A dilution fridge is the kind of refrigerator our lab uses to cool samples down to temperatures colder than deep space, typically around 10mK. It gets there using a mixture of two isotopes of helium that separate from each other as they cool, and pulling one isotope through the other in a controlled way pulls heat out of the system. We need temperatures this extreme because the quantum devices we build only show their real behavior when there is almost no stray heat around to interfere with them. At room temperature, or even inside an ordinary freezer, thermal energy would completely overwhelm the delicate effects we are trying to measure.

In June the lab installed two Proteox S dilution fridges from Oxford Instruments, which now anchor the whole experimental side of the lab. Each one is really a set of nested chambers, colder as you move inward. The outermost layer is the OVC, or outer vacuum chamber, pumped down to a near vacuum so heat cannot easily creep in from the room. Near the top sits a superconducting magnet, letting us apply strong magnetic fields to a sample once everything is cold. At the very center is the mixing chamber, the coldest point in the whole system and the place where the actual cooling happens. Samples go in from the bottom on a small platform called a puck, which lets us swap devices in and out without warming the entire fridge back up each time, something that would otherwise take days.

The chambers themselves are mostly machined aluminum and copper, chosen for how well they conduct heat and how easy they are to work with. Look closely and the outer surfaces are polished to a mirror shine, while the surfaces facing inward toward the colder stages are often left dull. A shiny, reflective surface bounces heat radiation away instead of soaking it up, which is exactly what you want facing the warm room outside. The duller inner surfaces do the opposite job, absorbing any stray radiation that slips past the outer shields so it never gets the chance to reach, and disturb, the sample sitting at the coldest point of all.