The hybridization of light and matter excitations in the form of polaritons has enabled major advances in understanding and controlling optical nonlinearities. Entering the quantum regime of strong interactions between individual photons has, however, remained challenging since the strength of achievable polariton interactions is typically limited by the available interactions in the material. Here, we investigate collisions between dark-state polaritons in three-level systems and discover a resonant process that yields effective interactions, which are much larger than the underlying interaction between their matter constituents. We systematically investigate the underlying mechanism and demonstrate a substantial enhancement of polariton interactions by several orders of magnitude. This suggests a promising approach to quantum nonlinear optics in a range of physical settings, from atomic gases to excitons in semiconductors and two-dimensional bilayer materials.

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• Exciton polariton
• Polaritons
• Scattering theory
• Ultracold collisions
• Feshbach resonance