Next generation measurement of the fine-structure constant

We present a next-generation atomic fountain designed to measure the fine-structure constant (alpha) as a test of the Standard model. Previous measurements in our group (Parker et al 2018) and at CNRS (Morel at al 2020) reported an accuracy of 200 parts-per-trillion and 80 parts-per-trillion respectively. Our improved atomic fountain seeks sensitivity to the 20 parts-per-trillion level. By driving Bragg diffraction using a beam with a large beam waist, systematic effects such as Guoy phase and effects from thermal motion of the atoms are minimized. A 5 meter tall, 50 cm wide vacuum chamber with optical baffles reduces the amount of scattered light and associated systematic effects. Offset simultaneous conjugate interferometer geometry cancels phase shifts from the gravity gradient and from diffraction phases. In order to achieve high momentum transfer with a larger beam area, we are constructing a kW-level quasi-CW laser system at 852 nm based on a high-efficiency free-space optical parametric amplifier.