“…Nonreciprocity means that some particles or waves exhibit different transmission properties when their sources and detectors are exchanged, such as the one-way electric conduction in the semiconductor p-n junctions. In a recent paper [6], some of us introduced the concept of nonreciprocity to investigate the transitions between different energy levels, and proposed 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 [6]. The nonreciprocal transition can be used for many applications, such as single-photon nonreciprocal transporter [6], nonreciprocal phonon devices [7,8], and the echo cancellation in quantum memory [9] and quantum measurements [10].…”