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 uncertainty associated with the measurement. 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. There is still a ton of work to do, and we're looking for passionate people to join our team! Feel free to contact any of our group members with questions about our research.

Team members

Richard Parker

Weicheng Zhong

Zachary Pagel

Eric Planz

Past team members

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).

  2. Controlling the Multiport Nature of Bragg Diffraction in Atom Interferometry. Richard H. Parker, Chenghui Yu, Brian Estey, Weicheng Zhong, Eric Huang, and Holger Müller, Phys. Rev. A 94, 053618 and arXiv:1609.06344.

  3. High resolution atom interferometers with suppressed diffraction phases. Brian Estey, Chenghui Yu, Holger Müller, Pei-Chen Kuan, and Shau-Yu Lan, Phys. Rev. Lett. 115, 083002 (2015) and arXiv:1410.8486.

  4. A clock directly linking time to a particle’s mass. Shau-Yu Lan, Pei-Chen Kuan, Brian Estey, Damon English, Justin Brown, Michael Hohensee, and Holger Müller, Science, 339, 554 (2013) with Science Perspective.