On the description of noise in quantum systems
A formalism of probability operators which generalizes the notion of density operator is introduced into the theory of noisy quantum systems. The Markov property and the connexion between Heisenberg and Schrödinger picture for systems undergoing an irreversible change are discussed in detail. The probability-operator treatment of noise is related to the Langevin method discussed byLax through a generalized Einstein-relation. The master equation for the quantum mechanical oscillator with linear damping is written down in a Fokker-Planck-type approximation. By means of the Einstein-relation the coefficients in the Fokker-Planck-equation are related to the parameters in the phenomenological equations.