2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/139474We prove a necessary and sufficient condition for the occurrence of entanglement in two two-level systems, simple enough to be of experimental interest. Our results are illustrated in the context of a spin star system analyzing the exact entanglement evolution of the central couple of spins.4 pages, Submitted to Physical Review LettersQuantum Physicstext