  Since Shor's discovery of an algorithm to factor numbers on a quantum
computer in polynomial time, quantum computation has become a subject of
immense interest. Unfortunately, one of the key features of quantum computers -
the difficulty of describing them on classical computers - also makes it
difficult to describe and understand precisely what can be done with them. A
formalism describing the evolution of operators rather than states has proven
extremely fruitful in understanding an important class of quantum operations.
States used in error correction and certain communication protocols can be
described by their stabilizer, a group of tensor products of Pauli matrices.
Even this simple group structure is sufficient to allow a rich range of quantum
effects, although it falls short of the full power of quantum computation.
