Colder than cold

To refrigerate a substance to very low temperatures one uses a cryostat (from Greek cryos = cold).
    The cooling is done in several steps. The first step cools the substance from room temperature to about 1 K.
    In a second step, from 1 K to about 10 mK, one normally uses a mixture of ⁴He and ³He in a dilution process.
    Temperatures down to about 1 mK can be reached by compressing ³He - the Pomeranchuk method.
    Even lower temperatures can be reached by magnetic methods.