Nonlinearity and constrained quantum motion
Dorje C. Brody, Anna C T Gustavsson, Lane P. Hughston · Journal of Physics A Mathematical and Theoretical · 2010
The dynamical equation satisfied by the density matrix when a quantum system is subjected to one or more constraints arising from conserved quantities is derived. The resulting nonlinear evolution of the density matrix has the property that it is independent of the specific composition of the pure-state mixture generating the initial state of the system.