 Another exotic model for quantum computing which has attracted a lot of attention, especially from Microsoft inc. (Freedman 1998), is the Topological Quantum Field Theory model. In contrast to the easily visualisable circuit model, this model resides in the most abstract reaches of theoretical physics. The exotic physical systems TQFT describes are topological states of matter. That the formalism of TQFT can be applied to computational problems was shown by Witten (1989) and the idea was later developed by others. The model has been proved to be efficiently simulatable on a standard quantum computer (Freedman, Kitaev, and Wang 2002; Aharonov, Jones, and Landau 2009). Its main merit lies in its high tolerance to the errors which are inevitably introduced in the implementation of a large scale quantum computer (see below). Topology is especially helpful here because many global topological properties are, by definition, invariant under deformation, and given that most errors are local, information encoded in topological properties is robust against them.