Heating effects and frequency shifts in a six-qubit Si/SiGe quantum processor*

As spin-based quantum processors grow in size and complexity, maintaining high fidelities and minimizing crosstalk will be essential for the successful implementation of quantum algorithms and error-correction protocols. In particular, recent experiments have highlighted pernicious transient qubit frequency shifts associated with microwave qubit control. Workarounds for small devices, including pre-pulsing with an off-resonant microwave burst to bring a device to a steady-state, wait times prior to measurement, and qubit-specific calibrations all bode ill for device scalability. Here, we make substantial progress in understanding and overcoming this effect. First, in a six-qubit silicon quantum processor, we report a surprising non-monotonic relation between device temperature and qubit frequency, with frequency shifts of a similar magnitude as those induced by microwave driving. Possible mechanisms are discussed. Second, we evaluate the robustness of rf-reflectometry in the context of heating. Last, we find a pragmatic solution to the heating effect: raising the device operating temperature to about 200 mK. We show this leads to stable qubit frequencies and eliminates the need for pre-pulsing and wait times without compromising qubit coherence.