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The performance of superconducting quantum circuits for quantum computing hasadvanced tremendously in recent decades; however, a comprehensive understandingof relaxation mechanisms does not yet exist. In this work, we utilize amultimode approach to characterizing energy losses in superconducting quantumcircuits, with the goals of predicting device performance and improvingcoherence through materials, process, and circuit design optimization. Usingthis approach, we measure significant reductions in surface and bulk dielectriclosses by employing a tantalum-based materials platform and annealed sapphiresubstrates. With this knowledge we predict and experimentally verify therelaxation times of aluminum- and tantalum-based transmon qubits. Weadditionally optimize device geometry to maximize coherence within a coaxialtunnel architecture, and realize on-chip quantum memories with single-photonRamsey times of 2.0$-$2.7 ms, limited by their energy relaxation times of1.0$-$1.4 ms. To our knowledge this is the highest coherence achieved in anon-chip quantum memory, and demonstrates an advancement towards a more modularand compact coaxial circuit architecture for bosonic qubits with reproduciblyhigh coherence.

Surpassing millisecond coherence times in on-chip superconducting quantum memories by optimizing materials, processes, and circuit design