  Superconducting qubits are a leading system for realizing large scale quantum
processors, but overall gate fidelities suffer from coherence times limited by
microwave dielectric loss. Recently discovered tantalum-based qubits exhibit
record lifetimes exceeding 0.3 ms. Here we perform systematic, detailed
measurements of superconducting tantalum resonators in order to disentangle
sources of loss that limit state-of-the-art tantalum devices. By studying the
dependence of loss on temperature, microwave photon number, and device
geometry, we quantify materials-related losses and observe that the losses are
dominated by several types of saturable two level systems (TLSs), with evidence
that both surface and bulk related TLSs contribute to loss. Moreover, we show
that surface TLSs can be altered with chemical processing. With four different
surface conditions, we quantitatively extract the linear absorption associated
with different surface TLS sources. Finally, we quantify the impact of the
chemical processing at single photon powers, the relevant conditions for qubit
device performance. In this regime we measure resonators with internal quality
factors ranging from 5 to 15 x 10^6, comparable to the best qubits reported. In
these devices the surface and bulk TLS contributions to loss are comparable,
showing that systematic improvements in materials on both fronts will be
necessary to improve qubit coherence further.
