An algebro-geometric study of two models of quantum computaiton

Abstract

We investigate geometric aspects of two models of quantum computation: starting with i. the computational complexity of the particle excitations of topological phases of matter in the restricted model of a Topological Quantum Field Theory in the sense of Turaev before leading to ii. where in analogy with ideas of Nielsen et. al., we propose and study a geometric model for grover's search algorithm. i. presents fundamental results in the mathematics and physics literature on conformal field theory as a model for quantum computation, phrased within the algebraic framework of the theory of tensor categories. Our main questions are 1. how does the computational power of these excitations change as a function of the genus of a fixed 2-dimensional space-time? and 2. independent of any particular space-time, what structural properties of a TQFT govern its computational power? When restricted to a space-time with space-like degrees of freedom represented by a smooth surface of genus g, we answer the first question by observing a q⁹-fold degeneracy in the ground state of the TQFT resulting from the presence of abelian anyons with exchange statistics a q-th root of unity. Such a resource is a topologically fault-tolerant quantum memory. The abelian character of the emergent particle statistics leads us to answer the second question via an algebraic realization of non-abelian anyonic excitations in the language of unitary modular tensor categories. Subsequently, ii. studies the quantum mechanical evolution of a particle within the Schröedinger wave-function formalism of quantum mechanics: our primary result is a purely geometric proof of the optimality of Grover's Search Algorithm on n qubits obtained via a study of the geometric structure of a homogenous space for the Unitary group of transformations acting on a single qubit.