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