Testing sub-gravitational forces on atoms from a miniature, in-vacuum source mass (1612.05171v1)

Abstract: Gravity is the weakest fundamental interaction and the only one that has not been measured at the particle level. Traditional experimental methods, from astronomical observations to torsion balances, use macroscopic masses to both source and probe gravitational fields. Matter wave interferometers have used neutrons, atoms and molecular clusters as microscopic test particles, but initially probed the field sourced by the entire earth. Later, the gravitational field arising from hundreds of kilograms of artificial source masses was measured with atom interferometry. Miniaturizing the source mass and moving it into the vacuum chamber could improve positioning accuracy, allow the use of monocrystalline source masses for improved gravitational measurements, and test new physics, such as beyond-standard-model ("fifth") forces of nature and non-classical effects of gravity. In this work, we detect the gravitational force between freely falling cesium atoms and an in-vacuum, centimeter-sized source mass using atom interferometry with state-of-the-art sensitivity. The ability to sense gravitational-strength coupling is conjectured to access a natural lower bound for fundamental forces, thereby representing an important milestone in searches for physics beyond the standard model. A local, in-vacuum source mass is particularly sensitive to a wide class of interactions whose effects would otherwise be suppressed beyond detectability in regions of high matter density. For example, our measurement strengthens limits on a number of cosmologically-motivated scalar field models, such as chameleon and symmetron fields, by over two orders of magnitude and paves the way toward novel measurements of Newton's gravitational constant G and the gravitational Aharonov-Bohm effect

Testing Sub-Gravitational Forces with In-Vacuum Source Masses

The research conducted by Jaffe et al. presents a significant advancement in the experimental investigation of gravitational interactions at the atomic scale, specifically with the use of a miniature in-vacuum source mass combined with state-of-the-art atom interferometry techniques. Traditional methods in testing gravitational forces have primarily relied on large macroscopic bodies to act as both the source and the probe of gravitational fields. In contrast, this paper makes strides in utilizing smaller source masses within a controlled environment, specifically a vacuum chamber, to explore forces at a microscopic level.

The experiment described utilizes cesium atoms, cooled and launched in free fall, to measure gravitational interactions with a tungsten cylinder of mere centimeters in size. Employing light-pulse atom interferometry, the researchers have demonstrated the potential to detect gravitational-strength couplings between atoms and a miniature source mass with unparalleled precision. This methodological refinement aids in enhancing the positioning accuracy of the source mass and opens the door for utilizing monocrystalline structures to achieve better control and measurement fidelity.

A noteworthy outcome of this paper is the strengthening of limits on scalar field models, such as chameleon and symmetron fields, by over two orders of magnitude. These models are of particular interest because they provide potential explanations for cosmologically significant phenomena like dark energy. The research sets up a framework for more precise measurements of Newton's constant $G$ and introduces the possibility of observing the gravitational Aharonov-Bohm effect.

The implications of these findings are manifold. Practically, the setup provides a highly sensitive platform to test beyond-standard-model forces and helps address fundamental questions in cosmology, such as the observed acceleration of the universe’s expansion. Theoretically, the ability to test ultra-weak fields and the mechanisms of screening within the vicinity of small source masses suggests new research pathways in both quantum mechanics and gravitational physics.

The experiment found an anomalous acceleration of $a_{\text{anomaly}} = 11 \pm 24$ nm/s$^2$, indicating no significant deviation from expected gravitational interactions at the tested scale. However, the sensitivity achieved, at 24 nm/s$^2$, aligns closely with that of the most precise atom-interferometric measurements to date. This result not only validates the efficacy of using atom interferometry with smaller masses but also enhances our understanding of gravity in less tested regimes.

Looking to the future, the authors propose further enhancements by integrating lattice interferometry within optical cavities and employing large momentum transfer beam splitters to overcome geometric limitations and improve sensitivity. Such upgrades could narrow the constraints on alternative theories further, potentially ruling out or verifying the existence of cosmologically-inspired scalar fields.

In conclusion, this research exemplifies a pivotal step in testing gravitational phenomena at microscopic scales, utilizing innovative interferometric techniques and miniature source masses. The experimental outcomes not only reinforce current gravitational theories but also pave the way for exploring exotic gravitational effects and clarifying erstwhile elusive cosmological questions.