  Description of neutrino oscillation in the case of Non-Standard neutrino
Interaction (NSI) is briefly presented. The NSI causes the entanglement between
internal degrees of freedom of neutrinos (mass, spin, flavour) and other
accompanying particles in the production and detection processes. In such case
neutrinos are mostly in the mixed states. Role of the density matrix in
description of neutrino oscillation process is shortly explained.
