Quantum spintronics using donors in isotopically engineered silicon

The exquisite control over materials fabrication and spin control techniques has reached a maturity where spintronics can go beyond purely classical effects and begin to fully exploit the unique quantum properties of superposition and entanglement. Potential applications arising from quantum spintronics range from quantum information processors, including the transmission of quantum information via itinerant electron spins, single microwave photon storage within spin ensembles, and new generation of sensors exploiting entanglement to yield fundamentally enhanced precision. Key ingredients for quantum spintronics include the preservation of spin coherence and the generation of high-purity entanglement. Through this collaborative research project, we propose to address both of these challenges using both the electron and nuclear spin of donors in isotopically engineered silicon. Our preliminary experiments show that such materials offer the greatest potential for high-purity entanglement and long coherence times, and their potential integration within conventional electronics is a further advantage.Through our initial collaboration, we have already demonstrated 'psuedo-entanglement' between the electron and nuclear spin associated with a P-donor in silicon at X-band (0.3 T) and 6K. We have used the fidelities which we achieved in those experiments to calculate thresholds for generating pure entanglement by this technique: for example moving to higher magnetic fields (>3.5T) and lower temperatures (<4K). We possess the major instrumentation to meet these requirements, and will demonstrate the controlled generation of pure spin entanglement within silicon as part of this project. We will then develop methods for preserving entanglement and understand the effect of spin transport.The isotopic purification of silicon to 28Si yields dramatic improvements in the donor electron spin coherence to tens of milliseconds. However, the nuclear spin is a powerful resource into which the coherent electron spin state may be temporarily stored and retrieved. We have demonstrated the use of the nuclear spin as a quantum memory in this way, yielding coherence times up to several seconds. We will understand the mechanisms for spin decoherence and, using materials refinement and active techniques such as dynamic decoupling and error-correction, we will push the limits of the longest spin coherences times in the solid state. The spin ensembles which we will be studying are capable of storing multiple bits of information in distributed states, analogous to holographic information storage. We shall build on our initial work, in which we stored and retrieved 100 coherent weak microwave excitations within an ensemble, to establish multimode quantum memories working i) at low applied magnetic fields ii) down to the single photon level iii) capable of storing coherent quantum states for several seconds.Throughout this project we will be performing an iterative development of instrumentation and materials: longer coherence times enabled by the isotopically engineered materials will push us to the limits of our instrumentation, and by improving the instrumentation we can then extract and understand intrinsic properties of the materials so that they may be further enhanced.At the end of the project we shall have established donor spins in isotopically engineered silicon as the forerunner material for quantum spintronics and demonstrated the essential components for a quantum spintronics device.

The research proposed here addresses important questions of fundamental science as well as seeking to establish key components of an emerging quantum spintronics technology. We will build on our experience in materials science and quantum control techniques to embark on a series of experimental investigations using spins in condensed matter which lay previously well in the realm of isolated atomic physics. Such a step is critical for the exploitation of quantum coherence and entanglement within solid state devices. We will focus on silicon-based spin systems, which offer convenient integration within conventional technologies. Early applications of quantum spintronics include entanglement-enhanced magnetic field sensors for small changes in magnetic fields. With sufficient refinement, the technologies we establish could be developed towards applications in quantum information processing. Quantum information processing is already profoundly affecting the fields of computing and physics, possessing a novelty and a richness that suggests the likelihood of great unanticipated impact. Such promise has lead to significant funding around the world (e.g. major 5-years grants have recently been awarded to the value of 63M in Singapore, 30M in Canada, at institutions with which we are collaborating) and was highlighted in a US Federal Vision published earlier this year. The recent concept of a hybrid quantum device, a recurrent theme throughout this proposal, promises revolutionary advantages by allowing fast operation speeds (using electron spins) while having the ability to store quantum information for long times (using nuclear spins). This project fully embraces the emerging discipline of quantum information technology - a field of research which the British embassy in Tokyo has singled out as one in which is ripe for UK-Japan collaboration through their UK-Japan Quantum Information Workshop on 23-24 January 2009. Support for this cooperative program will directly address the acute need for increased mobility and exchange of researchers between the two countries which was identified in the report following this workshop. We have found that engaging 'weirdness' of quantum mechanics, and the technologies which it promises have the potential to excite the wider public. Dr Morton won the Cavendish Medal at SET for BRITAIN 2009 in the House of Commons, where he described his research to MPs. Drs Morton and Benjamin exhibited their work at the Royal Society 2008 Summer Science Exhibition. Dr Benjamin, with input from Dr Morton, has produced an award-winning series of podcasts on Quantum Nanotechnology on itunes.ox.ac.uk. We thus have considerable experience in the public engagement in science, and we shall look for new opportunities to share the excitement of the field of this project. Throughout the program we shall seek to educate policymakers in government, commerce, and industry, about the prospects for quantum technologies, and to using quantum nanoscience to help revive interest in the physical sciences in schools. Support of this program will also have a significant positive impact on the academic careers of the investigators, each of whom is at an early state in their careers, and significantly broaden the horizons of the UK post-doctoral researchers and students who will be able to experience a Japanese research environment and establish their own links with Japan.