[en] In classical nonlinear media, interactions between single photons are negligible. However, a strong interaction can be reached using a single atom in a cavity. By driving a closed cycle in the atom’s level structure we show parametric four-wave-mixing at the single photon level and a Zeno-blockade in the effective energy level structure. Extending to a multi-mode cycle, we show a giant cross Kerr nonlinearity between individual photons. The new platform shows novel physics in quantum nonlinear optics and is promising for real-time simulation of quantum systems.