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IEEE Transactions on Applied Superconductivity

Development of Semi-Rigid Superconducting Coaxial Cables as Low-Pass Filters

A. Kushino, Soichi Kasai

Published Dec 26, 2016• 5 citations
Publisher

Abstract

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.

DOI
10.1109/tasc.2016.2645161

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•Aug 4, 2026•0 citations
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.
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.
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.
Electromagnetic noise is one of the main external factors decreasing superconducting qubit coherence. Matched coaxial filters can prevent microwave and IR photons' negative influence on superconducting quantum circuits. In this report, we describe the design and fabrication process of matched coaxial filters for noise-sensitive measurements at millikelvin temperatures. A robust transmission coefficient and ultralow reflection loss of −20 dB in the frequency range up to 20 GHz is achieved. Fabricated low-pass filters have linear transmission and reflection characteristics with 3 dB-cutoff frequency of 1.5–2.5 GHz. A method for extracting the propagation constant and filter impedance from scattering parameter measurements is demonstrated. This method is experimentally approved on a filter with a compound of Cu powder and Stycast epoxy resin and a filter filled with ECCOSORB CR-110 epoxy resin. The proposed design and assembly technology are versatile for various compounds and provide highly repeatable geometric and microwave characteristics. Furthermore, we demonstrate that these low-pass coaxial filters can be effectively utilized to improve superconducting qubit relaxation due to suppressing standing waves originating from reflections in control coaxial cables.
The interconnection between cryogenic quantum chips and room temperature measurement and control systems is a critical issue for quantum computing. In this paper, integrated filtering cryogenic flexible cables are designed to suppress external noise for the transmission of weak and high-frequency signal between low temperature and room temperature. They are made of flexible substrate (PI or LCP) with good flexibility and high integration, and lower heat loss than typical cryogenic coaxial cables. The low-pass filter is integrated in the cable with changing the line width, and the high-frequency noise can be effectively suppressed.
Electromagnetic filtering is essential for the coherent control, operation and readout of superconducting quantum circuits at milliKelvin temperatures. The suppression of spurious modes around transition frequencies of a few GHz is well understood and mainly achieved by on-chip and package considerations. Noise photons of higher frequencies - beyond the pair-breaking energies - cause decoherence and require spectral engineering before reaching the packaged quantum chip. The external wires that pass into the refrigerator and go down to the quantum circuit provide a direct path for these photons. This article contains quantitative analysis and experimental data for the noise photon flux through coaxial, filtered wiring. The attenuation of the coaxial cable at room temperature and the noise photon flux estimates for typical wiring configurations are provided. Compact cryogenic microwave low-pass filters with CR-110 and Esorb-230 absorptive dielectric fillings are presented along with experimental data at room and cryogenic temperatures up to 70 GHz. Filter cut-off frequencies between 1 to 10 GHz are set by the filter length, and the roll-off is material dependent. The relative dielectric permittivity and magnetic permeability for the Esorb-230 material in the pair-breaking frequency range of 75 to 110 GHz are measured, and the filter properties in this frequency range are calculated. The estimated dramatic suppression of the noise photon flux due to the filter proves its usefulness for experiments with superconducting quantum systems.
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.