Quantum decoherence in open systems

In the framework of the theory for open quantum systems, we determine the degree of quantum decoherence of a harmonic oscillator interacting with a thermal bath. It is found that the system manifests a quantum decoherence which is more and more significant in time. We also calculate the decoherence rime, which has the same scale as the time after which thermal fluctuations become comparable with quantum fluctuations.