In this thesis , at first we study solution of paradox of the Maxwell’s demon (MD) . Then we explain the Holevo bound on the accessible information . Then we study the physical mechanism of Maxwell’s demon and use it to do extra work from a heat engin in thermodynamic cycles . This engin is called " Szilard heat engin (SZHE)" , one chamber with one particle in a thermodynamic cycle . This cycle consists of four steps : insertion , measurement , controlled expansion , removing the wall so that the next thermodynamic cycle can be restarted . In this thesis we generalize this engin in step 2 and insert two walls insted of one wall in chamber . Here the Maxwell’s demon is modeled as a several-level system with quantum coherence or on a lower temperature than that of the heat bath of the particle . Number of levels depends on the states of engin . The Maxwell’s demon is for measuring and controlling the motion of the molecule. The quantum SZHE’s efficiency is evaluated in detail to prove the validity of second law of thermodynamics . It is illustrated that generalization of Szilard heat engin improve it’s efficiency. Moreover , in comparison with the ltr"
