If dark energy -which drives the accelerated expansion of the universeconsists of a light scalar field, it might be detectable as a "fifth force" between normalmatter objects, in potential conflict with precision tests of gravity. Chameleon fields and other theories with screening mechanisms, however, can evade these tests by suppressing the forces in regions of high density, such as the laboratory. Using a cesium matter-wave interferometer near a spherical mass in an ultra-high vacuum chamber, we reduce the screening mechanism by probing the field with individual atoms rather than bulk matter. Thus, we constrain a wide class of dark energy theories, including a range of chameleon and other theories that reproduce the observed cosmic acceleration.Cosmological observations have now firmly established that the universe is expanding at an accelerating pace, which can be explained by dark energy permeating all of space and accounting for ∼ 70% of the energy density of the universe (1). What constitutes dark energy, and why it has its particular density, remain as some of the most pressing open questions in physics. What is clear is that dark energy presents us with a new energy scale, of order meV. It is natural to speculate that new (usually scalar) fields might be associated with that scale that make up all or part of the dark energy density (2,3). String theory with compactified extra dimensions, for instance, features a plethora of scalar fields, which typically couple directly to matter fields unless protected by a shift symmetry as for axions (4,5). If the fields are light, this coupling would be observable as a "fifth force", in potential conflict with precision tests of gravity (6). 2Theories with so-called screening mechanisms, on the other hand, have features that suppress their effects in regions of high density, so that they may couple to matter but nonetheless evade experimental constraints (7). One prominent example is the chameleon field, whose mass depends on the ambient matter density (8,9). It is light and mediates a long-range force in sparse environments, such as the cosmos, but becomes massive and thus short-ranged in a high-density environment, such as the laboratory (see Fig. S1). This makes it difficult to detect by fifth-force experiments.Burrage, Copeland and Hinds (10) have recently proposed to use atom interferometers (11,12) to search for chameleons. An ultrahigh-vacuum chamber containing atomic test particles simulates the low-density conditions of empty space, liberating the chameleon field to become long-ranged and, thus, measurable. Here, we use a cavity-based atom interferometer (13,14) measuring the force between cesium-133 atoms and an aluminum sphere to search for a range of screened dark energy theories that can reproduce the current dark energy density (Fig. 1A, B).The chameleon dark energy field ϕ in equilibrium is determined by minimizing a potential density V(ϕ)+V int , which is the sum of a self-interaction term V(ϕ) and a term V int describing the interaction wit...
We review recent progress and future prospects of matter wave interferometry with complex organic molecules and inorganic clusters. Three variants of a near-field interference effect, based on diffraction by material nanostructures, at optical phase gratings, and at ionizing laser fields are considered. We discuss the theoretical concepts underlying these experiments and the experimental challenges. This includes optimizing interferometer designs as well as understanding the role of decoherence. The high sensitivity of matter wave interference experiments to external perturbations is demonstrated to be useful for accurately measuring internal properties of delocalized nanoparticles. We conclude by investigating the prospects for probing the quantum superposition principle in the limit of high particle mass and complexity.
Matter-wave interferometry with atoms 1 and molecules 2 has attracted a rapidly growing level of interest over the past two decades, both in demonstrations of fundamental quantum phenomena and in quantum-enhanced precision measurements. Such experiments exploit the non-classical superposition of two or more position and momentum states that are coherently split and rejoined to interfere 3-11 . Here, we present the experimental realization of a universal nearfield interferometer built from three short-pulse single-photon ionization gratings 12,13 . We observe quantum interference of fast molecular clusters, with a composite de Broglie wavelength as small as 275 fm. Optical ionization gratings are largely independent of the specific internal level structure and are therefore universally applicable to different kinds of nanoparticle, ranging from atoms to clusters, molecules and nanospheres. The interferometer is sensitive to fringe shifts as small as a few nanometres and yet robust against velocitydependent phase shifts, because the gratings exist only for nanoseconds and form an interferometer in the time domain.Recent progress in atom interferometry has been driven by the development of wide-angle beam splitters 14 , large interferometer areas 15 and long coherence times 16 . Most interferometers operate in a Mach-Zehnder 5,17 , Ramsey-Bordé 18 or Talbot-Lau 19 configuration, some of them also in the time domain 20,21 . Here we ask how to generalize these achievements to atoms, molecules, clusters or nanoparticles-irrespective of their internal states.Mechanical nanomasks 22 could be considered as universal if it were not for their van der Waals attraction on the traversing matter waves, which induces sizable dispersive, that is, velocity-dependent, phase shifts even for gratings as thin as 10 nm.Optical 9,14 or measurement-induced 23 gratings eliminate this effect, but most methods so far relied on closed transitions and required an individual light source for every specific kind of atom or molecule.It is possible to circumvent this restriction by using the spatially periodic electric dipole potential in an off-resonant standing light wave. Its field then modulates the phase of the matter wave rather than the amplitude. This implies, however, that the spatial coherence of the incident matter wave needs to be prepared by other means before-such as by collimation, cooling 24 or the addition of another absorptive (material) mask 2 .Here, we demonstrate a new method for coherence experiments with a wide class of massive particles and show how a sequence of ionizing laser grating pulses 12 can form a generic matter-wave interferometer in the time domain 13 . Figure 1 shows a schematic of the layout of our experiment, which we here realize specifically for clusters of anthracene (Ac) molecules. The molecules are evaporated in an Even-Lavie valve 25 Faculty of Physics, University of Vienna, VCQ, Boltzmanngasse 5, A-1090 Vienna, Austria. *e-mail: markus.arndt@univie.ac.at.that injects the organic vapour with a pulse wi...
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