Methods to achieve near-millisecond energy relaxation and dephasing times for a superconducting transmon qubit

Superconducting qubits are one of the most promising physical systems for implementing quantum computers. However, executing quantum algorithms of practical computational advantage requires further improvements in the fidelities of qubit operations, which are currently limited by the energy relaxation and dephasing times of the qubits. Here, we report our measurement results of a high-coherence transmon qubit with energy relaxation and echo dephasing times surpassing those in the existing literature. We measure a qubit frequency of 2.9 GHz, an energy relaxation time T₁ with a median of 425 μs and a maximum of (666 ± 33)μs, and an echo dephasing time $T_2^{\mathrm{echo}}$ with a median of 541 μs and a maximum of (1057 ± 138)μs. We report in detail our design, fabrication process, and measurement setup to facilitate the reproduction and wide adoption of high-coherence transmon qubits in the academia and industry.