Decoherence and continuous measurements: Phenomenology and models

Decoherence of a quantum system induced by the interaction with its environment (measuring medium) may be presented phenomenologically as a continuous (or repeated) fuzzy quantum measurement. The dynamics of the system subject to continuous decoherence (measurement) may be described by the complex-Hamiltonian Schroedinger equation, stochastic Schroedinger equation of a certain type or (nonselectively) by the Lindblad master equation. The formulation of this dynamics with the help of restricted path integrals shows that the dynamics of the measured system depends only on the information recorded in the environment. With the help of the complex-Hamiltonian Schroedinger equation, monitoring a quantum transition is shown to be possible, at the price of decreasing the transition probability (weak Zeno effect). The monitoring of the level transition may be realized by a long series of short weak observations of the system which resulting in controllable slow decoherence.