Quantum circuit dedicated PCB: a core carrier for low-temperature, low-loss quantum measurement and control interconnects

1. Overview

Quantum circuit PCBs are the core interconnect substrates of superconducting/silicon-based/ion trap quantum computers. They are divided into two main categories: low-temperature PCBs inside diluted refrigeration units and RF PCBs for room-temperature measurement and control. They are responsible for transmitting qubit microwave control signals, read signals, and DC bias signals, while also enabling multi-temperature (300K room temperature→ 4K liquid helium→ 10mK ultra-low temperature) signal cross-layer transmission. Unlike traditional FR-4 circuit boards, quantum PCBs must simultaneously meet four demanding specifications: milliaque-level low-temperature mechanical stability, ultra-low microwave loss at GHz, extreme electromagnetic shielding, and extremely low noise, directly determining qubit decoherence time T1/T2, gate control fidelity, and overall computing performance.

2. Core Application Scenarios of Quantum Circuit PCBs

1. Dilution chiller low-temperature substrate (mK temperature zone)

Directly equipped with a superconducting quantum chip, operating temperature as low as 10 millikelvin (-273.14°C), connected to a qubit Josephson junction, transmitting 4–8GHz microwave control pulses and reading resonant signals, it is the most core and most process-critical quantum PCB.

2. 4K temperature zone transition interconnect board

As an intermediate layer between room temperature and extremely low temperature, it undertakes thermal isolation and signal relay, reducing heat transfer at room temperature to the quantum chip, balancing mechanical toughness with low RF loss.

3. Room temperature measurement and control RF motherboard

Placed outside the chiller, it integrates signal source, filtering, amplification, and analog-to-digital conversion circuits, outputting high-precision control waveforms and connecting to high-end microwave instruments such as Siyi Technology.

4. Quantum chip packaging substrate

Ceramic/PTFE ultra-thin substrates, used for gold wire bonding of quantum chips and multi-bit array integration, suitable for hundred-bit and kilobit scale quantum computing architectures.

3. Key technical challenges of quantum PCBs

1. Mechanical failure risk at ultra-low temperatures

The CTE thermal expansion coefficient of ordinary FR-4 sheets differs greatly from that of copper foil. During hot and cold cycles (300K↔to 10mK), delamination, trace cracking, and brittle solder joints can occur; Epoxy resin becomes brittle at extremely low temperatures, with dielectric constant drift exceeding 15% and signal loss surging by over 30%, making it completely unusable for quantum systems.

2. Strict requirements for microwave ultra-low loss

The quantum control signal is a 4–8GHz microwave, and tiny dielectric losses can greatly reduce quantum gate fidelity. The loss factor Df of FR-4 ≈ 0.02, while the Df of quantum PCB substrates must be below 0.001, a difference of 10–100 times; For every 0.001 increase in loss, qubit coherence time is directly shortened by tens of microseconds.

3. Extremely low noise and anti-interference

Quantum states are highly susceptible to decoherence caused by electromagnetic noise, ground loops, and crosstalk interference. Any slotting, discontinuous ground plane, or trace coupling on the PCB introduces noise, so it is necessary to fully shield, partition, and eliminate ground loops.

4. Inhibition of heat conduction in multi-temperature zones

PCBs simultaneously span three temperature ranges: room temperature, 4K, and 10mK. The substrate requires low thermal conductivity to reduce heat flow into the chiller, lower the chiller load, and ensure stable ultra-low temperature environments for quantum chips.

4. Substrate Systems for Quantum PCBs (Selection Criteria)

(1) High-Frequency PTFE Series (Mainstream RF Layer)

Representative models: Rogers RO4003C, RT/duroid 5880

• Dk≈2.2–3.5, Df<0.0009, extremely low GHz band loss;

• CTE controllable, low-temperature cycling makes layering difficult;

• Applicable to: room temperature measurement and control board, 4K transient RF board, microwave signal transmission layer.

(2) Polyimide PI Flexible Panel (Low-Temperature Interconnect)

Strong low-temperature toughness, thermal stress buffering, suitable for tight internal wiring of chillers, used for flexible cross-temperature interconnection, Df < 0.003, halogen-free, CTI≥ high insulation grade of 600.

(3) Aluminum Nitride AlN Ceramic Substrate (10mK Ultra-Low Temperature Core Substrate)

Extremely low CTE (2.6 ppm/°C), low thermal conductivity, and ultra-high insulation perfectly match the thermal expansion of superconducting quantum chips, with no low-temperature cracking. It is the standard substrate for large-scale superconducting quantum chips and is used for kilobit array packaging.

(4) Prohibited Materials

Ordinary FR-4, high-halogen sheets, high-roughness copper foil, and ordinary low-temperature solder cannot meet the low-temperature and RF requirements of quantum systems.

5. Standard Design Specifications for Quantum PCBs

1. Lamination and impedance control

• The transmission line uniformly adopts a coplanar waveguide CPW structure, providing strong electromagnetic field constraints and low crosstalk;

• Strict 50Ω impedance control, tolerance within ±3%, avoiding signal reflection loss;

• Multilayer boards are symmetrically stacked to prevent warping at low temperatures; The inner layer must be complete and continuous in the ground plane; arbitrary grooving and dividing the ground plane is prohibited.

2. Layout of Partitioned Isolation (Noise Reduction Core)

• Physical isolation of three major areas: digital circuits, microwave RF, and DC bias, with copper walls of over 2mm reserved for isolation;

• Grounding protection grounding wires are laid on both sides of microwave-sensitive traces, with stitched ground via fences along the line to suppress lateral radiation crosstalk;

• Shortened path for sensitive quantum signals, reducing transmission loss and noise coupling.

3. Grounding System (Prevents Decoherence of Ground Loop)

• Single-point star grounding in analog/RF regions, eliminating closed ground loops;

• Densely arranged vias with vias spacing of ≤5mm to reduce ground impedance;

• Digital ground and quantum analog ground merge at a single point, prohibiting large-scale common-ground crosstalk noise.

4. Low-temperature interconnection process

• Solder selected is indium tin In-Sn and gold tin AuSn low-temperature alloy, maintaining toughness at 10mK, with no cracks after thousands of hot and cold cycles;

• The conductor uses high-purity, low-roughness copper foil to reduce microwave skin loss;

• Adhesive fills the bottom of key components to cushion thermal stress, and ceramic packaged components replace plastic-encapsulated components.

6. Current status of domestic industrial development

1. Research applications

Shenzhen International Quantum Research Institute, Guodun Quantum, Benyuan Quantum, Zhejiang Quantum Laboratory, and Chinese Academy of Sciences Quantum Innovation Institute have all mass-scale the use of custom low-temperature PTFE/ceramic quantum PCBs, supporting the development of 64-bit and 504-bit "Xiaohong" superconducting quantum chip measurement and control systems.

2. PCB manufacturers' technical layouts

Leading manufacturers such as Shenghong Technology, Jiechuang Electronics, and Aipi Circuits have joined forces with the Chinese Academy of Sciences and Southern University of Science and Technology to develop quantum-specific cryogenic PCBs, overcoming PTFE mixing, ceramic substrates, and ultra-low temperature thermal cycling verification processes, achieving domestic substitution and breaking the overseas monopoly of Rogers and Taconic in materials and boards.

3. Upstream and downstream supporting facilities

Upstream: Localization of low-loss PTFE substrates and AlN ceramic substrates;

Midstream: precision processing of quantum PCBs, low-temperature assembly of PCBA;

Downstream: Paired with Siyi Technology's microwave measuring instruments, dilution refrigeration units, and superconducting quantum chips, forming a complete quantum hardware industry chain.

7. Industry Development Prospects

1. Logic of demand explosion

The national 15th Five Plan's quantum technology strategy is advancing, with superconducting and silicon-based quantum computers moving from the laboratory to engineering; Mass production of hundred-bit and kilobit quantum chips will drive exponential growth in demand for low-temperature PCBs. Currently, the localization rate of high-end quantum PCBs is less than 20%, leaving huge room for import substitution.

2. Direction of technical iteration

1. Terahertz band ultra-low loss quantum PCB, compatible with 6G and next-generation qubit control;

2. Superconducting routing PCB (NbTi superconducting thin-film circuit) to achieve zero-loss signal transmission;

3. Integrated ceramic-PCB composite substrate, integrating cooling, shielding, and interconnection functions;

4. Flexible ultra-thin quantum FPC, compatible with miniature ion traps and silicon-based quantum dot chip packaging.

3. Long-term market potential

With the implementation of quantum simulation, quantum encryption, and quantum AI, quantum measurement and control hardware and low-temperature interconnect substrates will form a segment worth tens of billions. As essential core components, quantum PCBs will continue to benefit from the expansion of the entire industry chain in quantum computing, quantum sensing, and quantum communication.

8. Summary

Quantum circuit PCBs serve as the "bridge" connecting classical measurement and control systems with qubits. Their three core performances—low-temperature mechanical stability, microwave low loss, and ultra-low noise—directly determine the upper limit of quantum computers. Unlike ordinary RF PCBs, quantum PCBs form a complete technical system for dedicated substrates, wiring, grounding, and low-temperature assembly. With the acceleration of domestic quantum research and industrialization, domestic low-temperature, low-loss quantum PCBs are gradually achieving independent controllability. In the future, they will support the large-scale deployment of fault-tolerant quantum computers and become indispensable key foundational materials for the localization of quantum hardware.