  Low-loss waveguides are required for quantum communication at distances
beyond the chip-scale for any low-temperature solid-state implementation of
quantum information processors. We measure and analyze the attenuation constant
of commercially available microwave-frequency waveguides down to millikelvin
temperatures and single photon levels. More specifically, we characterize the
frequency-dependent loss of a range of coaxial and rectangular microwave
waveguides down to $0.005\,\rm{dB}/\rm{m}$ using a resonant-cavity technique.
We study the loss tangent and relative permittivity of commonly used dielectric
waveguide materials by measurements of the internal quality factors and their
comparison with established loss models. The results of our characterization
are relevant for accurately predicting the signal levels at the input of
cryogenic devices, for reducing the loss in any detection chain, and for
estimating the heat load induced by signal dissipation in cryogenic systems.
