Warner, M.; (2011) Beyond classical computing: towards organic quantum information processing. Doctoral thesis , UCL (University College London).

Abstract

This thesis examines the potential of a class of organic molecules, porphyrin derivatives, for quantum information purposes. The experiments in this thesis follow a simple progression: they begin by exploring the time-independent characteristics of the molecules, then investigate the dynamics and decoherence, and end with a consideration of the potential for controllable coupling. Whilst this methodology is obviously motivated by interest in quantum information processing (QIP), it is hoped that the results will have relevance for a larger group of fields including organic electronics, solar energy and spintronics. QIP is at the forefront of science, requiring an understanding of the coherent properties of materials - an understanding deeper than has ever been required before. Since copper phthalocyanine is a technologically relevant material, with uses in organic electronics and solar cells, and possible applications in the developing field of spintronics, this deep understanding can be fed back to provide interesting new developments. The first experimental chapter, Chapter 4, is such an example, where work focussed on understanding a novel phase of CuPc (the \eta-phase), through magnetometry and electron spin resonance led to the development of a method to measure the magnetic properties of liquids. It is hoped that this could open a path for solution based biological samples to be studied via magnetometry. A discussion of the details of the technique, the interpretation of the data, and a proof of principle are provided in this chapter. Another interesting technological development is discussed in Chapter 5, where the initial characterisation of the thin films of copper phthalocyanine (CuPc) led to a deeper understanding of their nanostructure, an important property for the development of solar cells. This consisted of continuous wave electron spin resonance (ESR) experiments on thin films of CuPc, varying both the percentage of copper and the orientation of the molecules, to allow the interpretation of the ESR spectra of mixed films of CuPc and C60, the mixture that is used in organic solar cells. I demonstrate that the CuPc and C60 form nano-clusters in these films, with a preferred orientation. Since the mixed films are known to be more efficient than a simple two layer device, this result provides new information, which can be used to improve the design of this type of solar cell. Chapter 6 consists of the measurement of the decoherence times of spins in CuPc films, and an understanding of the mechanisms of decoherence. Pulsed ESR is used on both powders and thin films of copper phthalocyanine and the field and temperature dependence is explored. Since it is unlikely that molecules in solution will ever be able to provide a scalable QIP solution, this move to the solid state represents a real advance. It is not trivial however, as the solid state can provide many additional decoherence routes. These times are the first step in proposing a material as a potential vehicle for QIP, and I demonstrate that the decoherence times are long enough that, given its other advantages, CuPc can be taken seriously as a candidate host for qubits. The important distinction of these measurements from those that have been made before on, for instance, molecular magnets is that these decoherence times are achieved in the solid state. The last results chapter is an attempt to make progress on a problem that is particularly relevant for molecular QIP. Since molecules can be made easily, they provide a attractive path to an individual qubit. The identical nature of the molecules comes at a price, though, as it makes it hard to address each qubit independently. Chapter 7 seeks to explore the physics of optically coupling two qubits (in this case TEMPO radicals) through a porphyrin linker. Steady state and time resolved optical absorption and pulsed and transient ESR are used to characterise the spin physics of the samples. I demonstrate that it is very likely a coupling can be controllably switched on (if not, as yet, o*). This chapter lays the foundations for a considerable number of future experiments, some of which are discussed at the end, that could demonstrate a truly switchable coherent coupling between qubits.

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