Entanglement is one of the most profound concepts to emerge from quantum mechanics, and a phenomenon whose implementation in real materials requires exceptional control over state preparation, coherence, coupling and measurement. The collaborators have already begun to address these individual challenges using the complementary advantages of both electron and nuclear spin degrees of freedom in a diverse range of materials, with notable successes including the coherent storage of the electron spin state in the nuclear spin to achieve coherence times of several seconds, and the true entanglement of an electron and nuclear spin with high fidelity. In this proposal, we will bring together these individual components, exploiting the transient nature of the electron spin in systems such as optically-excited molecules and silicon-based devices, in order to mediate entanglement between multiple nuclear spins. In addition to providing a key component of emerging quantum technologies within the solid state, this will lead to a new understanding of the mechanisms and correlations behind decoherence of electron and nuclear spins states, under different environments and processes.The key idea in this proposal is to use the transient electron spins in certain materials and devices, not only to understand and overcome spin decoherence mechanisms, but also to mediate the entanglement of multiple nuclear spins. Through experiment, density functional theory and modeling of open quantum systems, we will address long-standing questions behind spin decoherence in various condensed matter systems as well as new emerging questions such as the evolution and destruction of entangled states. We will address further technologically relevant questions such as the effect of interfaces on spin coherence in semiconductor devices, as well as the effect of removal or addition of an electron spin (by optical, or electrical means) on the coherent state of coupled nuclear spins. Six graduate students and postdocs will participate in a stimulating international collaboration between Oxford, Heriot-Watt and Princeton, supported by the fluid exchange of young researchers between the participating institutions, as well as interactions with their collaborators around the world. The project partners will continue to host undergraduate students in their laboratory on summer projects connecting with this research proposal. The Oxford and Heriot-Watt teams will continue to participate in enhancing the public understanding of science by, for example, presenting work at the Royal Society Summer Exhibit and hosting local high-school students in their laboratories. The Oxford investigators have experience producing a series of award-winning video podcasts and the collaboration will build on this experience to produce a joint series of podcasts, aimed a general audience, describing the basic science behind this proposal and the exciting applications which it promises.The grant will support and strengthen an existing and highly successful collaboration between researchers at Princeton, Oxford and Heriot-Watt. The groups have a strong track record performing the experiments and developing the techniques which motivate and enable the research proposed here. Together, the investigators bring together a range of expertise, including magnetic resonance (ESR, ENDOR), quantum information theory, density functional theory, semiconductor physics and organic chemistry. The collaborative nature of this proposed activity allows the focus to be on fundamental physical questions spanning a diverse range of physical quantum spin systems, increasing the impact of the experiments and range of beneficiaries in the scientific community. Finally, the complementary instrumentation across the three institutions provides the necessary set of experimental tools required for this challenging experimental program.