  We illustrate the existence of single-excitation bound states for propagating
photons interacting with $N$ two-level atoms. These bound states can be
calculated from an effective spin model, and their existence relies on
dissipation in the system. The appearance of these bound states is in a
one-to-one correspondence with zeros in the single-photon transmission and with
divergent bunching in the second-order photon-photon correlation function. We
also formulate a dissipative version of Levinson's theorem for this system by
looking at the relation between the number of bound states and the winding
number of the transmission phases. This theorem allows a direct experimental
measurement of the number of bound states using the measured transmission
phases.
