Open Access Publications from the University of California

## Precision measurement of the fine-structure constant with atom interferometry

• Author(s): Zhong, Weicheng
The fine-structure constant $\alpha$ is ubiquitous in physics, and a comparison among different experiments provides a powerful test of the Standard Model of particle physics. We have recorded the most accurate measurement of $\alpha = 1/137.035999046(27)$ at an accuracy of 0.20 parts per billion (ppb) via measuring $h/m_\text{Cs}$, the quotient of the Planck constant and the mass of a cesium-133 atom. Our tools are simultaneous conjugate Ramsey-Bord e atom interferometers based on a cesium atomic fountain. Using Bragg diffraction and Bloch oscillations, we have demonstrated the largest phase (12 million radians) of any Ramsey-Bord e interferometer and controlled the systematic effects at a level of 0.12 ppb. Comparing the Penning trap measurements with the Standard Model prediction of the electron gyromagnetic moment anomaly $a_e$ based on our $\alpha$ result, a 2.5-$\sigma$ tension has been observed. This tension provides hints for new physics beyond the Standard Model.
One of the largest systematic effects of our $\alpha$ measurement comes from the gravity gradient $\gamma$. In order to suppress this effect in the next-generation $\alpha$ measurement, we have demonstrated a new atom interferometer configuration - offset simultaneous conjugate interferometers (OSCIs). We create two pairs of simultaneous conjugate interferometers and precisely control the offset between them with Bragg diffraction and Bloch oscillations. The multichannel readouts of OSCIs allow us to not only cancel the effect of $\gamma$, but also reduce the undesired diffraction phase from Bragg diffraction beam splitters.
Other ongoing and planned upgrades of the next-generation $\alpha$ measurement have also been presented in this thesis, including an over-sized vacuum chamber and a high-power pulsed laser system. With these upgrades, we expect to improve the accuracy of $\alpha$ by one to two orders of magnitude.