A dilution refrigerator is a specialized cryogenic system that cools superconducting quantum processors to approximately 10-15 millikelvin (mK) — roughly 0.015 degrees above absolute zero and 200 times colder than outer space. At these temperatures, thermal energy is negligible compared to the quantum energy levels of superconducting qubits (operating at 4-6 GHz, corresponding to ~200 mK in energy), ensuring that qubits remain in their quantum ground states and are not excited by thermal noise.
The cooling mechanism exploits the thermodynamic properties of a helium-3/helium-4 mixture. At low temperatures, this mixture separates into two phases, and forcing helium-3 atoms from the concentrated phase into the dilute phase absorbs heat — similar to how evaporation provides cooling. This process is continuous and can maintain millikelvin temperatures indefinitely. The refrigerator has multiple cooling stages: a 4-kelvin stage (liquid helium), a 1-kelvin stage (pumped helium-4), a 100-millikelvin still, and the mixing chamber at 10-15 mK where the quantum processor sits.
Dilution refrigerators are significant infrastructure components for superconducting quantum computing. A single unit costs $500,000 to $2 million, weighs several hundred kilograms, stands 2-3 meters tall, requires substantial helium-3 (an increasingly scarce and expensive isotope), and takes 1-2 days to cool from room temperature to operating temperature. The wiring challenge is formidable: each qubit requires multiple microwave coaxial cables running from room-temperature electronics through attenuators at each thermal stage to the millikelvin processor, with careful thermal management to prevent heat from reaching the cold stage. Companies like Bluefors and Oxford Instruments are the primary suppliers, and both are scaling toward larger systems to accommodate processors with thousands of qubits.