  If a large Quantum Computer (QC) existed today, what type of physical
problems could we efficiently simulate on it that we could not simulate on a
classical Turing machine? In this paper we argue that a QC could solve some
relevant physical "questions" more efficiently. The existence of one-to-one
mappings between different algebras of observables or between different Hilbert
spaces allow us to represent and imitate any physical system by any other one
(e.g., a bosonic system by a spin-1/2 system). We explain how these mappings
can be performed showing quantum networks useful for the efficient evaluation
of some physical properties, such as correlation functions and energy spectra.
