Cryogenic high-Q passive hanger type drive-line notch filtering for leakage-resilient qubit control

Short control pulses broaden spectrally and drive unwanted ∣1↔2〉 transitions in weakly anharmonic qubits. We introduce a passive hardware mitigation: a capacitively coupled quarter-wavelength (λ/4) resonator on the qubit drive line, creating a notch at f12 while leaving f01 largely unaffected. Implemented as a superconducting device at millikelvin temperature, the filter exhibits high QL and a sharply localized, low-loss stopband. The coupling capacitance sets Qc, enabling lithographic tuning of notch depth and linewidth. Simulations and cryogenic S21 measurements confirm a well-defined minimum with minimal off-resonant distortion. Integrated in a qubit setup, the filter suppresses short-pulse leakage: spectroscopy reduces the f12 feature and IQ-plane data show a stable ∣e〉 cluster; at pulse length (Tₚ) = 6 ns the f₁₂ peak-height ratio drops by ∼7 dB. The approach is fully passive, wiring-free, and scalable, and complements derivative removal by adiabatic gate (DRAG) by allowing shorter envelopes or smaller DRAG weights.