Open-system quantum dynamics with correlated initial states, not completely positive maps, and non-Markovianity

Dynamical A and B maps have been employed extensively by Sudarshan and co-workers to investigate open system evolution of quantum systems. A canonical structure of the A-map is introduced here. It is shown that this canonical A-map enables us to investigate if the dynamics is completely positive (CP) or non-completely positive (NCP) in an elegant way and hence, it subsumes the basic results on open system dynamics. Identifying memory effects in open system evolution is gaining increasing importance recently and here, a criterion of non-Markovianity, based on the relative entropy of the dynamical state is proposed. The relative entropy difference of the dynamical system serves as a complementary characterization - though not related directly - to the fidelity difference criterion proposed recently. Three typical examples of open system evolution of a qubit, prepared initially in a correlated state with another qubit (environment), and evolving jointly under a specific unitary dynamics - which corresponds to a NCP dynamical map - are investigated by employing both the relative entropy difference and fidelity difference tests of non-Markovianity. The two qubit initial states are chosen to be (i) a pure entangled state, (ii) theWerner state, which exemplifies both entangled and separable states of qubits, depending on a real parameter, and (3) a separable mixed state. Both the relative entropy and fidelity criteria offer a nice display of how non-Markovianity manifests in all the three examples.