An atomic fountain for measuring the fine-structure constant

The fine-structure constant α describes the strength of the electromagnetic interaction. By measuring the recoil frequency (defined as the kinetic energy gained by an atom that has been kicked by a photon), we can make an improved measurement of the fine structure constant. By comparing such a measurement to the value of α as determined from measurements of the electron's gyromagnetic anomaly g-2, we are able to perform one of the most precise tests of quantum electrodynamics (QED) and the Standard Model of physics. Our measurement is sensitive to the existence of new particles, so we may even find new physics. In 2018, we published a measurement of α with a precision of 0.2 parts-per-billion, the most precise measurement to date, and our next generation experiment seeks to improve this measurement by an order of magnitude.

Below is a plot of our measurement in comparison with other measurements published in the past:

Comparison of precision measurements of the fine structure constant. ‘Zero’ on the plot is the CODATA 2014 recommended value. The green points are from photon recoil experiments; the red ones are from electron gyromagnetic anomaly measurements.

Comparison of precision measurements of the fine structure constant. ‘Zero’ on the plot is the CODATA 2014 recommended value. The green points are from photon recoil experiments; the red ones are from electron gyromagnetic anomaly measurements.

To measure α, we use a simultaneous conjugate Ramsey-Borde interferometer (SCI) geometry. This type of interferometer cancels the phase acquired from gravity while enhancing the kinetic phase from the atom's recoil when absorbing photons. We use standing light waves to transfer the momentum of hundreds of photons to the atoms using Bragg diffraction and Bloch oscillations. The sensitivity of the measurement depends on the total phase acquired in the interferometer, so we push the limits of how many photons can be coherently transferred to atoms without degrading the interferometer signal. Below is a diagram of the SCI configuration used to measure α.

SCI.png

Simultaneous-Conjugate Atom Interferometer

The solid lines denote the atoms’ trajectories, dashed lines indicate Bragg diffraction laser pulses, and the shaded region labeled BO represents Bloch oscillation pulses. |n〉 denotes a momentum eigenstate with momentum 2nℏk, where k is the laser wave number. In this figure gravity is neglected.

The next-generation precision measurement of α requires improvements in the sensitivity of the instrument as well as our systematic uncertainties. We have begun construction of a new experiment that aims for an order of magnitude improvement in our measurement of α. In particular, we are targeting systematic phase shifts associated with wavefront curvature of the laser, and we are developing a powerful new laser system to increase the momentum transferred to the atoms. Currently, we are assembling the 4.5m tall atomic fountain. The entire chamber will be vibration isolated, and will have an 8" aperture for the laser beam as well as homemade copper oxide baffles to prevent any stray light from reflecting off of wall of the chamber.

alpha chamber cross section

New vacuum chamber design

This is a cross section view of the new vacuum chamber. The UHV chamber is 0.5m diameter and 4.5m long, with lots of hardware in-vacuum, including a three-layer magnetic shield, solenoid coil, and an 8” diameter retroreflecting mirror that is actively rotated with piezos to cancel earth’s rotation.

Feel free to contact any of our group members with questions about our research.

Team members

Madeline Bernstein

Jack Roth

Nadia Sun

Past team members

Andrew Neely

Zachary Pagel

Yair Segev

Ocean Zhou

Stephanie Bie

Spencer Kofford

Weicheng Zhong

Niah Freeman

Eric Planz

Aini Xu

Chenghui Yu

Brian Estey

Jiafeng Cui

Eric Huang

Pei-Chen Kuan

Shau-Yu Lan

Publications

  1. Measurement of the fine-structure constant as a test of the Standard Model. Richard H. Parker, Chenghui Yu, Weicheng Zhong, Brian Estey, and Holger Müller, Science 360, 191-195 (2018).