arXiv:2007.06496 · IEEE Transactions on Applied Superconductivity
Flexible Coaxial Ribbon Cable for High-Density Superconducting Microwave Device Arrays
Jennifer Pearl Smith, Benjamin A. Mazin, Alex B. Walter, Miguel Daal, J.I. Bailey, Clinton Bockstiegel, Nicholas Zobrist, Noah Swimmer, Sarah Steiger, Neelay Fruitwala
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 ø0.076 mm NbTi inner conductor insulated with PFA (ø0.28 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 (ø1.6 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 be 20 nW per trace, approximately half the computed load from the smallest commercially available superconducting coax from CryoCoax.
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 ø0.076 mm NbTi inner conductor insulated with PFA (ø0.28 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 (ø1.6 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 be 20 nW per trace, approximately half the computed load from the smallest commercially available superconducting coax from CryoCoax.
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% Niobium (Nb)-47 wt% titanium (Ti) flexible coaxial ribbon cables. We used 3-D 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 to 0.005 in, which lowered the cable characteristic impedance from$\sim60to\sim53\Omega$. 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 to 1.5 dB of attenuation at 8 GHz. This loss is comparable to commercial superconducting coax and 3× lower than commercial NbTi-on-polyimide flex cables at 8 GHz. The nearest-neighbor forward crosstalk remained −50 dB. 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.
We describe progress and initial results achieved towards the goal of developing integrated multi-conductor arrays of shielded controlled-impedance flexible superconducting transmission lines with ultra-miniature cross sections and wide bandwidths (dc to >10 GHz) over meter-scale lengths. Intended primarily for use in future scaled-up quantum computing systems, such flexible thin-film niobium/polyimide ribbon cables could provide a physically compact and ultra-low thermal conductance alternative to the rapidly increasing number of discrete coaxial cables that are currently used by quantum computing experimentalists to transmit signals between the several low-temperature stages (from ∼4 K down to ∼20 mK) of a dilution refrigerator. We have concluded that these structures are technically feasible to fabricate, and so far they have exhibited acceptable thermo-mechanical reliability. S -parameter results are presented for individual 2-metal layer Nb microstrip structures having 50 Ω characteristic impedance; lengths ranging from 50 to 550 mm were successfully fabricated. Solderable pads at the end terminations allowed testing using conventional rf connectors. Weakly coupled open-circuit microstrip resonators provided a sensitive measure of the overall transmission line loss as a function of frequency, temperature, and power. Two common microelectronic-grade polyimide dielectrics, one conventional and the other photo-definable (PI-2611 and HD-4100, respectively) were compared. Our most striking result, not previously reported to our knowledge, was that the dielectric loss tangents of both polyimides, over frequencies from 1 to 20 GHz, are remarkably low at deep cryogenic temperatures, typically 100 × smaller than corresponding room temperature values. This enables fairly long-distance (meter-scale) transmission of microwave signals without excessive attenuation, and also permits usefully high rf power levels to be transmitted without creating excessive dielectric heating. We observed loss tangents as low as 2.2 × 10 −5 at 20 mK, although losses increased somewhat at very low rf power levels, similar to the well-known behavior of amorphous inorganic dielectrics such as SiO 2 . Our fabrication techniques could be extended to more complex structures such as multiconductor cables, embedded microstrip, 3-metal layer stripline or rectangular coax, and integrated attenuators and thermalization structures.
We present recent work on design, fabrication, and characterization of thin-film, flexible superconducting stripline cables. Fabrication details, dc, and microwave performance (up to 14 GHz) of cables with Al/Nb/Al conductors and polyimide (PI) dielectric are discussed. Thin Al was used as a barrier layer between Nb and polyimide to protect the Nb superconductivity during subsequent PI curing steps, which were performed at a reduced temperature. The dc performance, including critical transition temperature and critical current, of the stripline transmission lines is presented. We observed Tcvalues of ~8.8 and ~8.6 K for signal traces and ground planes, respectively. Microwave characterization, including cross-talk, was performed on stripline transmission lines and resonators. We observed reasonable impedance match (S11better than -15 dB), low insertion loss (S21better than 0.04 dB/cm) and cross-talk below -60 dB for stripline transmission lines measured up to 10 GHz at 4.2 K. Furthermore, similarly fabricated stripline resonators exhibited quality factors ~7 000 at 10 GHz and 1.2 K for stripline resonators. The results of these experiments provide insight into material stack-ups for robust, multi-layer superconducting flexible cables that can find use in future cryogenic electronics systems.
Abstract We describe a superconducting (SC) cable-to-cable connection method for thin-film flexible (Nb/polyimide) parallel transmission line cables that exhibits low loss from DC to microwave frequencies. Design, fabrication procedures, assembly methods and characterization results are reported. The connector assembly incorporates a monolithic array of short superconductive bridging traces that function as a ganged electrical splice between two ribbon cables. The assembly is micro-mechanically self-aligned and uses pressure to make simultaneous electrical contacts between multiple 50 μ m wide microstrip signal traces, as well as between ground planes. Each contacting surface comprises thin SC Nb overcoated with ultra-thin Au, providing nearly zero DC resistance and very good RF performance. A simulation model of the microwave performance of microstrip-to-microstrip interconnections at 4.2 K was successfully matched to measurement results. This new cable-to-cable connection approach exhibited an insertion loss <0.12 dB and a return loss >15 dB up to 14 GHz. The connections performed reproducibly throughout multiple thermal cycles and multiple disassembly/reassembly cycles.
This technical-proposal-type preprint presents the structure, physical design, integrated manufacturing process, and validation plan for a phonon-scattering-controlled seamless superconducting coaxial cable intended for cryogenic quantum-computing systems. As superconducting quantum computers scale up, wiring density inside dilution refrigerators increases rapidly, while heat leakage and radio-frequency loss across the 4 K to 20 mK stages become increasingly important constraints. The proposed cable combines phonon-scattering structures at nanometric and micrometric scales with an RF-smooth superconducting transmission surface. The proposed design includes: • a seamless NbTi outer conductor with controlled micro-corrugation;• a recommended corrugation pitch of approximately 80 µm within a design range of 25 µm to 160 µm;• a corrugation amplitude-to-pitch ratio A/Λ between 0.15 and 0.25;• a minimum curvature radius of approximately 10 µm;• equiaxed α-Ti precipitates in the approximate size range of 50 nm to 300 nm;• a residual dislocation density above 10^10 cm^-2;• a six-rib hollow or spiral PTFE dielectric with a conductor-contact fraction of approximately 15% to 25%;• a vacuum-gap thermal buffer;• a target characteristic impedance of 50 Ω ± 1.5 Ω;• a continuous manufacturing route based on drawing, extrusion, seamless-tube forming, skin-pass integration, rotary swaging or profile rolling, electropolishing, and connector termination. A preliminary Callaway-type thermal-conductivity integration suggests the possibility of a substantial reduction in phonon-mediated thermal transport across the millikelvin range. An indicative periodic transmission-line model is also presented to estimate RF behaviour over the intended operating band. These calculations are preliminary scaling estimates only and are not substitutes for full experimental measurements or three-dimensional electromagnetic simulation. The preprint provides a validation protocol covering SEM, EBSD, TEM, optical profilometry, thermal-conductivity measurements from 20 mK to 4 K, VNA measurements from 0.05 GHz to 20 GHz, full-wave HFSS/CST simulations, mechanical cycling, thermal cycling, and cryogenic outgassing analysis. This record includes the main English-language preprint together with machine-readable DXF and STEP reference geometries for the radial cable cross-section, axial corrugation profile, and an eight-period three-dimensional coaxial segment. The work is released as a technical disclosure under the Creative Commons Attribution 4.0 International License. It is intended to establish a dated public reference for the disclosed structure, manufacturing process, parameter ranges, and derivative variants. This document is not an experimental demonstration and does not constitute a legal opinion regarding patentability, freedom to operate, claim scope, or the enforceability of any present or future patent rights.
We developed thin semi-rigid coaxial cables for use as readout noise filters in low temperature experiments. The cables reported here had an outer diameter of 0.86 mm, and consisted of a standard cupronickel (CuNi) alloy as a seamless outer conductor and polytetrafluoroethylene (PTFE) as a dielectric material. The inner conductor was superconducting niobium-titanium (NbTi) covered by a CuNi cladding layer with a thickness of 0, 13, or 38 μm. The thermal conductance and transmission properties for the cables were measured at cryogenic temperatures and compared to those for a conventional CuNi semi-rigid cable. Between 1.4 and 8.4 K, the thermal conductance was similar to that for the conventional cable because the contribution from the cladding was estimated to be small. On the other hand, measured at about 3 K, the attenuation at high frequency up to 10 GHz differed in that the cutoff frequency increased with decreasing CuNi cladding thickness. For a cladding thickness of 38 μm, the 3 dB cutoff frequency for a 1-m-long cable was 100 MHz, whereas for a cladding thickness of 13 μm, it was 500 MHz. This type of semi-rigid cable can, therefore, be used as a low-pass filter whose attenuation characteristics can be tailored by changing the thickness of the cladding layer.