Suppressing decoherence in solid-state quantum information processing

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Quantum physics describes sub-atomic particles, their interactions with each other, and with external influences such as light. Quantum computation is a potentially ground-breaking technology which applies the laws of quantum physics to computational tasks, requiring an unprecedented level of control over the states of quantum particles, in order to perform algorithms that are impossible on any conventional computer. For example, such computers could provide an unmatched standard of secure communications and efficient database searches.However, in practise, quantum computers are confined to the laboratory and to small numbers of computational elements. Performing the factorisation of 15 (using a register of 7 elements) is state of the art. Although, at first glance, this does not seem a particularly impressive feat (conventional computers have now factorised a 200-digit number), closer examination of the problems inherent in building even a single quantum processor, never mind a register, reveals what a breakthrough this is.Conventional computers perform tasks by storing and processing information encoded in the form of binary bits which take a value of either 1 or 0 (corresponding to logical true or false). Quantum computers encode information in an analogous way (in quantum bits or qubits ). Qubits hold two important advantages over their classical counterparts: they may exist not only as 1 or 0 but as a combination of the two (known as a superposition) - and a register of qubits can share information out between each element due to a property known as entanglement (i.e. the qubits can talk to one another). It is these two properties that lead to the higher efficiency of quantum algorithms. Unfortunately, superposition and entanglement make quantum states extremely fragile. The information they hold is lost very quickly due to contact with the surrounding material of the processor, leading to errors in operation. This is a process known as decoherence. For any large scale quantum computer to be built this fundamental limitation must be overcome.The aim of my planned research is therefore to provide a better understanding of how information is lost from the computational elements of a quantum computer and to propose methods for protecting against such decoherence.