Adiabatic demagnetization
Cooling techniques at subKelvin temperature

Refrigeration can be approached in terms of an entropy reduction process. Entropy is a monotonic function of temperature and other variable parameters. Let's take X as an example of such a parameter. The curve below (figure 12 ) shows that a succession of processes can be imagined to lower the temperature. Note that, in accordance with the third principle of thermodynamics, entropy tends towards zero as the temperature approaches absolute zero, illustrating the difficulty of producing very low temperatures.