2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/91221We show that both information erasure process and trace process can be realized by projective measurement. And a partial trace operation consists to a projective measurement on a subsystem. We show that a nonunitary operation will destroy the wave-behavior of a particle. We give a quantum manifestation of Maxwell's demon and give a quantum manifestation of the second law of therodynamics. We show that, considering the law of memontum-energy conversation, the evolution of a closed system should be unitary and the von Neumann entropy of the closed quantum system should be least.8pages. Reported on the Conference of Chinese Physics Society (2003 autumn):Quantum information conncil houseQuantum Physicstext