Cryogenic wiring and readout amplifiers in quantum computers

How many lines does a superconducting quantum computer need?

About two per qubit in a common design, plus shared readout lines. ETH Zurich’s 50-qubit layout, with a drive and a flux line for every qubit and readout multiplexed six or seven qubits per line, needed 124 RF lines and used its large Bluefors refrigerator at full capacity, though the heat budget would have allowed 150 qubits had three times as many lines fit. PostQuantum, a trade publication, puts a 100-qubit system at 300–500 signal paths. Each drive line needs about 60 dB of attenuation, 20 dB each at 4 K, the cold plate and the mixing chamber in ETH’s scheme, to cut thermal photons at the chip to about one in a thousand. Bluefors, whose research says every qubit needs its own control line, fits up to 1,008 coax lines in an XLDsl refrigerator; Huber+Suhner’s multicoax carries 192 channels on one flange, and Delft Circuits’ flex puts eight on a 0.3 mm ribbon.

What does the readout chain do?

It carries a very weak signal up without sending noise down. A readout pulse leaves the chip at about −130 to −120 dBm, according to Japan’s 2025 supply-chain roadmap. In ETH Zurich’s output lines it passes a circulator and two isolators, giving more than 60 dB of isolation toward the chip, a traveling-wave parametric amplifier at the mixing chamber with 20–30 dB of gain across 3–12 GHz, and a HEMT amplifier with about 40 dB of gain at 4 K, before amplifiers at room temperature. The traveling-wave amplifier’s gigahertz bandwidth is what lets one line read several qubits at once. The chain grows with the machine: the roadmap estimates that reading 1,000 qubits four per line would take 250 HEMT amplifiers drawing about 2 to 4.4 W at 4 K, and PostQuantum reports lead times for new HEMTs that can stretch to months.

Who makes the wiring and amplifiers, and how much is listed?

A mix, with much of it private. Huber+Suhner sells NbTi, copper-nickel and stainless coax, non-magnetic connectors and its 192-channel multicoax, and is developing interconnects with Bluefors. Amphenol owns XMA, whose cryogenic attenuators the Japanese roadmap ranks among the most used, and Narda-MITEQ, which sells amplifiers qualified to 4 K. The roadmap calls JAE’s non-magnetic SMPM connectors almost the only option at their size, Hirose’s SER unit sells NbTi multi-coax cables, QuantumCTek sells complete cable assemblies with attenuators, filters and parametric amplifiers, and Qnity’s Eccosorb absorber is cast into infrared filters. AmpliTech, in the readout sector, markets 4 K HEMT amplifiers for quantum computing without a named customer. Private makers include Bluefors, Delft Circuits, COAX, TOTOKU and CryoCoax for wiring, Low Noise Factory for HEMTs, QuinStar for circulators and Silent Waves for traveling-wave amplifiers, and no listed maker of cryogenic isolators was verified.

How to research a cryogenic wiring or readout company

1. Place the part in the chain: input lines carry coax, attenuators, filters and connectors, while output lines carry isolators, a parametric amplifier, a HEMT and a room-temperature amplifier, often from different makers.
2. Rate the evidence: Japan’s AIST–RIKEN–Fujitsu–NEC roadmap describes what Japanese research groups use, not purchases by a named quantum-computer maker.
3. Look inside groups: XMA and Narda-MITEQ belong to Amphenol, SER to Hirose Electric and Laird’s Eccosorb to Qnity, and each is a small part of its parent; XMA had about $15 million of annual sales when Amphenol bought it.
4. Treat generic and early roles as such: TE and Bel Fuse’s Cinch make micro-D connectors the roadmap names for dense DC wiring, QTREX’s printed interconnect is not yet commercial, and Quanta Computer may develop flexible cables for Rigetti.
5. Watch the architecture: flex cables, denser coax and cryogenic or on-chip control could cut the lines per qubit and move content between cable, connector and electronics makers.
6. Use the list as a starting point: it is curated, not exhaustive, and the model is illustrative, not a bill of materials.

Sources

• Krinner et al. (ETH Zurich): engineering cryogenic setups for 100-qubit scale superconducting circuits (arXiv, 2018)

  124 RF lines for a 50-qubit processor at the refrigerator’s full capacity, about 60 dB of drive-line attenuation, a TWPA, circulator, isolators and HEMT on each output line, and room for 150 qubits with three times the line capacity.

• AIST, RIKEN, Fujitsu and NEC: supply-chain technology roadmap for superconducting quantum computers (Japanese PDF), Sept 9, 2025

  Readout pulses of about −130 to −120 dBm; 250 HEMT amplifiers drawing about 2 to 4.4 W at 4 K for 1,000 qubits read four per line; XMA attenuators among the most used; JAE SMPM connectors almost the only option.

• PostQuantum (trade press): quantum cryogenic infrastructure and helium-3, May 23, 2026

  300–500 signal paths for a 100-qubit system and HEMT lead times that can stretch to months, in trade press.

• Bluefors: research derives the limits of quantum computer wiring density, Sept 9, 2026

  Every qubit needs its own control line, so fidelity requirements set a minimum line pitch and a maximum number of connections.

• Bluefors: High-Density Wiring

  Up to 168 lines per side-loading port and 1,008 on an XLDsl system.

• Bluefors: Microwave Readout Module

  A traveling-wave parametric amplifier at the millikelvin stage, a HEMT at 4 K and filtering and isolation components in one package.

• Delft Circuits: Cri/oFlex technology

  Eight channels per 0.3 mm flex with NbTi or silver striplines and built-in attenuators and filters.

• Huber+Suhner: cryogenic semi-rigid cables for quantum computing

  NbTi coax under 0.5 dB/m at 4 K and 10 GHz, plus copper-nickel, copper-beryllium and stainless steel lines.

• Huber+Suhner: multicoax systems for quantum computing

  Up to 192 channels on an ISO flange from room temperature down to the mixing chamber.

• Huber+Suhner: management report 2025

  Work with Bluefors on interconnects for next-generation cryogenic quantum applications.

• XMA: cryogenic products

  Cryogenic attenuators, terminations, DC blocks and IR filters that operate at 4 K, for quantum computing.

• Amphenol: Q3 2023 results (SEC exhibit 99.1), Oct 25, 2023

  Amphenol closed its acquisition of XMA, with about $15 million of annual sales.

• Narda-MITEQ: cryogenic low-noise amplifiers

  CLNA amplifiers qualified down to 4 K, of interest for radio astronomy and quantum computing.

• AmpliTech Group: Form 10-K for 2025

  Cryogenic amplifiers that operate as low as 4 K for applications including quantum computing.

• Hirose SER: connector cables for quantum computers (Japanese)

  NbTi multi-channel coax connector cables for quantum computing, from DC to 40 GHz.

• QuantumCTek: cryogenic cable assemblies for superconducting quantum computing (Chinese)

  Copper-nickel, stainless, aluminum and NbTi coax by stage, with attenuators, filters, hermetic feedthroughs and parametric amplifiers.

• Laird (Qnity): microwave absorbers

  Eccosorb castable and machinable absorbers under the Laird brand of Qnity.