Complete population inversion by a phase jump: an exactly soluble model

An exact analytic solution to the time-dependent Schrodinger equation is presented for a two-state quantum system coherently driven by a nonresonant pulsed external field. The pulse has a hyperbolic-secant shape, with a sign jump (i.e. a phase step of pi) at its maximum, which nullifies the overall pulse area. It is shown that off resonance, for detunings larger than the pulse bandwidth, the transition probability approaches unity as the field intensity increases. The population inversion is robust against small-to-moderate variations in the detuning and the intensity, a feature reminiscent of adiabatic passage. The population inversion, however, is not of adiabatic nature; it is induced by delta-function-shaped nonadiabatic coupling with an area of nearly pi. An estimate of the required experimental resources shows that implementation with shaped femtosecond pulses is feasible.