984) that one could express the transition amplitude as the product of its first-order approximant and a factor which is determined by the eigenvalues and eigenfunctions of a certain nonHermitian operator. In the present work, it is demonstrated that explicit knowledge of the eigenfunctions is not needed-it is sufficient to know the ensemble of eigenvalues. As an illustration, the transition probability induces by a Lorentzian pulse slightly detuned from resonance is calculated. Unlike most pulse shapes, which exhibit power broadening, it is found that the Lorentzian is neutral to first order in the detuning, and exhibits power narrowing when quadratic terms in the detuning are included.