Decoherence and the theory of continuous quantum measurements

The decoherence of a quantum system, i.e., its becoming partly classical, results from its interaction with the environment and is well described in terms of continuous quantum measurement theory. In the present paper, various approaches to CQM theory are reviewed, of which that using effective complex Hamiltonians is discussed in detail. The effective complex Hamiltonian is obtained from the restricted path integral, the latter highlighting the role of information in the dynamics of the measured system, and is applied to the energy measurement of a two-level system. For such a measurement, quantum transition monitoring is shown to be possible and its back-action on the transition probability analyzed. The performance of such a measurement using a long series of soft observations is described.