Evolution of quantum fidelity for two-level systems is studied in the context of periodic echo. From a general treatment for time independent case, we obtain a simple condition on the governing Hamiltonians under which the systems display periodic quantum echo. For a specific time dependent problem the quantum fidelity is shown to exhibit Rabi oscillation. This may be considered as a simple mechanism to generate periodic echo, except for a specific initial superpositional state in which case the fidelity remains invariant.
@article{arxiv.nlin/0506018,
title = {Quantum dynamical echo in two-level systems},
author = {R. Sankaranarayanan and Jane H. Sheeba and M. Lakshmanan},
journal= {arXiv preprint arXiv:nlin/0506018},
year = {2007}
}