Complex quantum system

This approach was developed originally as an approximate method, if the wave functions of isolated atoms are taken as a basis wave functions Wannier functions. Only in the last case the expansion (1) and the Hamiltonian (2) are exact, but some extension to the arbitrary basis functions is possible. In principle, the TB model is reasonable only when local states can be orthogonalized. The method is useful to calculate the conductance of complex quantum systems in combination with ab initio methods. It is particular important to describe small molecules, when the atomic orbitals form the basis. 

V.M. Akulin et al. (eds.), Decoherence, Entanglement and Information Protection in Complex Quantum Systems, 15—16. 

Although its actual implementation remains far away, it has been known for a long time that the hypothetical quantum computer should be a much more powerful tool than its classical analog [Ekert 1996 Steane 1998], In particular, it might be able to simulate the behaviour of complex quantum systems far more efficiently than classical machines [Feynman 1996], For example, to simulate an N interacting spin system classically requires about 2N bits therefore, such classical computations become intractable when N exceeds 40 spins. 

Collisional decoherence is a universal mechanism which occurs for objects of any size. In contrast to that, the following section describes a mechanism which only appears for complex quantum systems, namely decoherence,thermal due to the emission of their own heat radiation. 

Akulin V. M., Dynamics of Complex Quantum systems, Chapt. 4, (Springer, 2004). 

Decoherence, Entanglement and Information Protection in Complex Quantum Systems... 

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