Abstract:

Anyons, comprising a special type of particles that exhibit braid statistics,
have received considerable attention in the past decade, from physicists,
mathematicians, and computer scientists. It turns out that socalled modular
tensor categories naturally appear in the description of the key properties of
anyons, which partly explains the interest of mathematicians in the subject.
In this thesis, we study the way such modular tensor categories emerge in two
classes of models, viz. quantum field theory in lowdimensional Minkowski
spacetime, and quantum spin systems on an infinite spatial lattice. Although
the nature of these two classes of models is quite different, we show in this
thesis that they may both be discussed in a similar (mathematically rigorous)
framework. This approach is inspired by the work of Dpplicher, Haag and
Roberts (DHR) in algebraic quantum field theory.
The DHR approach leads to braided tensor categories, which describe the charges
in the theory (in our case, these are anyons). These categories need not be
modular. In this thesis we present a method to remove an obstruction for
modularity, in the case of a spacetime of dimension three. We then study (a
variant of) Kitaev's toric code, regarded as a quantum spin model on an
infinite lattice. Using techniques inspired by the DHR programme, we show that
one can obtain a modular tensor category describing all relevant properties of
the anyons in this system. Finally, some generalisations to nonabelian models
are discussed.
