The purpose of this informal workshop is to review the landscape of cold atom technologies being developed to explore of dark sector physics - especially light dark matter, gravitational effects - particularly gravitational waves in the mid-frequency band, and searches for new fundamental interactions - which offer several options for ground-breaking discoveries. One important goal of the workshop will be to assemble a White Paper outlining the physics case for a future space-based cold atom detector in response to the ESA Voyager 2050 call  (deadline August 5th). This White Paper will propose a space interferometer experiment using cold atoms to detect ultra-light dark matter, and measure gravitational waves in the frequency range between the most sensitive ranges of the LISA and LIGO/Virgo/KAGRA/Einstein Telescope experiments. This experiment, called Atomic Experiment for Dark Matter and Gravity Exploration (AEDGE), would build upon the technologies now being developed for terrestrial experiments using cold atoms, and would benefit from the space experience obtained with LISA and cold atom experiments on the ISS. AEDGE would complement other planned searches for dark matter, and would have many synergies with other gravitational wave detectors. The Workshop will focus on the physics opportunities and connections between the fundamental research areas of particle physics and gravitational physics using technologies based on cold atom expertise. In this context, we will review several terrestrial atom interferometer projects to demonstrate the capabilities of different technologies, which are under construction, planned or proposed, including those listed below . To make the strongest possible science case, we aim to establish a diverse and large support across communities. This Workshop will be an opportunity to bring together representatives of the cold atom, particle physics and gravitational communities in Europe, to work towards a strong science case that will build the foundation for future space-based as well as terrestrial projects. Workshop Organisers: Kai Bongs(CA), Philippe Bouyer(CA), Oliver Buchmueller(PP), Albert De Roeck(PP), John Ellis(PP, Theory), Peter Graham (CA, Theory), Jason Hogan (CA), Wolf von Klitzing(CA), Guglielmo Tino(CA) and AtomQT [https://atomqt.eu] PP=Particle Physics CA=Cold Atoms  https://www.cosmos.esa.int/web/voyage-2050 : The Matter-wave laser Interferometric Gravitation Antenna (MIGA) Experiment: http://miga-project.org/. MIGA aims at demonstrating precision measurements of gravity with cold atom sensors in a large-scale instrument and at studying associated applications in geosciences and fundamental physics. In particular, it will assess future potential applications of atom interferometry to gravitational wave detection in the mid-frequency band between 0.1 and 10 Hz, intermediate between LISA and LIGO/Virgo/KAGRA/INDIGO/ET. The MAGIA-Advanced Project funded by the Italian Ministry for Research and Istituto Nazionale Fisica Nucleare for a large scale atom interferometer based on ultracold Rb and Sr atoms. In additions to lab R&D activity, the team is investigating the possibility of a 100-300 m atom interferometer to be installed in a vertical shaft on Sardinia island. Main goals are GW observation and search for DM. The Zhaoshan long-baseline Atom Interferometer Gravitation Antenna (ZAIGA) https://arxiv.org/pdf/1903.09288.pdf ZAIGA is an underground laser-linked interferometer facility under construction near Wuhan, China. It has an equilateral triangle configuration with two 1-km-apart atom interferometers in each arm, a 300-meter vertical tunnel equipped with an atom fountain and atomic clocks, and 1-km-arm-length optical clocks linked by locked lasers. It is designed for experimental research on gravitation and related problems including gravitational wave detection and high-precision tests of the equivalence principle. The Matter-wave Atomic Gradiometer Interferometric Sensor (MAGIS) project https://arxiv.org/pdf/1812.00482.pdf The MAGIS project in the US plans a series of interferometers using cold atoms with progressively increasing baselines of 10m, 100m, and 1km. The first step is funded and under construction at Stanford, the second step is being prepared at Fermilab, and the third step is planned for the Sanford Underground Research Facility. ELGAR ELGAR is a European initiative to build a terrestrial infrastructure for cold atoms for gravitational wave detection with potential applications also for dark matter. Configurations similar to MAGIS/AION are included in the proposal, and a White Paper about this infrastructure is being prepared [https://indico.obspm.fr/event/58/contributions/214/attachments/88/98/Slides-bouyer2018_06_21_MIGA_GDR.pdf] The UK Atom Interferometry Observatory and Network (AION) project http://www.hep.ph.ic.ac.uk/AION-Project/ The AION project in the UK proposes a series of atom interferometers with baselines of 10m, 100m, and 1km, similar to MAGIS. The first stage would be located in Oxford, with sites for the subsequent steps awaiting study. The Bose-Einstein Condensate and Cold Atom Laboratory (BECCAL) TheBECCAL is a bilateral project of NASA and DLR for a multi-purpose facility on the international space station, based in the heritage of drop tower (QUANTUS) and sounding rocket experiments (MAIUS). It will enable a variety of experiments in atom optics and atom interferometery to cover a broad spectrum ranging from fundamental physics to studies for applications in earth observation. It is intended as a pathfinder for future space missions. The Space Atomic Gravity Explorer (SAGE) http://arxiv.org/abs/1907.03867 The SAGE mission was proposed to the European Space Agency in 2016 in response to a Call for “New Ideas” (https://www.cosmos.esa.int/web/new-scientific-ideas). It has the scientific objective to investigate Gravitational Waves, Dark Matter, and other fundamental aspects of gravity as well as the connection between gravitational physics and quantum physics combining quantum sensing and quantum communication based on recent impressive advances in quantum technologies for atom interferometers, optical clocks, microwave and optical links.