  We study a generic open quantum system where the coupling between the system
and its environment is of an energy-preserving quantum nondemolition (QND)
type. We obtain the general master equation for the evolution of such a system
under the influence of a squeezed thermal bath of harmonic oscillators. From
the master equation it can be seen explicitly that the process involves
decoherence or dephasing without any dissipation of energy. We work out the
decoherence-causing term in the high and zero temperature limits and check that
they match with known results for the case of a thermal bath. The decay of the
coherence is quantified as well by the dynamics of the linear entropy of the
system under various environmental conditions. We make a comparison of the
quantum statistical properties between QND and dissipative types of evolution
using a system of two-level atom and a harmonic oscillator.
