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Structure and Integrated Manufacturing Process of a Phonon-Scattering-Controlled Seamless Superconducting Coaxial Cable for Cryogenic Quantum Computing

Taisuke Michikaze

Published Aug 4, 2026• 0 citations
Publisher

Abstract

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.

DOI
10.5281/zenodo.21793826

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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.
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$\sim60to60 to60to\sim535353\Omega$. 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.
Abstract Superconducting quantum computers require microwave control lines running from room temperature to the mixing chamber of a dilution refrigerator. Adding more lines without preliminary thermal modeling to make predictions risks overwhelming the cooling power at each thermal stage. In this paper, we investigate the thermal load of SC-086/50-SCN-CN semi-rigid coaxial cable, which is commonly used for the control and readout lines of a superconducting quantum computer, as we increase the number of lines to a quantum processor. We investigate the makeup of the coaxial cables, verify the materials and dimensions, and experimentally measure the total thermal conductivity of a single cable as a function of the temperature from cryogenic to room temperature values. We also measure the cryogenic DC electrical resistance of the inner conductor as a function of temperature, allowing for the calculation of active thermal loads due to Ohmic heating. Fitting this data produces a numerical thermal conductivity function used to calculate the static heat loads due to thermal transfer within the wires resulting from a temperature gradient. The resistivity data is used to calculate active heat loads, and we use these fits in a cryogenic model of a superconducting quantum processor in a typical Bluefors XLD1000-SL dilution refrigerator, investigating how the thermal load increases with processor sizes ranging from 100 to 225 qubits. We conclude that the theoretical upper limit of the described architecture is approximately 200 qubits. However, including an engineering margin in the cooling power and the available space for microwave readout circuitry at the mixing chamber, the practical limit is approximately 140 qubits.
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.
Since superconducting quantum computers use microwave signals to control and readout their qubits, microwave transmission lines and circuits that can be used under cryogenic conditions are required. For microwave transmission lines, thermal insulation is essential to enhance wiring density and to prevent parasitic heating that can lead to increased computational errors. Thus, here we design and characterize a scalable, thermally insulated microwave transmission line. The transmission line designed for superconducting qubit readout at 5 GHz band achieves both thermal insulation and low loss, by employing resonator-based impedance matching in a microstripline structure. A flexible cable including eight planar transmission lines was designed, fabricated and characterized using S-parameter measurement, demonstrating low crosstalk below 40dB and signal attenuation of -23.5±0.07dB; this coincides with electromagnetic simulations. These microwave properties remained stable across a wide temperature range from room temperature down to 4 K, including steep gradients. The integration of chip attenuators further reduces thermal noise and protects sensitive cryogenic components. Our approach offers a promising solution for compact, high-density microwave wiring using flat cables under thermal gradients, necessary for large scale quantum computing.
Title A Modular High-Density Cryogenic Architecture for 10⁴–10⁵ Qubit Superconducting Quantum Computers (Version 1.0 – Open Proposal, December 2025) Abstract (copy-paste this in the abstract field) This document proposes a practical, fully specified cryogenic wiring and packaging architecture that enables 10 000 to 40 000 high-quality superconducting qubits inside a single commercial dilution refrigerator (Bluefors XLD or Oxford Triton class) using only components and fabrication processes demonstrated in 2024–2025. The central idea is a large vertical “Wall” panel at the 10 mK stage combined with a vertical coaxial routing scheme that reduces conductive heat load to ≤ 0.58 µW per line (measured on NbTi/CuNi aerogel-filled lines with two-point anchoring). No new materials, no unproven refrigeration cycles, and no speculative physics are required. The design is deliberately conservative and offered as an open contribution for community validation. Description (copy-paste this in the “Notes” / description box on Zenodo) I am an independent researcher sharing this detailed engineering proposal in the hope that groups with real dilution refrigerators and clean-room access will find it useful enough to test. Key points that I believe make this worth a quick look: Uses only off-the-shelf cryogenic coax, PEEK 3D printing, OFHC braids, Kevlar suspension, and standard 2025 transmon/fluxonium processes. The thermal budget calculations and the ≤ 0.58 µW/line number are derived from publicly available NbTi/CuNi data and simple two-point anchoring – nothing exotic. A subscale prototype (e.g. 0.5 m × 0.5 m sector, 2 000–5 000 lines) could be built and cooled down in 2026 by almost any serious lab. Full mechanical drawings, spacer STL files, cabling lists, and test protocols will be uploaded here as soon as at least one group expresses interest in trying it. I am not claiming that everything will work perfectly on the first try – only that the path looks technically conservative and testable today. Feedback, criticism, and especially experimental results (positive or negative) are very warmly welcomed. If you run a quantum hardware lab and would like the CAD files or discuss a small joint prototype, please feel free to reach out, I am easy to find. License: CC-BY-4.0 (share, adapt, and publish results freely, just attribute). The specific vertical routing geometry may be patentable, but I am happy to grant royalty-free use for academic and non-commercial research. Thank you for your time, Nadia Sahraoui Independent researcher ORCID 0009-0002-7049-9794 December 2025
Scaling superconducting quantum processors to hundreds of qubits is essential for demonstrating quantum advantage and implementing quantum error correction codes with larger distances. A key challenge is integrating control and readout wiring without compromising cryogenic or electronic performance. To address this, we performed a systematic analysis of the thermal budget within a dilution refrigerator and developed high-density 0.5 mm SCuNi-CuNi coaxial cable sets optimized for heat load management. This enables the integration of 696 control lines and 40 readout amplification chains into a dilution refrigerator with a cooling power of 1 mW at 100 mK, maintaining a stable base temperature of ∼8 mK for over a year. We validated the platform's performance with two large-scale quantum processors: a 540-qubit processor with an average T1 of 35 μs and a 156-qubit processor with 182 tunable couplers, achieving average fidelities of 99.9% for single-qubit gates and 99.0% for two-qubit gates. This study demonstrates a viable high-density wiring solution for controlling and measuring processors at the 500-qubit scale and provides critical engineering insights for larger-scale quantum computing systems.