Architectures for quantum computation under restricted control

In this thesis, we introduce several new, architectures for quantum computing under restricted control. We consider two specific cases of restricted control; systems in which local control of individual qubits is impossible, and systems in which qubits are connected by a probabilistic entangling mechanism. First we will- consider densely packed spin chains and introduce a novel scheme for performing universal quantum computing within this system using only global control. Global control avoids local manipulation of individual qubits, using instead identical operations on each qubit. We present an experimental demonstration of this scheme using NMR techniques. Next we propose an architecture for implementing fault-tolerant computation using only global control. We will consider a chain consisting of repeating patterns of three distinct species. The structure of this chain allows error correction to be performed in parallel. We describe the necessary operations required to construct a universal set of fault-tolerant operations, and prove the existence of a fault-tolerance threshold. vVe finish our discussion of global control with a look at the ultimate limits of control within quantum systems. We describe a technique for calculating an upper bound on the number of accessible qubits within any quantum system, and derive upper bounds on the number of usable qubits in a range of spin networks. Next we will propose an architecture for distributed quantum computing using linear optics to generate entanglement between matter qubits. Using the graph state formalism we show that such entanglement can be generated in a deterministic manner when linking systems consisting of coupled pairs of qubits. vVe finish our discussion of distributed quantum computing by presenting an algorithm for reducing the number of qubits required to implement a graph state calculation. This algorithm efficiently removes redundant qubits from the measurement pattcrn used to perform the computation.EThOS - Electronic Theses Online ServiceGBUnited Kingdo