Superconducting Flexible Cables For Cryogenic Applications

Abstract

In systems for cryogenic electronics experiments and applications containing multiple temperature stages down to the milliKelvin temperature range, such as in quantum computing research performed in dilution refrigerators, significant effort is undertaken to limit thermal leakage between the various temperature stages. In order to route many RF/microwave/high-speed signals, a large number of semi-rigid coaxial cables are generally used. From a practical perspective the number of microwave connections is limited; due to the thermal load imposed on the cryogenic system, as well as space and volume constraints within the refrigerator. As advances in cryogenic electronics are made, the number of connections that can be achieved could potentially limit its scalability. Consequently, there is a need for high-density, low-loss, low thermal leakage cables. Polyimide has been shown to have exceptional mechanical and thermal properties at cryogenic temperatures. Polyimide as a substrate material with Nb traces has been commonly used in relatively low-frequency (< MHz) applications such as superconducting focal plane arrays. This work addresses the need for even lower loss microwave interconnects by using thin-film processing methods to fabricate controlled-impedance superconducting microstrip cables on 20 um thick PI-2611 polyimide and on 50 um thick E-series Kapton films with 250 nm thick Nb traces and ground planes. In this work, we also studied embedded microstrip transmission lines and Nb cladded with Cu and Al microstrip transmission lines. We also implemented distributed element models to match measurements that took into account the kinetic inductance effects on the microwave performance. The fabricated microstrip flex cables were measured at 4.2 K, in liquid He, and up to a frequency of 14 GHz.