The Dilution Refrigerator

The dilution refrigerator is the machine that keeps qubits colder than deep space — and it runs on helium-3.

In brief

• Reaches ~2–10 mK continuously, unlike one-shot coolers.
• Cooling comes from the enthalpy of mixing helium-3 into helium-4 across a phase boundary.
• The helium-3 is circulated and recovered in a closed loop — but an initial charge is required per machine.
• Scaling qubit counts drives larger fridges and more helium-3.

The detail

Below about 0.87 K, a helium-3/helium-4 mixture separates into two phases: a helium-3-rich phase floating on a dilute phase. Forcing helium-3 across that boundary into the dilute phase absorbs heat — the same way evaporation cools, but effective all the way down to a few millikelvin. A dilution refrigerator circulates helium-3 through this mixing chamber continuously, which is why it can hold a quantum processor at its operating temperature for weeks. The helium-3 charge is the enabling consumable, and it is the isotope Kronos's breeder produces.