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Final Report Summary - SAWQUBIT (SAW-driven single-electron quantum devices with optical readout of the spin)

The aim of the Fellowship was to develop a method of emitting and detecting polarised photons of light from quantum dots at temperatures less than 1°C above absolute zero. Quantum dots are “boxes” that can contain one or more electrons, and the dots here are special, in that they are formed and driven across a piece of gallium arsenide (GaAs) material by a surface acoustic wave (SAW). This is a moving strain wave that causes the potential energy to oscillate along with the strain. Electrons residing in a layer just below the surface are caught in the minima of this potential and can be carried along, even being dragged up a potential hill from an n-type region of electrons to a p-type region of holes. Each electron should then recombine with a hole to give out a photon of light.
Such a single-photon source will be useful in its own right, in quantum cryptography (sending an encrypted message or key with the ability to know if anyone is eavesdropping) and eventually in quantum computing (where the ability of quantum particles to be in more than one state at a time could give a vast speed improvement for complex calculations). In addition, the ability to polarise each photon based on the spinpolarisation of the emitting electron will offer extra capability in quantum cryptography, and provides a method of reading out the result of a quantum computation performed with electrons in quantum dots.
While developing such devices, it is vital to be able to work at extremely low temperatures close to absolute zero (–273°C or 0 Kelvin). Getting light out of a cryostat that provides such low temperatures is a difficult task, so it was necessary to design and build a low-temperature, scanning microscope connected to an optical fibre to get the light out. This was a major part of the work in this two-year Fellowship. In addition, ways of making the samples themselves had to be developed. Regions of electrons and holes (missing electrons that behave like positively-charged electrons) have to be produced on the same piece of material. This makes it impossible to dope the whole piece of material uniformly to provide the charges. A technique for inducing the charges by metal “gates” on the surface has been refined and shown to work in these samples. There will be a large potential slope up which the SAW has to drag the electrons. The SAW potential therefore needs to be as large as possible. We have shown that, by depositing a crystalline layer of zinc oxide (ZnO) on the surface, the potential can be increased by a factor of up to 20. This is a very significant achievement, and it required much optimisation of the crystal growth parameters. As a spin-off result, this type of high-quality ZnO layer is now being developed for use in other applications such as biological sensors.
The various aspects of the work will be described in more detail in the subsequent sections.

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THE CHANCELLOR, MASTERS AND SCHOLARS OF THE UNIVERSITY OF CAMBRIDGE
United Kingdom
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