A dilution refrigerator is the machine that cools a quantum processor to the extreme temperatures it needs to work. Many types of qubit, including silicon spin qubits, must be held at a few thousandths of a degree above absolute zero, colder than the depths of interstellar space. At these temperatures the thermal vibrations that would otherwise disturb the qubits are strongly suppressed.
The cooling relies on a mixture of two isotopes of helium, helium-3 and helium-4. When these separate into two phases, moving helium-3 across the boundary between them absorbs heat, and it is this process that reaches the millikelvin temperatures required. The qubit chip sits at the coldest stage, at the base of the refrigerator.
The dilution refrigerator also shapes one of the central challenges of scaling quantum computers. Every qubit controlled from room-temperature electronics needs wiring running down into the cold, where space and cooling power are limited. This is a key motivation for Cryo-CMOS, which moves control electronics into the refrigerator alongside the qubits to reduce the wiring burden.