Microwave Output Stabilization of a Qubit Controller via Device-Level Temperature Control

We present the design and performance of QuEL-1 SE, which is a multichannel qubit controller developed for superconducting qubits. The system incorporates the active thermal stabilization of critical analog integrated circuits, such as phase-locked loops, amplifiers, and mixers, to suppress the long-term amplitude and phase drift. To evaluate the amplitude and phase stability, we simultaneously monitor 15 microwave output channels over 24 h using a common analog-to-digital converter. Across the channels, the normalized amplitude exhibits standard deviations of 0.09\%--0.22\% (mean: 0.15\%), and the phase deviations are 0.35$^\circ$--0.44$^\circ$ (mean: 0.39$^\circ$). We further assess the impact of these deviations on quantum gate operations by estimating the average fidelity of an $X_{\pi/2}$ gate under the coherent errors corresponding to the deviations. The resulting gate infidelities are $2\times 10^{-6}$ for amplitude errors and $2\times 10^{-5}$ for phase errors, which are significantly lower than typical fault-tolerance thresholds such as those of the surface code. These results demonstrate that the amplitude and phase stability of QuEL-1 SE enables reliable long-duration quantum operations, thus highlighting its utility as a scalable control platform for superconducting and other qubit modalities.