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New Measurements at Bluefors Test the Performance of Cryogenic Wiring for Qubit Control

Accurate qubit control requires good microwave performance of cryogenic wiring components. Bluefors and Keysight Technologies have collaborated on new cryogenic measurements that directly probe the scattering parameters of the attenuators and cables that comprise qubit drive lines. We aim to fill a major gap in the availability of measurement and detection methods that are native to the millikelvin environment and can address its unique measurement challenges.

A challenge faced across the industry is the difficulty of obtaining accurate microwave measurements in situ, and therefore most measurements are taken at room temperature and the results are extrapolated to low temperatures.  Rapid progress of engineered quantum systems has exposed an industry-wide lack of microwave measurement and detection methods native to the millikelvin environment. Direct measurement of microwave scattering parameters of individual microwave components placed at the base temperature of a dilution refrigerator brings challenges related to the lack of stable calibration standards that are commercially available.

In microwave pulse driven qubit architectures single-qubit gate fidelity becomes highly susceptible to any impedance mismatch in the microwave line since such imperfections lead to pulse distortions.  The resulting reflections interfere with the original pulse causing distortions in both phase and amplitude.

New cryogenic microwave technology must be ready and able to meet requirements of low operational power dissipation within the cryogenic environment, the ability to correct for the offset from room temperature test equipment by meter-long scale interconnects, temperature-dependent impedance changes and the practical challenge of the lack of commercially available cryogenically compatible RF components.

In our article in Applied Physics Letters we report direct measurements of the scattering parameters of qubit drive line components over a wide frequency band at 30 mK using the 1-port SOL method.  We measured cryogenic attenuators and two different types of cryogenic semi-rigid coaxial cables made from NbTi and SCuNi  This work was performed in collaboration with Keysight’s Quantum Engineering Solutions team.

The article is an opportunity to present accurate measurement results of cryogenic attenuators and coaxial cables to the qubit measurement community. Furthermore, we discuss the significance of the measurement results in qubit state preparation fidelity and demonstrate the importance of microwave measurement for the quantum computing research community.

Contact Bluefors experts for advice and guidance on cryogenic microwave wiring.

Keep cool and carry on.