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Dielectric loss is one of the major decoherence sources of superconductingqubits. Contemporary high-coherence superconducting qubits are formed bymaterial systems mostly consisting of superconducting films on substrate withlow dielectric loss, where the loss mainly originates from the surfaces andinterfaces. Among the multiple candidates for material systems, a combinationof titanium nitride (TiN) film and sapphire substrate has good potentialbecause of its chemical stability against oxidization, and high quality atinterfaces. In this work, we report a TiN film deposited onto sapphiresubstrate achieving low dielectric loss at the material interface. Through thesystematic characterizations of a series of transmon qubits fabricated withidentical batches of TiN base layers, but different geometries of qubitshunting capacitors with various participation ratios of the materialinterface, we quantitatively extract the loss tangent value at thesubstrate-metal interface smaller than $8.9 \times 10^{-4}$ in 1-nm disorderedlayer. By optimizing the interface participation ratio of the transmon qubit,we reproducibly achieve qubit lifetimes of up to 300 $\mu$s and quality factorsapproaching 8 million. We demonstrate that TiN film on sapphire substrate is anideal material system for high-coherence superconducting qubits. Our analysesfurther suggest that the interface dielectric loss around the Josephsonjunction part of the circuit could be the dominant limitation of lifetimes forstate-of-the-art transmon qubits.

Titanium Nitride Film on Sapphire Substrate with Low Dielectric Loss for Superconducting Qubits