Hardware & Errors
Dilution Refrigerator
Specialized cooling equipment used to bring superconducting quantum processors down to near absolute zero temperatures.
Definition
A dilution refrigerator is a piece of specialized cryogenic equipment capable of cooling materials to temperatures just a few thousandths of a degree above absolute zero — the extreme cold required for superconducting qubits to exhibit their quantum properties reliably.
In depth
A dilution refrigerator is the machine that cools superconducting qubits and some other quantum devices to about 10 millikelvin, a hundred times colder than outer space. It uses a mixture of two helium isotopes, helium-3 and helium-4. Below about 0.8 kelvin the mixture separates into a helium-3-rich phase and a dilute phase, and helium-3 atoms moving from the rich phase into the dilute phase absorb heat, the way evaporation cools water. A closed-cycle system pumps and recirculates the mixture to cool continuously.
The refrigerator is a stack of stages. A pulse-tube cooler brings the system to about 4 kelvin, and the dilution unit takes it down through plates at roughly 1 kelvin, 100 millikelvin, and finally the mixing chamber at around 10 millikelvin. Each stage can remove only a limited amount of heat: roughly a watt at 4 kelvin, a few milliwatts at 100 millikelvin, and tens of microwatts at 20 millikelvin. Every cable that reaches the qubits brings heat, so signal lines are filtered, attenuated, and thermalized at each stage, and quantum-limited amplifiers sit at the cold stages.
The size and cost of these systems are central limits on scaling. A large machine such as IBM's Goldeneye or the Bluefors KIDE platform holds a few thousand qubits' worth of wiring, and building a machine with a million qubits will need either many refrigerators, much more efficient wiring using cryo-CMOS controllers, or modular designs with links between cryostats. Helium-3 supply is limited, which is a further concern. Vendors include Bluefors, Oxford Instruments, Maybell, and Leiden Cryogenics.
Technical Definition
Dilution refrigerators achieve their extreme cooling using a mixture of two helium isotopes (helium-3 and helium-4) that undergo a phase separation process at low temperatures, absorbing heat in the process — a multi-stage process that takes many hours to reach base temperature.
Visual Explanation: An Analogy
Think of a dilution refrigerator like an extremely elaborate, multi-stage version of a household refrigerator, except instead of keeping food a few degrees above freezing, it's designed to reach temperatures colder than the vacuum of deep space.
Key facts
| Item | Detail |
|---|---|
| Base temperature | About 10 millikelvin |
| Working fluid | A mixture of helium-3 and helium-4 |
| Cooling power | About 1 W at 4 K and tens of microwatts at 20 mK |
| Stages | Roughly 50 K, 4 K, 1 K, 100 mK, and the mixing chamber |
| Makers | Bluefors, Oxford Instruments, Maybell, Leiden Cryogenics |
Where the idea comes from
The dilution cooling principle was proposed by Heinz London in 1951, and the first working refrigerators were built in the 1960s. Dry, cryogen-free versions using pulse-tube precooling became standard for quantum computing in the 2010s.
Real-World Use Cases
- Essential infrastructure for operating superconducting qubit systems like those from IBM and Google
- A significant cost and size factor in superconducting quantum computer installations, unlike room-temperature approaches
How it differs from related ideas
- A dilution refrigerator reaches millikelvin temperatures, while laser cooling brings atoms to microkelvin in a vacuum chamber.
- Neutral atom and ion machines do not need dilution refrigerators.
- Cryo-CMOS control chips aim to reduce the wiring heat load on the refrigerator.
Frequently asked questions
What is a dilution refrigerator?
A cryogenic machine that uses a helium-3 and helium-4 mixture to cool devices to about 10 millikelvin, used for superconducting quantum computers.
Why are such low temperatures needed?
To keep the circuits superconducting and prevent thermal energy from disturbing the qubits.
What limits scaling?
Cooling power at the coldest stage, wiring heat load, size, cost, and the supply of helium-3.
Related Terms
Keep exploring
- Algorithm: Quantum Error-Correcting Codes (Shor Code, Surface Code)
- Hardware: IBM Heron (r1, r2, and r3)
- Hardware: Google Willow