Quantum ergodic theorem originated by John von Neumann is a strong result arising from the mere mathematical structure of quantum mechanics. The QET is a precise formulation of termed normal typicality, i.e. the statement that, for typical large systems, every initial wave function ψ ψ 0 from an energy shell is ‘normal’: it evolves in such a way that ψ ψ t for most t, is macroscopically equivalent to the micro-canonical density matrix.