Coherent quantum control of discrete and continuous variable systems

In any protocol aimed at manipulating or transmitting information, symbols are encoded in states of some physical system. If this system is quantum mechanical rather than classical, as is bound to become common if the miniaturisation of processing units persists at the current rate, the laws of information processing are different than the ones governing ordinary classical computers. Quantum Information (QI) science investigates these laws, exploring the novel, potentially revolutionary, possibilities offered by quantum mechanical systems across the whole spectrum of information technologies, from the possibility to run unique and faster algorithms on a quantum computer to applications in high precision metrology, and in general providing a new insight into the foundation of quantum mechanics. In the last two decades we have witnessed the rise of QI science from the realm of theoretical conjecture to that of actual technological application, with the prominent example of operating quantum cryptographic systems. Yet, a major obstacle still stands in the way of the day to day realisation of QI protocols: preserving quantum coherence, as necessary for QI processing, in most scalable systems at room conditions is always a very difficult challenge, mainly due to the undesired interactions between the small quantum system and its large environment. The field of quantum control offers one of the most promising ways to cope with the problem of decoherence (the unwanted demise of coherent quantum properties). Quantum control consists in the application of a series of externally controlled actions and manipulations on the system where the information is being stored or processed, such that, typically, decoherence is delayed, or specific quantum states, useful as resources for QI, can be prepared. Although the area of quantum control is well-established and has shown considerable promise, its application to QI science is still in its first steps, and the impact control techniques could have on the path towards daily QI processing is arguably still underestimated. This research project intends to study, develop and apply novel quantum control and feedback techniques, and to optimise them for specific quantum information tasks and for specifically promising physical systems (in particular atomic ensembles, trapped ions and solid state devices). The project is divided in three main objectives: i) the analysis of local controllability for continuous-variable and hybrid many-body systems; ii) the analysis of the entanglement enhancement by means of the interspersing of ''fast'' local operations and classical communication; iii) the generalisation of entanglement optimisation feedback techniques to noisy systems. The results obtained pursuing these objectives will hopefully prompt experimental groups, working on promising quantum technologies for quantum information, to use the developed techniques for their purposes.

The proposed research project will surely contribute to strengthen the leadership of UK in two strategic areas of interest for EPSRC as quantum information science and research at nano/microscopic scale. Indeed, since the birth of quantum information, the possibility of coherently control quantum systems was recognised as the main challenge to possible implementations. Thus theoretical investigations on the controllability properties of emerging and promising quantum technologies will certainly enhance the UK community competitiveness in these promising contexts, contributing to make a step further towards the ''day to day realisation'' of quantum information protocols. At a long-term the proposed research is potentially of the highest impact for the society as a whole. In fact, the implementation of quantum computers and in general the processing of quantum information would permit to realise tasks that are inaccessible to the actual technologies. Furthermore, quantum information is also providing a new insight into the foundation of quantum mechanics and the history of science teaches that its impact on the society fully discloses only many years after the discoveries themselves and in usually unpredictable ways. Thus, it is reasonable to guess that we still do not know which will be the major applications of quantum information/computation in the everyday life and we will discover them only in the next years.