Cryogenic Wiring and Connectors

In a superconducting quantum computer, the cables and connectors that link the room-temperature control electronics to the quantum chip, located in the millikelvin (mK) stage of a dilution refrigerator, are a crucial yet extremely challenging component. They must not only transmit precise control and readout signals but also minimize heat leak and noise interference across an extreme temperature gradient. A successful cryogenic wiring system must strike a balance among multiple conflicting requirements.

Core Challenges and Design Requirements

Challenge Area Specific Requirement & Description
Thermal Management Core Challenge: Good electrical conductors are typically also good thermal conductors.
  • Ultra-Low Thermal Conductivity: Cables must have extremely low thermal conductivity to minimize heat transfer from room temperature (~300K) to the mK stage. Excessive heat load can overwhelm the cooling capacity of the refrigerator, severely limiting the number of supported qubits.
  • Thermalization: Signal lines must be sufficiently thermalized at each temperature stage, typically using thermal anchors and attenuators to bring the noise temperature of the cable down to the physical temperature of that stage.
Signal Integrity Core Challenge: Maintaining signal integrity without distortion over long distances and extreme temperature gradients.
  • Broadband Performance: Provide low-loss, low-dispersion transmission characteristics over a wide frequency range (DC to several GHz) to ensure the fidelity of control pulses and readout signals.
  • Impedance Matching: Maintain good impedance matching (typically 50ฮฉ) at all temperature stages to reduce signal reflection.
  • Low Crosstalk: High signal isolation is required to prevent interference between different channels.
Noise Mitigation Core Challenge: Protecting qubits from decoherence caused by noise.
  • Suppressing Thermal Noise: Cables act as black-body radiators; this thermal noise must be suppressed using thermalization and in-line attenuators.
  • Electromagnetic Shielding: Cables require excellent shielding to block environmental electromagnetic interference (EMI).
  • Infrared Filtering: Infrared filters are often integrated to block high-frequency thermal radiation from propagating down the dielectric layers.
Scalability & Compactness Core Challenge: The "wiring bottleneck" is one of the main obstacles to scaling up quantum computers.
  • High-Density Integration: As the number of qubits increases, cables and connectors must be high-density and compact to accommodate hundreds or thousands of channels within the limited space of a cryostat.
Mechanical & Material Reliability Core Challenge: Materials must withstand extreme temperature changes and repeated thermal cycling.
  • Cryogenic Compatibility: Materials must not become brittle, crack, or suffer significant performance degradation at low temperatures. Differences in thermal expansion coefficients are a key design consideration.
  • Flexibility: Flexible cables help simplify the time-consuming and error-prone process of installation within a complex refrigerator.
  • Non-Magnetic Requirement: Materials with extremely low magnetic susceptibility are required to avoid interfering with magnetically sensitive quantum components (like SQUIDs).
  • Hermeticity: Feedthroughs that pass through the vacuum shroud must meet ultra-high vacuum standards to prevent leaks.
Cost Core Challenge: Due to specialized materials, complex manufacturing processes, and an emerging market, high-performance cryogenic cables and components are currently very expensive.

Key Components, Materials, and Technologies

To address the challenge of correctly transmitting microwave pulses across extreme temperature gradients while minimizing thermal load, researchers and vendors have developed several types of cryogenic wires, connectors, and related components. Each type represents a different trade-off between electrical performance, thermal properties, mechanical reliability, and scalability. The key components are summarized below:

Component Category Specific Type Key Features & Design Considerations Main Application
Wiring & Cables Low-K Coaxial Cables
  • Structure: Available in semi-rigid forms for better shielding or flexible forms for easier routing.
  • Materials: Conductors use low-thermal-conductivity alloys like Stainless Steel (SS) or CuproNickel (CuNi); at the coldest stages, superconducting materials like Niobium-Titanium (NbTi) are used to achieve zero resistance and low thermal conductivity.
 The traditional solution for RF signal transmission, optimized for cryogenic use. Suitable for systems with fewer channels or where cost is a major consideration.
Flexible Ribbon Cables / PCBs
  • Structure: Multi-channel microstrip lines printed on a flexible substrate like Polyimide.
  • Advantages: High density, excellent flexibility, low thermal load, and the ability to integrate passive components directly.
 The modern, mainstream solution for addressing high-density and ease-of-installation challenges.
Twisted Pairs
  • Structure: Two insulated wires twisted together.
  • Features: Simple structure, low cost, with some resistance to common-mode noise.
 Primarily used for transmitting low-frequency analog signals or DC bias signals. RF performance is limited.
Optical Fibers
  • Features: Extremely low thermal conductivity and extremely high bandwidth potential.
  • Advantage: Fundamentally solves problems of heat leak and electromagnetic interference (EMI).
 Considered a highly promising future alternative, especially for applications requiring massive data throughput or that are extremely sensitive to heat load.
Connectors & Interconnects Standard RF Coaxial Connectors
(Cryogenic/Non-Magnetic)
  • Types: Cryogenic, non-magnetic versions of standard interfaces like SMA, SMP, 2.92mm (K-type), etc.
  • Materials: Made from non-magnetic stainless steel, Beryllium-Copper (BeCu), etc., with PEEK or PTFE insulators. Often gold-plated.
 Used for single-line connections.
High-Density Connectors
  • Types: Multiport coaxial connectors, board-to-board/cable-to-board interfaces based on spring-loaded contact arrays.
  • Advantage: Significantly increases connection density, simplifying assembly and maintenance of large-scale systems.
 Addresses the challenge of a growing number of channels.
Hermetic Feedthroughs
  • Function: The critical interface for bringing signal lines from the ambient pressure environment into the ultra-high vacuum, cryogenic environment.
  • Technology: Uses glass-to-metal or ceramic-to-metal sealing. Bodies are often made of Kovar or stainless steel, with standard vacuum flange interfaces (like KF, CF).
 Required for all lines that must pass through the vacuum shroud.
Passives & Thermal Management Attenuators & Filters
  • Function: Precisely integrated into signal lines to attenuate thermal noise from higher temperature stages, filter out unwanted frequencies, and accurately control the signal power delivered to the quantum chip.
 Indispensable components in the signal chain for noise management and signal conditioning.
Thermal Anchors / Sinks
  • Function: Anchoring cables at various temperature stages of the dilution refrigerator (e.g., 4K, Still, Cold Plate) to dissipate the heat conducted down the line.
 A core method for managing cable thermal load, crucial for maintaining stable mK temperatures.


Originally written in Chinese by the author, these articles are translated into English to invite cross-language resonance.

Professional portraitPeir-Ru Wang Hardware Guide (2025)