Abstract
Recently, Rostovtsev et al. (Phys Rev A 79: 063833, 2009) discussed possible experimental applications for the interaction between the atom and field without using the rotating-wave approximation (RWA). Using this result, here we study qualitatively the quantum Fisher information (QFI) in a two-level atom interacting with an external classical field for arbitrary pulse shape without using RWA. We use this exact method (without RWA) to reexamine the variation of QFI and compare the result with the RWA considering two kinds of the external field. We show that it is possible to control the system precision by choosing suitable parameters of the atom-field system. Interestingly, we find that the variation of the QFI in the time is unchanged and it differs as we gets farther from the resonance. Moreover, we show that the increase in the Rabi frequency seriously increase the QFI and thus enhances the precision of the parameter estimation during the time evolution. The solution is very suitable for atom-field systems, especially as the system gets farther away from the resonance, so that the RWA method break down.