2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/114548The unitary time evolution of a critical quantum spin chain with an impurity is calculated, and the entanglement evolution is shown. Moreover, we show that the reduced density matrix of a part of the chain evolves such that the fidelity of its spectrum is very high with respect to a state in thermal equilibrium. Hence, a thermal state occurs through unitary time evolution in a simple spin chain with impurity.8 pages, 7 figuresQuantum Physicstext