The working principle of dilution refrigerators relies on the properties of helium-3 and helium-4. At very low temperatures, these isotopes separate into two phases: a concentrated helium-3 phase and a dilute phase of helium-3 in helium-4. When helium-3 atoms cross from the concentrated phase to the dilute phase, they absorb energy, thus cooling the system. This continuous process, known as the "dilution process," can maintain temperatures as low as 2 millikelvin.
