Quantum Computing and Quantum RF Cryogenic Microwave Engineering Informational

How do I select the appropriate coaxial cable type for each temperature stage of a cryostat?

Q: Why not use copper cable for lowest microwave loss?

Copper has excellent microwave properties (lowest loss of all common metals) but terrible cryogenic thermal properties: its thermal conductivity is 100-1000× higher than stainless steel at cryogenic temperatures (400 W/m-K at 4K vs 0.3 W/m-K for SS). A single copper coaxial cable from 300K to 4K would conduct approximately 10 mW of heat, consuming most of the 1.5W cooling capacity at 4K for just one cable. A 200-cable system would need 2W of cooling per cable, far exceeding any practical refrigerator. The only situation where copper is used: very short bus bars or thermal straps within a single temperature stage, where both ends are at the same temperature and thermal conduction is desired for heat sinking.

Q: What is SMP and why is it used in cryostats?

SMP (Sub-Miniature Push-on) is a blind-mate coaxial connector designed for high-density applications. It is smaller than SMA (3.2 mm diameter vs 6.4 mm for SMA coupling nut) and connects by simply pushing together without threading. In cryostats, SMP enables: (1) Higher cable density through stage plates (2× more cables per unit area compared to SMA). (2) Faster assembly and disassembly (no torque wrench needed). (3) Blind-mate capability for stacked-plate cryostat architectures. SMP performance: DC to 40 GHz, VSWR < 1.3 at 18 GHz, 500+ mating cycles. Disadvantage: lower repeatability than SMA (phase may shift by 1-3° between matings) and lower retention force (may disconnect under vibration if not properly constrained).

Q: How do I thermally anchor cables at each stage?

Three methods in order of thermal effectiveness: (1) Wrap and clamp: coil the cable 1-2 turns around a gold-plated copper bobbin bolted to the stage plate, then clamp with a copper saddle. Contact length: 3-10 cm. Thermal conductance: 10-100 mW/K. Best for stages with abundant cooling power (4K, 50K). (2) Clamp only: press the cable against the stage plate using a copper saddle with thermal grease (Apiezon N) or indium foil. Contact length: 1-2 cm. Adequate for intermediate stages. (3) SMA bulkhead: route the cable through an SMA bulkhead adapter threaded into the stage plate. The outer conductor of the SMA bulkhead provides galvanic contact with the plate. Moderate thermal anchoring but also provides a convenient connection point for stage-to-stage cable segments with different materials.

Coaxial cable selection for each temperature stage of a cryostat involves trading off thermal conductivity (heat leak), microwave loss, mechanical properties, and cost. Recommended cables by stage: Room temperature to 50K: UT-085-SS (stainless steel semi-rigid, 50 ohm, 0.085" OD). Thermal conductivity: ~0.3 W at 300K-to-50K per cable. Loss: 1.5 dB/m at 5 GHz. Cost: ~$10/m. 50K to 4K: UT-085-SS or CuNi semi-rigid. Same thermal and loss properties. Brief section (15-20 cm), so total loss is tolerable. 4K to still (800 mK): NbTi/NbTi or NbTi/CuNi superconducting coaxial. Thermal conductivity: ~0.03 W from 4K to 0.8K per cable. Loss: <0.1 dB/m at 5 GHz below Tc. Cost: ~$100-200/m. Still to cold plate to mixing chamber: NbTi superconducting coaxial throughout. Thermal load per cable: ~0.5 μW from 0.8K to 20 mK. Signal loss: <0.05 dB per section. For output (readout) lines: use NbTi from 4K to MC and the lowest-loss stainless steel available from 300K to 4K, or accept the loss penalty and compensate with HEMT gain. Some labs use superconducting aluminum ribbon cable for the MC-to-chip section (zero loss below Tc). For DC bias and flux lines: twisted-pair or single-conductor resistive wire (phosphor bronze, manganin) with low thermal conductivity, filtered at each stage with RC or LC low-pass filters.

Category: Quantum Computing and Quantum RF

Updated: April 2026

Product Tie-In: Cryogenic Components, Attenuators, Circulators, Cables

Cryostat Cable Selection Guide

Cable selection is one of the most impactful decisions in cryogenic microwave system design. The wrong cable at a critical stage can either exceed the thermal budget (too much heat leak) or degrade signal quality (too much loss), with consequences that are difficult to correct without a complete re-wiring of the cryostat.

Parameter	Option A	Option B	Option C
Performance	High	Medium	Low
Cost	High	Low	Medium
Complexity	High	Low	Medium
Bandwidth	Narrow	Wide	Moderate
Typical Use	Lab/military	Consumer	Industrial

Technical Considerations

Key cable materials and their properties: Stainless Steel 304 (UT-085-SS): thermal conductivity integral 300K-4K ≈ 30 W·cm/m. Microwave surface resistance at 5 GHz ≈ high (resistive at all temperatures). Good mechanical strength. CuNi (70/30 copper-nickel): thermal conductivity integral 300K-4K ≈ 20 W·cm/m (slightly lower than SS). Similar microwave loss. More flexible than SS. BeCu (beryllium copper): thermal conductivity integral ≈ 100 W·cm/m (much higher). Low microwave loss. NOT recommended for cold stages due to high heat leak. Used only for vibration-sensitive connections at room temperature. NbTi (niobium-titanium): superconducting below 10K. Thermal conductivity integral 4K-0.1K ≈ 0.3 W·cm/m (100× lower than SS). Zero DC resistance, near-zero microwave surface resistance below Tc. The standard for all cable sections below 4K. Cost: 10-20× more than SS.

Performance Analysis

Different signal types have different cable requirements: (1) Qubit drive (input) lines: moderate loss acceptable (can increase DAC output power to compensate). Prioritize low thermal conductivity. Use SS from 300K-4K, NbTi below 4K, with attenuators at each stage. (2) Readout output lines: loss must be minimized (every dB degrades system noise figure). Use the lowest-loss cable available at each stage. NbTi from MC to 4K, and the shortest possible SS section from 4K to 300K. Or use a second HEMT at 50K to compensate for cable loss above 4K. (3) Flux bias lines: carry DC and low-frequency signals (<1 GHz). Twisted pair resistive wire (phosphor bronze, ~40 AWG) is sufficient and has very low thermal conductivity. Filter with RC networks at each stage to prevent RF noise injection. (4) Pump lines (for JPA/TWPA): require stable amplitude (-70 to -60 dBm at the device). Use the same cable routing as qubit drive lines with appropriate attenuation.

Performance verification: confirm specifications against the application requirements before finalizing the design
Environmental factors: temperature range, humidity, and vibration affect long-term reliability and parameter drift
Cost vs. performance: evaluate whether the application demands premium components or standard commercial grades

Interface compatibility: verify impedance, connector type, and mechanical form factor match the system architecture

Design Guidelines

When ordering cryogenic cables, specify: (1) Cable type and diameter (e.g., SC-086/50-NbTi-NbTi). (2) Length with ±5 mm tolerance. (3) Connector types at each end (SMA male, SMA female, or SMP push-on for high-density applications). (4) Bend radius constraints if pre-formed routing is needed. (5) Non-magnetic certification: all materials including connectors, nuts, and solder must be non-magnetic for qubit applications. Standard SMA connectors often use nickel-plated brass; specify gold-over-BeCu for non-magnetic. Cable assembly quality: verify VSWR <1.3 at 8 GHz, and perform time-domain reflectometry (TDR) to check for impedance discontinuities from poor connector installation or cable damage during assembly.

Common Questions

Frequently Asked Questions

Why not use copper cable for lowest microwave loss?

Copper has excellent microwave properties (lowest loss of all common metals) but terrible cryogenic thermal properties: its thermal conductivity is 100-1000× higher than stainless steel at cryogenic temperatures (400 W/m-K at 4K vs 0.3 W/m-K for SS). A single copper coaxial cable from 300K to 4K would conduct approximately 10 mW of heat, consuming most of the 1.5W cooling capacity at 4K for just one cable. A 200-cable system would need 2W of cooling per cable, far exceeding any practical refrigerator. The only situation where copper is used: very short bus bars or thermal straps within a single temperature stage, where both ends are at the same temperature and thermal conduction is desired for heat sinking.

What is SMP and why is it used in cryostats?

SMP (Sub-Miniature Push-on) is a blind-mate coaxial connector designed for high-density applications. It is smaller than SMA (3.2 mm diameter vs 6.4 mm for SMA coupling nut) and connects by simply pushing together without threading. In cryostats, SMP enables: (1) Higher cable density through stage plates (2× more cables per unit area compared to SMA). (2) Faster assembly and disassembly (no torque wrench needed). (3) Blind-mate capability for stacked-plate cryostat architectures. SMP performance: DC to 40 GHz, VSWR < 1.3 at 18 GHz, 500+ mating cycles. Disadvantage: lower repeatability than SMA (phase may shift by 1-3° between matings) and lower retention force (may disconnect under vibration if not properly constrained).

How do I thermally anchor cables at each stage?

Three methods in order of thermal effectiveness: (1) Wrap and clamp: coil the cable 1-2 turns around a gold-plated copper bobbin bolted to the stage plate, then clamp with a copper saddle. Contact length: 3-10 cm. Thermal conductance: 10-100 mW/K. Best for stages with abundant cooling power (4K, 50K). (2) Clamp only: press the cable against the stage plate using a copper saddle with thermal grease (Apiezon N) or indium foil. Contact length: 1-2 cm. Adequate for intermediate stages. (3) SMA bulkhead: route the cable through an SMA bulkhead adapter threaded into the stage plate. The outer conductor of the SMA bulkhead provides galvanic contact with the plate. Moderate thermal anchoring but also provides a convenient connection point for stage-to-stage cable segments with different materials.

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