  One of the missing elements for realising an integrated optical circuit is a
rectifying device playing the role of an optical diode. A proposal based on a
pair of two-level atoms strongly coupled to a one-dimenisonal waveguide showed
a promising behavior based on a semi-classical study [Fratini et al., Phys.
Rev. Lett. 113, 243601 (2014)]. Our study in the full quantum regime shows
that, in such a device, rectification is a purely multi-photon effect. For an
input field in a coherent state, rectification reaches up to $70\%$ for the
range of power in which one of the two atoms is excited, but not both.
