  We have measured the excited state lifetimes in Josephson junction phase and
transmon qubits, all of which were fabricated with the same scalable
multi-layer process. We have compared the lifetimes of phase qubits before and
after removal of the isolating dielectric, SiNx, and find a four-fold
improvement of the relaxation time after the removal. Together with the results
from the transmon qubit and measurements on coplanar waveguide resonators,
these measurements indicate that the lifetimes are limited by losses from the
dielectric constituents of the qubits. We have extracted the individual loss
contributions from the dielectrics in the tunnel junction barrier, AlOx, the
isolating dielectric, SiNx, and the substrate, Si/SiO2, by weighing the total
loss with the parts of electric field over the different dielectric materials.
Our results agree well and complement the findings from other studies,
demonstrating that superconducting qubits can be used as a reliable tool for
high-frequency characterization of dielectric materials. We conclude with a
discussion of how changes in design and material choice could improve qubit
lifetimes up to a factor of four.
