Decoherence of a two-level system in a coherent superposition of two dephasing environments

Decoherence of a two-level system is studied when it interacts with one of two environments via a dephasing coupling, where the environments are treated as classical and quantum systems. The situation is considered in which the environment that actually interacts with the two-level system is determined by a state of an ancillary two-level system. When information conveyed by the ancillary two-level system is discarded, the reduced dynamics of the relevant two-level system is described by a statistical mixture of two dephasing channels, which shows mixing-induced non-Markovianity. When projective measurement is performed on the ancillary two-level system to extract some information, the diagonal elements of the reduced state of the two-level system depend on time in spite of the dephasing process. Furthermore, it is found that the coherence of the two-level system can exceed its initial value. The parameter region is clarified in which the measurement on the ancillary two-level system can improve the coherence of the relevant two-level system. In particular, when the two environments are equivalent and the ancillary two-level system is in the maximally coherent state, the coherence of the two-level system can be always enhanced by measurement.