Probing the TLS density of states in thin a-SiO films using superconducting lumped element resonators

In the context of low-loss materials needed for superconducting qubits, we investigated the dielectric loss in the volume of a-SiO thin films at mK temperatures and single photon power levels. Our broadband measurement setup employs multiplexed lumped element resonators as well as suitable power combiner and low noise amplifier. This enables measurements on all resonators to be carried out in one cool down cycle. The results are in good agreement with the temperature and power dependence of the dielectric losses predicted for atomic two-level tunneling systems (TLSs). We find indication that the TLS density of states increases with frequency, which had not been seen in previous loss measurements.