In contrast to particle physics, which is also called high energy physics because of phenomena such as the Big Bang, condensed matter physics is the physics of low energy. There is an “overlap” between these two branches of physics. For example, they both study Dirac fermions described by Dirac equation. In particle physics, all solutions to the Dirac equation are called Dirac fermions. In condensed matter physics, however, only the massless ones deserve the name.
Helium-3 also plays a role in both physics. It is a clean fuel for nuclear fusion. Just imagine how high the energy when a nuclear fusion happens! No wonder He3 is the subject of high energy physics. It is clean because there are no dangerous neutrons produced during the fusion. He3 is very rare on the earth since it’s escaping. People produce He3 by nuclear reaction if they want some. The commercial He3 one bought may be the byproduct of a nuclear weapon. People always say there is more He3 on the moon and we can mine it. In condensed matter physics, He3 is an interesting material which is a fermion and shows the superfluidity at low temperatures. The superfluidity is a result of Bose-Einstein condensation (a property of bosons). In order to obtain low temperatures, one needs a fridge. In many cases, He3 or its cousin He4, or both, is the refrigerant. If we use He4, we get 4.2 K (4.2 degrees above absolute zero). If we pump He4, we achieve ~ 1 K. We pump He3, we have 300 mK. We pump the He3-He4 mixture, the temperature may be less than 10 mK. The thermal energy of 1 K is about 0.1 meV. So low is the energy compared to a He3-He3 fusion (10 MeV)!