  We study the loss rate for a set of lambda/2 coplanar waveguide resonators at
millikelvin temperatures (20 mK - 900mK) and different applied powers (3E-19 W
- 1E-12 W). The loss rate becomes power independent below a critical power. For
a fixed power, the loss rate increases significantly with decreasing
temperature. We show that this behavior can be caused by two-level systems in
the surrounding dielectric materials. Interestingly, the influence of the
two-level systems is of the same order of magnitude for the different material
combinations. That leads to the assumption that the nature of these two-level
systems is material independent.
