UDC-3: A Tunable Composite Pulse for Mixed-Axis Drift Suppression in Single-Qubit Gates

We introduce UDC-3, a compact three-pulse composite control sequence designed to suppress mixed-axis coherent drift in single-qubit rotations. Standard composite pulses such as SK1 and BB1 primarily target symmetric amplitude errors, assuming that the control Hamiltonian remains aligned with the intended rotation axis. However, real quantum hardware often exhibits distorted effective rotation axes due to asymmetric calibration drift in multiple control channels.

UDC-3 uses a short XYX prefix whose rotation angles are numerically optimized to maximize worst-case fidelity over two-axis drift (εx, εy ∈ [−0.12, 0.12]). Our optimized sequence improves worst-case fidelity from 66% (unoptimized UDC-3) to 98.79%, while preserving near-perfect performance at zero drift (99.91%). The optimized pulse also displays complete immunity to pure Y-axis drift, achieving 0% fidelity drop, outperforming SK1 and matching BB1 in symmetric regions while using fewer pulses.

These results suggest that UDC-3 is an adaptable, hardware-aware correction primitive for coherent error suppression in near-term quantum devices. The sequence is compact, tunable, and suitable for calibration-limited platforms such as superconducting qubits, trapped ions, and spin qubits.