Microwave calibration of qubit drive line components at millikelvin temperatures

  1. Slawomir Simbierowicz,
  2. Volodymyr Y. Monarkha,
  3. Suren Singh,
  4. Nizar Messaoudi,
  5. Philip Krantz,
  6. and Russell E. Lake
Systematic errors in qubit state preparation arise due to non-idealities in the 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, 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+3−2 dB, 33+3−2 dB, 34+3−2 dB, and 29+2−1 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 a 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 & Y) through a 2-element Fabry-Pérot cavity with 400-mm path length directly preceding the qubit, and establish that the return loss of its reflective elements must be >9.42 dB (> 14.3 dB) to obtain 99.9 % (99.99 %) gate fidelity.

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