4 K. Enough for to make "low temperature superconductors" like regular metals superconducting, e.g. the superconducting temperature of aluminum if 1.2 K.
Contrast with liquid nitrogen, which is much cheaper but only goes to 77K.
Surprisingly, it can also become a superfluid even though each atom is a fermion! This is because of Cooper pair formation, just like in superconductors, but the transition happens at lower temperatures than superfluid helium-4, which is a boson.
aps.org/publications/apsnews/202110/history.cfm: October 1972: Publication of Discovery of Superfluid Helium-3 contains comments on the seminal paper and a graph which we must steal.
Also sometimes called helium II, in contrast to helium I, which is the non-superfluid liquid helium phase.
Superfluid helium Resonance Experiment by Dietterich Labs (2019)
Source. Articles by others on the same topic
Liquid helium is the liquid phase of helium, a noble gas that is colorless, odorless, tasteless, non-toxic, and inert. Helium is the second lightest and second most abundant element in the observable universe, after hydrogen. Liquid helium has several unique properties, particularly at very low temperatures. It is primarily known for its extremely low boiling point, which is around -269 degrees Celsius (-452 degrees Fahrenheit), making it one of the coldest substances known.