We present a transmon qubit fabrication technique that yields systematic improvements in T₁ relaxation times. We encapsulate the surface of niobium and prevent the formation of its lossy surface oxide. By maintaining the same superconducting metal and only varying the surface, this comparative investigation examining different capping materials, such as tantalum, aluminum, titanium nitride, and gold, as well as substrates across different qubit foundries demonstrates the detrimental impact that niobium oxides have on coherence times of superconducting qubits, compared to native oxides of tantalum, aluminum or titanium nitride. Our surface-encapsulated niobium qubit devices exhibit T₁ relaxation times 2–5 times longer than baseline qubit devices with native niobium oxides. When capping niobium with tantalum, we obtain median qubit lifetimes above 300 μs, with maximum values up to 600 μs. Our comparative structural and chemical analysis provides insight into why amorphous niobium oxides may induce higher losses compared to other amorphous oxides.

我们提出了一种 transmon 量子位制造技术，可以系统地改进T ₁弛豫时间。我们封装铌的表面并防止其有损表面氧化物的形成。通过保持相同的超导金属并仅改变表面，这项比较研究检查了不同的覆盖材料（例如钽、铝、氮化钛和金）以及不同量子位铸造厂的基材，证明了氧化铌对相干性的有害影响与钽、铝或氮化钛的天然氧化物相比，超导量子位的倍数。我们的表面封装铌量子位器件的T ₁弛豫时间比采用天然铌氧化物的基线量子位器件长 2-5 倍。当用钽覆盖铌时，我们获得了超过 300 μs 的中值量子位寿命，最大值高达 600 μs。我们的比较结构和化学分析提供了关于为什么非晶铌氧化物与其他非晶氧化物相比可能引起更高损耗的见解。