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arXiv:2502.01945 · EPJ Quantum Technology

Cryogenic thermal modeling of microwave high density signaling

Naomi Raicu, Tom Hogan, Xian Wu, Mehrnoosh Vahidpour, David Snow, Matthew I. Hollister, Mark Field

Published Oct 30, 2025• 8 citations
arXivHTMLPDF

Abstract

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.

DOI
10.1140/epjqt/s40507-025-00427-1

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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

•Nov 18, 2025•0 citations
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.
A robust cryogenic infrastructure in form of a wired, thermally optimized dilution refrigerator is essential for solid-state based quantum processors. Here, we engineer a cryogenic setup, which minimizes passive and active heat loads, while guaranteeing rapid qubit control and readout. We review design criteria for qubit drive lines, flux lines, and output lines used in typical experiments with superconducting circuits and describe each type of line in detail. The passive heat load of stainless steel and NbTi coaxial cables and the active load due to signal dissipation are measured, validating our robust and extensible concept for thermal anchoring of attenuators, cables, and other microwave components. Our results are important for managing the heat budget of future large-scale quantum computers based on superconducting circuits.
Installing semi-rigid coaxial cables and microwave components in a dilution refrigerator is essential to readout and control superconducting quantum processors. Here we discuss passive and active heat loads generated in the refrigerator and explain techniques to minimize them. Scaling up the size and cooling power of the refrigerator is required in order to operate largely integrated superconducting processors. We also discuss problems impeding the scale up of the refrigerators in Japan and activities in the world.
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.
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.
One of the challenges for building a quantum computer with superconducting qubits is being able to integrate a sufficient density of microwave lines in a cryogenic environment. Traditionally this has been limited by the pitch of SMA cables. When trying to go beyond this approach there are two problems to address. The first is outfitting a dilution refrigerator with enough lines to run from room temperature to the base temperature stage. The second is building a package for a quantum integrated circuit that houses enough microwave lines itself. We propose a solution to both of these problems by using RF flexible circuits to increase signal density in the dilution refrigerator and package while maintaining signal integrity. We also discuss the thermal, mechanical and RF considerations that go into designing such a system. Finally we show a proof-of-concept integration of the entire system including measurements with superconducting qubits.
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