Characterizing Precision Coaxial Air Lines as Reference Standards for Cryogenic S-Parameter Measurements at Milli-kelvin Temperatures

Accurate S-parameter characterization of coaxial microwave devices and interfaces at milli-kelvin (mK) temperatures is essential for building high performance quantum computing systems. Coaxial air-dielectric transmission lines (air lines) are used routinely as primary reference standards for S-parameter measurements at room temperature. For the first time, the feasibility of utilizing such air lines as primary reference standards at mK temperatures is investigated. However, for air lines to be used at such temperatures, they must first be characterized accurately in terms of their characteristic impedance and propagation constant. This paper describes techniques for characterizing air lines at both room temperature (taken here to be 296 K) and mK temperature (around 25 mK). The measurement results demonstrate the feasibility of using air lines as primary reference standards at mK temperatures.