Refrigeration by adiabatic nuclear demagnetization

Temperature is one of the most fundamental physical quantities, and low temperature environments serve as a typical approach for frontier scientific explorations. Lower temperature environments help to discover novel phenomena in low-energy scale quantum manybody systems, and to achieve higher experimental resolutions at lower thermal noise levels. There are two basic missions for low temperature experimentalists: discovering new physics at low temperatures, and maintaining low temperature environments or creating better ones. Precooling by liquid helium or cryogen-free methods, refrigeration principles such as adiabatic nuclear demagnetization have been proposed and verified. The process of cryogen-free, is as important as the liquefaction of helium and the dilution refrigeration for low temperature refrigeration. In 1908, helium was first liquefied while China was able to achieve stable ~ 4 K environments in the 1960s. During that time, dilution refrigerators were invented elsewhere. China is investing on how to build them nowadays. This article covers the principles of adiabatic nuclear demagnetization and introduces what has led to the cryogen-free adiabatic nuclear demagnetization technique. Additionally, it describes a successful attempt to build such a refrigerator in China, which is the lowest temperature cryogen-free refrigerator in the world.