  We propose a novel scheme of realizing an optical diode at the few-photon
level. The system consists of a one-dimensional waveguide coupled
asymmetrically to a two-level system. The two or multi-photon transport in this
system is strongly correlated. We derive exactly the single and two-photon
current and show that the two-photon current is asymmetric for the asymmetric
coupling. Thus the system serves as an optical diode which allows transmission
of photons in one direction much more efficiently than the opposite.
