Improved Flexible Coaxial Ribbon Cable for High-Density Superconducting Arrays

TL;DR

This paper presents design improvements to superconducting flexible coaxial cables, reducing microwave loss and crosstalk while maintaining low thermal load, enhancing performance for high-density cryogenic arrays.

Contribution

The authors introduce a modified design of superconducting NbTi coaxial cables with increased conductor diameter and improved transition, achieving lower loss and crosstalk at cryogenic temperatures.

Findings

Microwave attenuation reduced from 6 dB to 1.5 dB at 8 GHz.

Heat load per trace estimated at 5 nW from 1 K to 100 mK.

Crosstalk remained below -40 dB at 8 GHz.

Abstract

Superconducting arrays often require specialized, high-density cryogenic cabling capable of transporting electrical signals across temperature stages with minimal loss, crosstalk, and thermal conductivity. We report improvements to the design and fabrication of previously published superconducting 53 wt% Nb-47 wt% Ti (Nb47Ti) FLexible coAXial ribbon cables (FLAX). We used 3D electromagnetic simulations to inform design changes to improve the characteristic impedance of the cable and the connector transition. We increased the center conductor diameter from 0.003 inches to 0.005 inches which lowered the cable characteristic impedance from $\sim$60 $\Omega$ to $\sim$53 $\Omega$. This change had a negligible impact on the computed heat load which we estimate to be 5 nW per trace from 1 K to 100 mK with a 1-ft cable. This is approximately half the heat load calculated for the smallest commercially available superconducting coax. We also modified the transition board to include a capacitive coupling between the upper ground plane and signal traces that mitigates the inductive transition. We tested these changes in a 5-trace, 1-ft long cable at 4 K and found the microwave transmission improved from 6 dB to 1.5 dB of attenuation at 8 GHz. This loss is comparable to commercial superconducting coax and 3$\times$ lower than commercial NbTi-on-polyimide flex cables at 8 GHz. The nearest-neighbor forward crosstalk remained less than -40 dB at 8 GHz. We compare key performance metrics with commercially available superconducting coax and NbTi-on-polyimide flex cables and we share initial progress on commercialization of this technology by Maybell Quantum Industries.