2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/81018Evolution 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.6 pagesExactly Solvable and Integrable SystemsQuantum Physicstext