Beyond the quantum Markov approximation, we calculate the geometric phase of a two-level system driven by a quantized magnetic field subject to phase dephasing. The phase reduces to the standard geometric phase in the weak-coupling limit, and it involves the phase information of the environment in general. In contrast with the geometric phase in dissipative systems, the geometric phase acquired by the system can be observed on a long time scale. We also show that with the system decohering to its pointer states, the geometric phase factor tends to a weighted sum over the phase factors pertaining to the pointer states.

• Received 14 December 2004

DOI:https://doi.org/10.1103/PhysRevA.71.044101