  Entanglement is a special feature of the quantum world that reflects the
existence of subtle, often non-local, correlations between local degrees of
freedom. In topological theories such non-local correlations can be given a
very intuitive interpretation: quantum entanglement of subsystems means that
there are "strings" connecting them. More generally, an entangled state, or
similarly, the density matrix of a mixed state, can be represented by
cobordisms of topological spaces. Using a formal mathematical definition of
TQFT we construct basic examples of entangled states and compute their von
Neumann entropy.
