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Title: Applications of graph theory to quantum computation
Author: Al-Shimary , Abbas
Awarding Body: University of Leeds
Current Institution: University of Leeds
Date of Award: 2013
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Systems with topologically ordered ground states are considered to be promising candidates for quantum memories. These systems are characterised by a degenerate ground eigenspace separated by an energy gap from the rest of the spectrum. Consequently, topologically ordered systems are resilient to local noise since local errors are suppressed by the gap. Often, knowledge of the gap is not available and a direct approach to the problem is impractical. The first half of this thesis considers the problem of estimating the energy gap of a general class of Hamiltonians in the thermodynamical limit. In particular, we consider a remarkable result from spectral graph theory known as Cheeger inequalities. Cheeger inequalities give an upper and lower bound on the spectral gap of discrete Laplaeians defined on a graph in terms of the geometric characteristics of the graph. We generalise this approach and we employ it to determine if a given discrete Hamiltonian is gapped or not in the thermodynamic limit. A large class of 2D topologically ordered systems, including the Kitaev toric code, were proven to be unstable against thermal fluctuations. There systems can store information for a finite time known as the memory lifetime. The second half of this thesis will be devoted to investigating possible theoretical ways to extend the lifetime of thc 2D toric code. Firstly, we investigate the effect lattice geometry has on the lifetime of the qubit toric code. Importantly, we demonstrate how lattice geometries can be employed to enhance topological systems with intrinsically biased couplings due to physical implementation. Secondly, we improve the error correction properties and lifetime of the generalised 2D toric code by using charge-modifying domain walls. Specifically, we show that we can inhibit the propagation of anyons by introducing domain walls, provided the masses of the anyon types of the model are imbalanced.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available