Creating and manipulating single photons is key to future quantum technologies based on light. However, light from most light sources typically contains a random number of photons that is difficult to control. In an article in Physical Review Letters, Ole Iversen and Thomas Pohl from the Center for Complex Quantum Systems show that sending such light through a chain of three-level atoms can generate an effective repulsive interaction between the photons, such that they exit the medium one photon at a time.

Strong photon-photon interactions are difficult to achieve and repulsive photons occur even more scarcely. For example, optical fibers or an optical otherwise equivalent chain of two-level atoms are known to have a focusing effect or bunch photons together. The results from the Aarhus team suggests that this difference originates from a new type of quantum mechanical eigenstate that occurs for the scattering of two photons from three-level atoms, but is absent for a two-level emitter. For example, this yields interesting perspectives for exploring transport and possible self-organization phenomena of quantum light fields.

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