In quantum physics, indistinguishable particles are particles that cannot be distinguished from one another. This is the case of bosons. This notion of indiscernibility has a fundamental consequence at the origin of condensates: it encourages the gregarious behaviour of particles. This means that the probability of accumulating bosons in a same level is higher than what classical physics predicts, because in the case of classical physics, two particles are always distinguishable from one another. If we take a look at the following scheme, we can see that when we cannot follow the particles by their number and the particles are indistinguishable, the chance that they might be in the same level and hence “condensate”, is higher.
A condensate is hence characterized by a large number of particles in the same quantum state at a very low temperature. All the particles hence occupy the same quantum wavefunction. Thus, the phases of the waves associated to each particle are identical and make the gas condensate a phase coherent object.
By analogy with optics, the phase coherence of gas condensates was proved by the existence of interferences (voir l’image ci-contre). These phase coherence properties are at the origin of different fields of research: atomic interferometry, atom metrology and atom lasers.
Pictures of interferences (dark and shiny fringes) in Bose-Einstein gas condensates. As in the case of light, the presence of these fringes proves that the condensate is coherent: it is indeed a unique wave! [Crédits Ketterle group, MIT]
