What is the role of eccosorb and other microwave absorbers in the cryogenic environment?
Cryogenic Microwave Absorbers
Microwave absorbers are a critical but often overlooked component of the cryogenic microwave environment. Their proper use can improve qubit T1 (energy relaxation time) by 2-10× by eliminating parasitic photon sources.
- 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
- Margin allocation: include sufficient design margin to account for manufacturing tolerances and aging effects
Frequently Asked Questions
Does Eccosorb work at millikelvin temperatures?
Yes: Eccosorb CR (castable) has been extensively tested and used at millikelvin temperatures in quantum computing labs worldwide. It maintains its absorptive properties at cryogenic temperatures. The main concerns: thermal contraction (Eccosorb contracts slightly when cooled; ensure it is mechanically secure to prevent cracking or delamination), outgassing (in a vacuum environment, Eccosorb should be pre-baked to remove moisture and volatiles before installation in the fridge), and thermal conductivity (Eccosorb is a poor thermal conductor; ensure good thermal contact with the enclosure walls for thermalization).
How much absorber do I need?
For a sample box (qubit enclosure): line all internal surfaces except the qubit chip area with 2-5 mm of Eccosorb CR. This provides 20-50 dB of attenuation for cavity modes and stray radiation. For radiation shields: a thin layer (1-2 mm) on the inner walls is usually sufficient. For cable entries: a ring of absorber (5-10 mm thick) around each cable penetration. Over-absorbing is generally not harmful (more absorption = less stray radiation). The downside: excess material adds mass and may complicate thermal cycling.
What about carbon-loaded coatings?
Alternative to bulk Eccosorb: carbon-loaded paint or epoxy (e.g., Aquadag, carbon black mixed with Stycast epoxy) can be applied as a thin layer to internal surfaces. Advantages: very thin (less than 0.5 mm), easy to apply, low mass, and effective for broadband absorption. Disadvantages: lower attenuation per layer than bulk Eccosorb (approximately 5-10 dB), and may need multiple coats. These coatings are commonly used for: coating the inside of radiation shields, where space is limited and a thin absorber is preferred.