Dynamical echo in two-state quantum systems

Evolution of quantum fidelity for two-level systems is studied in the context of periodic echo. For time independent case, we obtain a formal condition on the governing Hamiltonians under which the systems display periodic quantum echo. In addition, a revisit of single spin-1/2 system exposed to uniform rotating magnetic field elucidates that the well known Rabi oscillation is a simple mechanism to generate such echoes.