In order to reach temperatures even lower than 1 K, other ingenious techniques are used in laboratories. Today, matter can be cooled to temperatures of a few microkelvins, millionths of a degree from absolute zero.
The most used technique is called “dilution cryostat”, or “dilution fridge”. Its principle is based on the difference in the quantum properties of the two helium isotopes, helium 4 and helium 3. These are two atoms with identical electrons, but slightly different nuclei (the nucleus of helium 3 is lighter). When these two kinds of helium are liquid and mixed, the blend divides in two parts: one only composed of helium 3, the other mainly composed of helium 4 and a little diluted helium 3. At a very low temperature, helium 4 becomes superfluid, but helium 3 does not.
When it becomes superfluid, helium 4 becomes “orderly” and no longer presents any entropy. As far as helium 3 is concerned, there is a blend with pure liquid helium 3 on the one hand, and a few particles of helium 3 lost in superfluid on the other, as if they were free in air.
This situation is similar to that of a cup full of liquid coffee with coffee steam over it. In order to cool it, you have to “blow” on the cup; when you dispel the steam particles, you make the liquid evaporate again, which cools it. In a dilution cryostat, the particles of the pseudo-gas helium 3 are pumped, which cools the liquid helium 3. This enables to reach temperatures of millikelvins (0.001 degree above absolute zero).
Other techniques can be used to cool this blend again: adiabatic demagnetization for instance.
Recent advances have also enabled the conception of refrigerators that can cool matter almost as efficiently as cryogenic liquids and that require neither liquid nitrogen nor liquid helium. These systems work in a closed circuit, like a refrigerator, and use advanced thermodynamic processes: Joule-Thomson effect, Stirling or Gifford-McMahon cycles, pulse tubes… In all these systems, a mechanical work pumps heat out of the object in order to cool it.
