  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 novel, clear and
unambiguous signature that these bound states are truly bound. To this end we
consider a new configuration of close-by waveguides, each chirally coupled to
two-level emitters. We show that in this system the photon bound states behave
like 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.
