  We propose a controllable non-reciprocal transmission model. The model
consists of a Mobius ring, which is connected with two one-dimensional
semi-infinite chains, and with a two-level atom located inside one of the
cavities of the Mobius ring. We use the method of Green function to study the
transmittance of a single photon through the model. The results show that the
non-reciprocal transmission can be achieved in this model and the two-level
atom can behave as a quantum switch for the non-reciprocal transport of the
single photon. This controllable non-reciprocal transmission model may inspire
new quantum non-reciprocal devices.
