We propose in this White Paper a concept for a space experiment using cold atoms to search for ultra-light dark matter, and to detect gravitational waves in the frequency range between the most sensitive ranges of LISA and the terrestrial LIGO/Virgo/KAGRA/INDIGO experiments. This interdisciplinary experiment, called Atomic Experiment for Dark Matter and Gravity Exploration (AEDGE), will also complement other planned searches for dark matter, and exploit synergies with other gravitational wave detectors. We give examples of the extended range of sensitivity to ultra-light dark matter offered by AEDGE, and how its gravitational-wave measurements could explore the assembly of super-massive black holes, first-order phase transitions in the early universe and cosmic strings. AEDGE will be based upon technologies now being developed for terrestrial experiments using cold atoms, and will benefit from the space experience obtained with, e.g., LISA and cold atom experiments in microgravity.KCL-PH-TH/2019-65, CERN-TH-2019-126

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SummaryMatter-wave microscopy can be dated back to 1932 when Max Knoll and Ernst Ruska published the first image obtained with a beam of focussed electrons. In this paper a new step in the development of matter-wave microscopy is presented. We have created an instrument where a focussed beam of neutral, ground-state atoms (helium) is used to image a sample. We present the first 2D images obtained using this new technique. The imaged sample is a free-standing hexagonal copper grating (with a period of about 36 μm and rod thickness of about 8 μm). The images were obtained in transmission mode by scanning the focussed beam, which had a minimum spot size of about 2.0 μm in diameter (full width at half maximum) across the sample. The smallest focus achieved was 1.9 ± 0.1 μm. The resolution for this experiment was limited by the speed ratio of the atomic beam through the chromatic aberrations of the zone plate that was used to focus. Ultimately the theoretical resolution limit is set by the wavelength of the probing particle. In praxis, the resolution is limited by the source and the focussing optics.

In the Poisson-spot experiment, waves emanating from a source are blocked by a circular obstacle. Due to their positive on-axis interference an image of the source ͑the Poisson spot͒ is observed within the geometrical shadow of the obstacle. In this paper we report the observation of Poisson's spot using a beam of neutral deuterium molecules. The wavelength independence and the weak constraints on angular alignment and position of the circular obstacle make Poisson's spot a promising candidate for applications ranging from the study of large molecule diffraction to patterning with molecules. DOI: 10.1103/PhysRevA.79.053823 PACS number͑s͒: 37.20.ϩj, 03.75.Ϫb, 37.25.ϩk Diffraction experiments played a crucial role in establishing the existence of de Broglie matter waves ͓1-3͔. Today, matter-wave diffraction is used, among other applications, to investigate quantum interference of large molecules ͓4͔, enabling the study of quantum decoherence ͓5͔ and its role in the quantum-to-macroscopic-world transition. These experiments have mostly been carried out with free-standing material gratings ͓6,7͔ or light wave gratings ͓8͔. The largest molecules so far ͑Ͼ3 nm͒ for which quantum interference was successfully demonstrated were perfluoroalkylfunctionalized azobenzenes in a Kapitza-Dirac-Talbot-Lau interferometer ͓9͔. Scaling such experiments to even larger objects, such as macromolecules or perhaps even viruses, is a tantalizing prospect. In principle, this should be possible to some degree with a Kapitza-Dirac-Talbot-Lau interferometer. However, as the size of the molecule and/or object approaches the distance between grating bars difficulties arise. In the case of material gratings, van der Waals ͑vdW͒ forces increasingly limit interference contrast by adding a locally varying coherent phase shift. In fact, even blocking may occur. In the case of light gratings spontaneous emission and photon absorption are likely to perturb coherence. Furthermore, for the Talbot-Lau configuration the distance between the three gratings is a function of wavelength, and thus requires wavelength selection. This limits effective intensity of the commonly used thermal sources because only a fraction of the emitted molecules can be used in the experiment. In the case of clusters, the necessity of mass selection constrains effective source intensity additionally. Finally, alignment of the gratings, both with respect to each other and the vertical, is challenging, and misalignment can cause classical Moiré fringes which differ from expected interference patterns only in visibility and wavelength dependence.In this paper we make use of the Poisson-spot configuration to demonstrate quantum interference in a beam of molecules. The Poisson spot refers to a classical-optics experiment, in which a point light-source is blocked by a circular obstacle. Wave theory predicts that the intensity on the optical axis within the geometrical shadow is the same as without the blocking obstacle due to the cylindrical symmetry ͓11͔, resulting in a bright i...