Dilution Refrigerator
A machine that makes ten millikelvin — a temperature the universe has never reached on its own.
Provides the silent, ultra-cold environment a quantum processor needs. Stages step the temperature down: 50 K → 4 K → 1 K → 100 mK → 10 mK. The bottom stage holds qubits, sensors, and detectors at temperatures where thermal noise is too weak to scramble quantum states.
Two stable isotopes of helium — the common ⁴He and rare ³He — separate into two phases below 0.87 K. Pumping the lighter phase removes ³He atoms across the boundary; that boundary-crossing is endothermic, just like sweat evaporating cools your skin. Continuous circulation of ³He through a heat exchanger at the bottom of the fridge drives the temperature down to ~10 mK indefinitely. Above the mixing chamber, a series of pulse-tube and Joule–Thomson stages handles the higher-temperature precooling.
In plain terms
Imagine a thermos inside a thermos inside a thermos, each one colder than the last. The innermost one uses a magic trick: when you mix two flavors of helium, one of them gets dramatically colder — and that's the part you put your chip on.
Also called: dil fridge, DR.
Why it matters
- Without it, no superconducting quantum computer can operate.
- The only practical way to keep a sample below ~300 mK continuously and indefinitely.
- Same hardware enables ultralow-noise sensors for dark-matter experiments and CMB telescopes.
Where it lives
- IBM Quantum System One / Two
- South Pole Telescope detector arrays
- ADMX axion dark-matter search
- Almost every superconducting-qubit lab on Earth
Key numbers
- Base temp: 5–10 mK
- Cooling power @ 100 mK: 300–600 µW
- Stages: 5 (50 K, 4 K, 1 K, 100 mK, 10 mK typical)
- Cooldown time: 12–48 h
Surprising facts
- The base temperature of a modern dilution fridge — about 10 millikelvin — is roughly 30,000 times colder than the cosmic microwave background, the leftover heat of the Big Bang.
- The cooling power at the coldest stage is around 20 microwatts. A single human cell, metabolizing, produces more heat.
- ³He is so rare on Earth that most dilution fridges are filled with helium that was produced as a byproduct of US nuclear weapons stockpile maintenance during the Cold War.
- The technique was first proposed in 1962 by Heinz London — and only worked reliably in 1965 because no one had pure enough ³He before that.
Open questions
- Mysterious heat leaks below 10 mK: Even in the most carefully designed dilution refrigerators, the electron temperature measured on-chip is often significantly higher than the mixing-chamber thermometer reads — sometimes by a factor of two to five. Sources include residual vibrations, infrared photons leaking through coaxial cables, and poorly understood heat pathways through crystal lattice defects. No single mechanism has been shown to account for all observed excess heating across different experiments.
- Persistent quantum fluid below 0.87 K: Below 0.87 K, a mixture of ³He and ⁴He spontaneously separates into two liquid phases that coexist indefinitely — one rich in ³He floating on a ⁴He-dominated phase. This phase separation does not freeze into a solid at any pressure (a consequence of quantum zero-point motion), making helium the only substance that remains liquid at absolute zero under its own vapor pressure. The dilution refrigerator is the only machine that exploits this quantum anomaly as an engineering resource.
A short history
Heinz London proposed using a ³He/⁴He mixture for refrigeration in 1962. The first working dilution refrigerator was built in 1965 by Das, de Bruyn Ouboter and Taconis at Leiden. By the 1980s commercial systems from Oxford Instruments and Janis brought sub-100 mK research into reach for ordinary university labs; today 'dry' (cryogen-free) versions from Bluefors and Oxford dominate the quantum-computing era.
Sources
- Pobell, Matter and Methods at Low Temperatures (Springer, 3rd ed.) — definitive reference (https://link.springer.com/book/10.1007/978-3-540-46360-3)
- Bluefors XLD product spec sheet (https://bluefors.com/products/dilution-refrigerator-measurement-systems/xld-dilution-refrigerator-measurement-system/)