Superfluidity and superconductivity are striking examples of how quantum mechanics can affect the collective behaviour of large numbers of identical particles. When the temperature of a many-particle fluid is lowered below a critical threshold, thermal fluctuations are no longer able to prevent it from collapsing into a purely quantum state. If the particles are electrically neutral, like helium–3 and helium–4 atoms, such a 'coherent' state can flow without friction and is known as a superfluid. If, on the other hand, the particles are charged, the coherent state loses its resistance to electrical current and becomes a superconductor.
