Microwave calibration of qubit drive line components at millikelvin temperatures

Abstract

Systematic errors in qubit state preparation arise due to non-idealities in qubit control lines such as impedance mismatch. Using a data-based methodology of short-open-load calibration at a temperature of 30 mK, we report calibrated 1-port scattering parameter data of individual qubit drive line components. At 5 GHz, cryogenic return losses of a 20-dB-attenuator, a 10-dB-attenuator, a 230-mm-long 0.86-mm silver-plated cupronickel coaxial cable, and a 230-mm-long 0.86-mm NbTi coaxial cable were found to be 35−2+3 dB, 33−2+3 dB, 34−2+3 dB, and 29−1+2 dB, respectively. For the same frequency, we also extract cryogenic insertion losses of 0.99−0.04+0.04 dB and 0.02−0.04+0.04 dB for the coaxial cables. We interpret the results using master equation simulation of all XY gates performed on a single qubit. For example, we simulate a sequence of two 5 ns gate pulses (X and Y) through a two-element Fabry–Pérot cavity with 276-mm path length directly preceding the qubit and establish that the return loss of its reflective elements must be >9.7 dB (>14.7 dB) to obtain 99.9% (99.99%) gate fidelity.