Within optical quantum information processing, the quantum bits are encoded on single photons and their quantum mechanical properties are exploited to build new functionality. A prime example is the quantum computer, which can be built simply from single-photon sources and detectors, and simple optical components. However for scalable optical quantum computing involving hundreds of photons, the performance requirements for the single-photon source are daunting: the source must feature near-unity efficiency and near-unity indistinguishability simultaneously! Today, all known source designs suffer from inherent trade-offs between efficiency and indistinguishability and their performance is insufficient for scalable quantum computing. The project objective is to realize a source of single indistinguishable photons with performance of ground-breaking nature. The break-through lies in the simultaneous realization of near-unity efficiency and indistinguishability, a combination which overcomes the limitations of present state-of-the-art and ventures far into the regime of scalable quantum computing. As an expert in single-photon source engineering I find myself in a unique position to address this challenge. Since it is unknown how to design such a source, I will first establish a new understanding of the physics of the near-unity regime, where phonon-induced decoherence represents a main limitation for the indistinguishability. I will then advance state-of-the-art in optical engineering by proposing a novel design, where all physical parameters can be controlled independently. The modelling of the near-unity performance source is extremely demanding, and the analysis requires additional advances within optical simulations and open quantum systems theory. Once this is achieved, I will fabricate a prototype and test it in a multi-photon interference boson sampling experiment to unambiguously prove that scalable optical quantum information processing is indeed within reach.

Hydrogen is an alternative future energy carrier. However, the drawback associated with its compact storage is still a scientific and technological challenge. Metal hydrides offer a suitable combination weighing both safety and cost. In particular, magnesium hydride (MgH2) is an ideal candidate with a high gravimetric capacity of 7.6 wt %, low cost, and abundance in nature. However, the high stability of Mg-H is a significant limitation for practical application. Although, recently, interface and strain induced-modification is proposed as a strategy to reduce the MgH2 stability in Mg nanoalloys. Nonetheless, they are not well understood in Mg nanoalloys. Moreover, understanding and interpreting these effects on a single nanoparticle (NP) from bulk measurement techniques is a significant problem. Since the effect of averaging and low spatial resolution plagues the collected data, it prevents in resolving the intrinsic impact of size, strain, and interface on a structure-property relationship of single NPs. Therefore, we propose (i) to use STEM-EELS with insitu gas holder(H2) at operando conditions in an aberration-corrected microscope to unravel the metal-hydride phase transition of individual Mg nanoalloys. (ii) apply state of the art iDPC and 4D-STEM to resolve the role of the interface and precise measurement of strain to identify the effect of destabilization on individual Mg nanoalloys. Moreover, advanced training on insitu TEM at DTU, iDPC, and 4D-STEM techniques @secondment and other transferable skills will diversify my competence further and positively impact my future career prospects and networking across Europe. The infrastructure/expertise at DTU, my experience, and knowledge in NP synthesis and hydrogen storage, along with the DTU support office, will ensure the successful implementation of the proposal. Finally, disseminating research and communication to the stakeholders and the general public will ensure the maximum impact of the project's results.