  We have investigated dielectric losses in amorphous SiO thin films under
operating conditions of superconducting qubits (mK temperatures and low
microwave powers). For this purpose, we have developed a broadband measurement
setup employing multiplexed lumped element resonators using a broadband power
combiner and a low-noise amplifier. The measured temperature and power
dependences of the dielectric losses are in good agreement with those predicted
for atomic two-level tunneling systems (TLS). By measuring the losses at
different frequencies, we found that the TLS density of states is energy
dependent. This had not been seen previously in loss measurements. These
results contribute to a better understanding of decoherence effects in
superconducting qubits and suggest a possibility to minimize TLS-related
decoherence by reducing the qubit operation frequency.
