Photon bound states in coupled waveguides

Photon bound states have been identified as particular solutions to the scattering of two photons from a single emitter, but from these results the full nature of these states remains elusive. We study a clear and unambiguous signature that these bound states are truly bound. To this end we consider an alternative configuration of close-by waveguides, each chirally coupled to two-level emitters. We show that in this system the photon bound states behave as rigid molecules, where photons do not tunnel individually but rather collectively, such that there is rarely a single photon in each waveguide. We further identify new classes of bound states in this system.