Periodic Reporting for period 1 - SPIRAL (Single Photons from Isotopically-pure Rubidium Atoms in a Long fibre) Reporting period: 2018-07-01 to 2020-02-29 Summary of the context and overall objectives of the project Quantum technology is set to revolutionise communication, computing, and sensing. Many upcoming technologies will require single-photon sources. When compatible with atomic systems these single-photon sources may open the door to many possibilities both for fundamental research but also for quantum technology. However, many atomic compatible single-photon sources previously developed lack the efficiency required to make them practical for quantum technology. The aim of this project was to take the first steps in boosting the efficiency for an atomic based single-photon source. The projects main goals were to investigate the possibility of using a new class of optical fibre, namely hollow-core photonic crystal fibres, to more efficiently generate light via so called “four-wave mixing”. Furthermore, complications in filtering the photons mean that another major goal of the project was to investigate a possible enabling technology, the so-called atomic Faraday dichroic beam splitter. Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far We have developed two key component devices and have successfully interfaced them. The first device is the new class of “hollow-core fibre cells”, which are comprised of hollow-core photonic crystal fibres fully encapsulated in an isotopically pure 87Rb vapour cell were made. These cells allow the hollow-core fibres to be loaded with 87Rb atoms quickly, and in a persistent manner by simply heating the cell. Our cells have long fibres mounted which allow the four-wave mixing process to be achieved at very low pump powers. The second key device is the atomic Faraday dichroic beam splitter, which was designed by computational optimisation before being built and tested. We found that the Faraday beam splitter works exceptionally well, and when interfaced with light generated from the hollow-core fibre cell we find clear evidence of four-wave mixing occurring even at unprecedentedly low laser powers for the type of atomic scheme.The results of the project have been disseminated to the scientific community through international conferences and meetings, as well as an academic article submitted to a peer-reviewed journal. Furthermore, we have participated in outreach activities to showcase our research to younger physics and school students (through lab tours) to inspire a new generation of scientists and engineers. Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far) By using computational optimisation to efficiency guide experimental efforts, this project has created an atomic Faraday dichroic beam splitter of unprecedented performance in spectral selectivity. It is also the first to be designed and demonstrated for application of separating light generated from four-wave mixing in an atomic ensemble. This project has therefore laid the groundwork for creating efficient heralded single photon sources that are compatible with atomic systems and are efficient enough to encourage their use for quantum technology. As such this project has provided results perfectly in line with the agenda of the EU Quantum Flagship, which aims to “kick-start a competitive European industry in Quantum Technologies”. Illustration of the Faraday dichroic beam splitter.