Evolution of a quantum system during measurement and decoherence

The process of measurement and decoherence is studied with a model consisti ng of a measuring apparatus in interaction with both an oscillator and the environment. In the present approach, the Weisskopf-Wigner approximation is used in order to calculate the evolution operator of the complete system, and to construct directly the density operator. The quantum fluctuations of the zero-point held make an important contribution of the decoherence; thi s process is described in an explicit form.