  Dielectric loss is one of the major decoherence sources of superconducting
qubits. Contemporary high-coherence superconducting qubits are formed by
material systems mostly consisting of superconducting films on substrate with
low dielectric loss, where the loss mainly originates from the surfaces and
interfaces. Among the multiple candidates for material systems, a combination
of titanium nitride (TiN) film and sapphire substrate has good potential
because of its chemical stability against oxidization, and high quality at
interfaces. In this work, we report a TiN film deposited onto sapphire
substrate achieving low dielectric loss at the material interface. Through the
systematic characterizations of a series of transmon qubits fabricated with
identical batches of TiN base layers, but different geometries of qubit
shunting capacitors with various participation ratios of the material
interface, we quantitatively extract the loss tangent value at the
substrate-metal interface smaller than $8.9 \times 10^{-4}$ in 1-nm disordered
layer. By optimizing the interface participation ratio of the transmon qubit,
we reproducibly achieve qubit lifetimes of up to 300 $\mu$s and quality factors
approaching 8 million. We demonstrate that TiN film on sapphire substrate is an
ideal material system for high-coherence superconducting qubits. Our analyses
further suggest that the interface dielectric loss around the Josephson
junction part of the circuit could be the dominant limitation of lifetimes for
state-of-the-art transmon qubits.
