Few-photon optical diode

We propose a 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 multiphoton 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.