Abstract
Electromagnetically induced transparency (EIT) and electromagnetically enhanced absorption (EEA), the phenomena due to quantum interference, have attracted much attention in the field of the nonlinear optics. In this work, the EIT, EEA systems of 87Rb have been studied theoretically by the density matrix approach to provide the predictions of experiments. The main contribution here is the derivation of Hamiltonian with choosing quantum axis in arbitrary direction. At the end, we discuss the measuring of magnetic field by EEA effect and some topics worth developing in the future.