  We formulate the unitary rational orbifold conformal field theories in the
algebraic quantum field theory framework. Under general conditions, we show
that the orbifold of a given unitary rational conformal field theories
generates a unitary modular category. Many new unitary modular categories are
obtained. We also show that the irreducible representations of orbifolds of
rank one lattice vertex operator algebras give rise to unitary modular
categories and determine the corresponding modular matrices, which has been
conjectured for some time.
