Correlated fluctuation in relaxation times of superconducting qubits with T₁ > 400 μs

Scalable quantum computing with superconducting qubits requires stringent constraints on their coherence times, not only on long relaxation times but crucially on their stability. However, a longer relaxation time is more sensitive to small fluctuations in its environment. Nevertheless, a detailed analysis of the fluctuations may provide us with additional information about the loss mechanisms, enabling us to further improve the relaxation times. Here, we develop superconducting transmon qubits with a maximum relaxation time exceeding > 400 μs and average > 200 μs, enabled by a single Al/Ox/Al Josephson junction shunted by Nb capacitance electrodes on a Si substrate, and measure the stability of the relaxation times of multiple qubits over a long period of time (> 1 week). We observe that the fluctuations in the relaxation times are correlated, implying that the relaxation times can be limited by two-level systems, whose frequencies are scrambled by a common process, such as background ionizing radiations.