Error budget of parallel single-qubit gates in a superconducting processor

We analyze the performance of single-qubit gates operated both in isolation and in parallel on a superconducting qubit processor. While isolated single-qubit gates are coherence-limited, parallel operation introduces significant microwave crosstalk that degrades gate fidelity. We identify and characterize the dominant crosstalk mechanisms, including AC Stark–induced phase shifts, coherent cross-driving errors, and crosstalk included leakage. Our results show that crossdriving between neighboring qubits in the computational subspace (|0⟩–|1⟩ transitions) is largely mitigated through frequency detuning practices that avoid crowding. However, |0⟩–|1⟩ and |1⟩–|2⟩ transitions are not always sufficiently detuned, leading to substantial crosstalk-induced leakage during parallel gate execution. In addition to experimental characterization, we develop analytical models and numerical simulations that capture these effects and support our findings. This combined approach highlights the limitations of parallel single-qubit performance and provides guidance for calibration strategies in larger-scale processors.