Phase Distributions in Open Quantum Systems
Raman Research Institute, Bangalore.
We motivate and discuss the quantum description of phases using phase distributions in open quantum systems. Open quantum systems can be broadly classified into two categories: (A). Quantum non-demolition (QND), which involves decoherence without any dissipation and (B). Quantum dissipative systems, where decoherence is accompanied with dissipation. In the parlance of quantum information theory, the noise generated by a QND open system would be a ``phase damping channel'', while that generated by a dissipative (Lindblad) evolution would be a ``(generalized) amplitude damping channel''. A brief description of the underlying dynamics of QND as well as quantum dissipative processes is provided and their connection to quantum information processes detailed. We then provide a unified description of phases for both QND as well as dissipative systems, starting from a number of interesting initial conditions of practical relevance, using the phase distribution approach. The phase distributions conditioned on various initial system states are used to study the corresponding phase dispersion, a measure of quantum phase fluctuations and also used to discuss the `number-phase' complementarity for atomic systems.