Flexible Coaxial Ribbon Cable for High-Density Superconducting Microwave Device Arrays

Superconducting electronics often require high-density microwave interconnects capable of transporting signals between temperature stages with minimal loss, cross talk, and heat conduction. We report the design and fabrication of superconducting 53 wt% Nb-47 wt% Ti (Nb47Ti) FLexible coAXial ribbon cables (FLAX). The ten traces each consist of a circle divide 0.076 mm NbTi inner conductor insulated with PFA (circle divide 0.28 mm) and sheathed in a shared 0.025 mm thick Nb47Ti outer conductor. The cable is terminated with G3PO coaxial push-on connectors via stainless steel capillary tubing (circle divide 1.6 mm, 0.13 mm thick) soldered to a coplanar wave guide transition board. The 30 cm long cable has 1 dB of loss at 8 GHz with -60 dB nearest neighbor forward cross talk. The loss is 0.5 dB more than commercially available superconducting coax likely due to impedance mismatches caused by manufacturing imperfections in the cable. The reported cross talk is 30 dB lower than previously developed laminated NbTi-on-Kapton microstrip cables. We estimate the heat load from 1 K to 90 mK to he 20 nW per trace, approximately half the computed load from the smallest commercially available superconducting coax from CryoCoax.

Automated summary

High-density microwave interconnects are often used in superconducting electronics for signal transportation with minimal loss, crosstalk, and heat conduction. A superconducting 53 wt% Nb-47 wt% Ti (Nb47Ti) FLexible coAXial ribbon cable (FLAX) was designed and fabricated, with a 0.076 mm NbTi inner conductor insulated with PFA and shared Nb47Ti outer conductor. Although the cable has 1 dB loss at 8 GHz with -60 dB nearest neighbor forward crosstalk, there are impedance mismatches likely caused by manufacturing imperfections. Heat load from 1 K to 90 mK per trace is estimated to be 20 nW, half of the load from the smallest commercially available superconducting coax.