  We study two two-level systems (TLSs) interacting with a reservoir of guided
modes confined in a rectangular waveguide. For the energy separation of the
identical TLSs far away from the cutoff frequencies of transverse modes, the
delay-differential equations are obtained with single excitation initial in the
TLSs. The effects of the inter-TLS distance on the time evolution of the
concurrence of the TLSs are examined.
