  We assess independently the impact of high-temperature substrate annealing
and metal deposition conditions on the coherence of transmon qubits in the
standard 2D circuit-QED architecture. We restrict our study to devices made
with aluminum interdigital capacitors on sapphire substrates. We record more
than an order-of-magnitude improvement in the relaxation time of devices made
with an annealed substrate, independent of whether a conventional evaporator or
molecular beam epitaxy chamber was used to deposit the aluminum. We also infer
similar levels of flux noise and photon shot noise through dephasing
measurements on these devices. Our results indicate that substrate annealing
plays a primary role in fabricating low-loss qubits, consistent with the
hypothesis that substrate-air and substrate-metal interfaces are essential
factors limiting the qubit lifetimes in superconducting circuits.
