Macroscopic Quantum Phenomena
Usually quantum mechanics deals with matter on the scale of atoms and atomic particles. However, at low temperatures, there are phenomena that are manifestations of quantum mechanics on a macroscopic scale. The most well-known effects are superfluidity of helium and superconductivity which both show spectacular behavior. E.g. in both cases matter can flow with zero flow resistance. In rotating helium so-called quantum vortices are formed which are all equally strong and which can organize in beautiful patterns. A similar effect shows up in superconductors where an applied magnetic field is squeezed in bundles each containing the same amount of magnetic flux.
Macroscopic quantum effects are among the most elegant phenomena in physics. Chapter 21 of the Feynman Lectures on Physics on this topic starts with “This lecture is only for entertainment.” In the period from 1996 to 2003 four Nobel prizes were given for work which was related to macroscopic quantum phenomena. Macroscopic quantum phenomena can be observed in superfluid helium and in superconductors, but also in dilute quantum gases and in laser light. Although these media are very different, their behavior is very similar as they all show macroscopic quantum behavior.
The phenomena which are called macroscopic quantum phenomena are macroscopic in two ways:
The quantum states are occupied by a large number of particles (typically Avogadro’s number).
The quantum states involved are macroscopic in size (up to km size in superconducting wires).