Fig. 3: Qubit measurement results.

a T₁ comparison of the five sets of qubit devices that were prepared on sapphire substrates. All four Nb/Ta qubits on the chip show T₁ > 100 μs, and the largest T₁ measured for Nb control qubits is ~50 μs. Boxes mark the 25th percentile and the 75th percentile of the measurement distribution over the course of 10 h of consecutive measurements. The line inside each box represents the median value. Error bars represent 95% confidence interval and circles represent outliers. b Measured T₁ values for test devices fabricated on silicon substrates. We observe a clear improvement in terms of the median T₁ value following surface capping of Nb with Ta. c Dependence of T₁ standard deviation, σ_T1, on the average T₁, μ_T1. Different colors correspond to the different encapsulation groups shown in (a). Dashed line shows the best fitting of \({\sigma }_{{T}_{1}}\propto {\mu }_{{T}_{1}}^{3/2}\), according to ref. ³⁶. It also shows that the Nb/Ta qubit has 5-10 times improvement of TLS loss compared with the Nb qubit, after converting the number of TLS into the tangent loss, δ_TLS. d Best T₁= 198 μs. e Statistics for T₁ consecutively measured over 10 h, the average T₁ (μ_T1) is 161 μs and the standard deviation (σ_T1) is 15 μs. The star shows the iteration that yielded the best T₁ (see (d) for the decay curve). f Histogram of the T₁ values in (e), with a Gaussian fit.