Dilution refrigerator
The cooling rig that holds superconducting qubits at around 15 millikelvin — colder than intergalactic space.
The principle rests on a mixture of helium-3 and helium-4 separating into two phases below a certain temperature. Moving helium-3 across the phase boundary costs energy, much as evaporation cools skin, and the process can be driven continuously. It is the only known method for sustaining millikelvin temperatures around the clock.
The gold chandelier-like structures used to illustrate quantum computers are these cryostats. Each tier is a colder temperature stage, and the cabling running down must carry signal but not heat. The quantum chip itself sits at the bottom and is roughly the size of a thumbnail.