  We study the coherent transport of one or two photons in a 1D waveguide
chirally coupled to a nonlinear resonator. Analytic solutions of the one-photon
and two-photon scattering is derived. Although the resonator acts as a
non-reciprocal phase shifter, light transmission is reciprocal at one-photon
level. However, the forward and reverse transmitted probabilities for two
photons incident from either the left side or the right side of the nonlinear
resonator are nonreciprocal due to the energy redistribution of the two-photon
bound state. Hence, the nonlinear resonator acts as an optical diode at
two-photon level.
