Cryogenic Thermal Modeling for Scalable High Density Signaling*

Upscaling quantum computers based on superconducting qubits necessitates the addition of many microwave signal lines to a dilution refrigerator without overwhelming the available cooling power at each stage with heat loads. As each signal line is composed of several different materials, calculating these heat loads requires models of the thermal conductivities versus temperature for the static load of each material component in addition to models of the electrical resistances as a function of temperature for the Ohmic losses associated with each component. While comprehensive thermal conductivity models exist for various grades of copper and aluminum, models for other materials, such as alloys like cupronickel, remain to be synthesized.

In this study, we construct a cryogenic to room temperature thermal conductivity model for 70-30 (C71500) cupronickel, a material found in semi-rigid coaxial cables that are commonly used in dilution refrigerators. SC-086/50-SCN-CN coaxial cable samples from Coax Co., Ltd. were deconstructed, and the thermal conductivities of the conductors were measured. This data was then compared with the limited data available in literature.