Calibrated microwave measurements of qubit drive line components at millikelvin temperature

Systematic errors in qubit state-preparation arise due to non-idealities in the qubit control lines such as impedance mismatch. Using a databased short-open-load calibration technique at a temperature of 30 mK, we determine the impedance matching of individual qubit drive line components: microwave attenuators and coaxial cables. We report calibrated 1-port S-parameters and interpret the results using a master equation simulation of all XY gates performed on a single qubit. Considering only drive pulse distortion due to reflections as the non-ideality, the data and simulations establish the required impedance matching threshold to enable a single-qubit gate fidelity of over 99.9%.