  Stimulated emission and absorption are two fundamental processes of
light-matter interaction, and the coefficients of the two processes should be
equal in general. However, we will describe a generic method to realize
significant difference between the stimulated emission and absorption
coefficients of two nondegenerate energy levels, which we refer to as
nonreciprocal transition. As a simple implementation, a cyclic three-level atom
system, comprising two nondegenerate energy levels and one auxiliary energy
level, is employed to show nonreciprocal transition via a combination of
synthetic magnetism and reservoir engineering. Moreover, a single-photon
nonreciprocal transporter is proposed using two one dimensional semi-infinite
coupled-resonator waveguides connected by an atom with nonreciprocal transition
effect. Our work opens up a route to design atom-mediated nonreciprocal devices
in a wide range of physical systems.
