How can low temperatures for superconductivity be reached?
The most commonly used refrigerant is liquid helium, which has a boiling point of 4.2 K. But how does one make liquid helium?
Essentially, this process is exactly the way a normal household refrigerator works, except instead of using a fluorocarbon as the working fluid, you use helium gas.
Most gases cool when they expand and heat up when compressed. The trick is to compress them to high T, high P and then cool the compressed gas. When expanded again, the gas becomes colder than it originally started. This gas can be used to pre-cool incoming gas, so that it doesn't get as hot when compressed, and gets even colder than the first gas.
If you make a multi-stage device, the cold gas from one stage can be used to cool the compressed gas from the next stage, and the gases get even colder.
In this way, you can make liquid air, liquid nitrogen, liquid helium and many other liquified gases. The colder you go, the more difficult it gets.