  Coupling photons to Rydberg excitations in a cold atomic gas yields
unprecedentedly large optical nonlinearities at the level of individual light
quanta, where the formation of nearby dark-state polaritons is blocked by the
strong interactions between Rydberg atoms. This blockade mechanism, however,
realizes an inherently dissipative nonlinearity, which limits the performance
of practical applications. In this work, we propose a new approach to strong
photon interactions via a largely coherent mechanism at drastically suppressed
photon losses. Rather than a polariton blockade, it is based on an interaction
induced conversion between distinct types of dark-state polaritons with
different propagation characteristics. We outline a specific implementation of
this approach and show that it permits to turn a single photon into an
effective mirror with a robust and continuously tuneable reflection phase. We
describe potential applications, including a detailed discussion of achievable
operational fidelities.
